path: root/doc/rfc/rfc2083.txt

Network Working Group                                T. Boutell, et. al.
Request for Comments: 2083                             Boutell.Com, Inc.
Category: Informational                                       March 1997


             PNG (Portable Network Graphics) Specification
                              Version 1.0

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

IESG Note:

   The IESG takes no position on the validity of any Intellectual
   Property Rights statements contained in this document.

Abstract

   This document describes PNG (Portable Network Graphics), an
   extensible file format for the lossless, portable, well-compressed
   storage of raster images.  PNG provides a patent-free replacement for
   GIF and can also replace many common uses of TIFF.  Indexed-color,
   grayscale, and truecolor images are supported, plus an optional alpha
   channel.  Sample depths range from 1 to 16 bits.

   PNG is designed to work well in online viewing applications, such as
   the World Wide Web, so it is fully streamable with a progressive
   display option.  PNG is robust, providing both full file integrity
   checking and simple detection of common transmission errors.  Also,
   PNG can store gamma and chromaticity data for improved color matching
   on heterogeneous platforms.

   This specification defines the Internet Media Type image/png.

Table of Contents

   1. Introduction ..................................................  4
   2. Data Representation ...........................................  5
      2.1. Integers and byte order ..................................  5
      2.2. Color values .............................................  6
      2.3. Image layout .............................................  6
      2.4. Alpha channel ............................................  7
      2.5. Filtering ................................................  8
      2.6. Interlaced data order ....................................  8
      2.7. Gamma correction ......................................... 10



Boutell, et. al.             Informational                      [Page 1]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      2.8. Text strings ............................................. 10
   3. File Structure ................................................ 11
      3.1. PNG file signature ....................................... 11
      3.2. Chunk layout ............................................. 11
      3.3. Chunk naming conventions ................................. 12
      3.4. CRC algorithm ............................................ 15
   4. Chunk Specifications .......................................... 15
      4.1. Critical chunks .......................................... 15
          4.1.1. IHDR Image header .................................. 15
          4.1.2. PLTE Palette ....................................... 17
          4.1.3. IDAT Image data .................................... 18
          4.1.4. IEND Image trailer ................................. 19
      4.2. Ancillary chunks ......................................... 19
          4.2.1. bKGD Background color .............................. 19
          4.2.2. cHRM Primary chromaticities and white point ........ 20
          4.2.3. gAMA Image gamma ................................... 21
          4.2.4. hIST Image histogram ............................... 21
          4.2.5. pHYs Physical pixel dimensions ..................... 22
          4.2.6. sBIT Significant bits .............................. 22
          4.2.7. tEXt Textual data .................................. 24
          4.2.8. tIME Image last-modification time .................. 25
          4.2.9. tRNS Transparency .................................. 26
          4.2.10. zTXt Compressed textual data ...................... 27
      4.3. Summary of standard chunks ............................... 28
      4.4. Additional chunk types ................................... 29
   5. Deflate/Inflate Compression ................................... 29
   6. Filter Algorithms ............................................. 31
      6.1. Filter types ............................................. 31
      6.2. Filter type 0: None ...................................... 32
      6.3. Filter type 1: Sub ....................................... 33
      6.4. Filter type 2: Up ........................................ 33
      6.5. Filter type 3: Average ................................... 34
      6.6. Filter type 4: Paeth...................................... 35
   7. Chunk Ordering Rules .......................................... 36
      7.1. Behavior of PNG editors .................................. 37
      7.2. Ordering of ancillary chunks ............................. 38
      7.3. Ordering of critical chunks .............................. 38
   8. Miscellaneous Topics .......................................... 39
      8.1. File name extension ...................................... 39
      8.2. Internet media type ...................................... 39
      8.3. Macintosh file layout .................................... 39
      8.4. Multiple-image extension ................................. 39
      8.5. Security considerations .................................. 40
   9. Recommendations for Encoders .................................. 41
      9.1. Sample depth scaling ..................................... 41
      9.2. Encoder gamma handling ................................... 42
      9.3. Encoder color handling ................................... 45
      9.4. Alpha channel creation ................................... 47



Boutell, et. al.             Informational                      [Page 2]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      9.5. Suggested palettes ....................................... 48
      9.6. Filter selection ......................................... 49
      9.7. Text chunk processing .................................... 49
      9.8. Use of private chunks .................................... 50
      9.9. Private type and method codes ............................ 51
   10. Recommendations for Decoders ................................. 51
      10.1. Error checking .......................................... 52
      10.2. Pixel dimensions ........................................ 52
      10.3. Truecolor image handling ................................ 52
      10.4. Sample depth rescaling .................................. 53
      10.5. Decoder gamma handling .................................. 54
      10.6. Decoder color handling .................................. 56
      10.7. Background color ........................................ 57
      10.8. Alpha channel processing ................................ 58
      10.9. Progressive display ..................................... 62
      10.10. Suggested-palette and histogram usage .................. 63
      10.11. Text chunk processing .................................. 64
   11. Glossary ..................................................... 65
   12. Appendix: Rationale .......................................... 69
      12.1. Why a new file format? .................................. 69
      12.2. Why these features? ..................................... 70
      12.3. Why not these features? ................................. 70
      12.4. Why not use format X? ................................... 72
      12.5. Byte order .............................................. 73
      12.6. Interlacing ............................................. 73
      12.7. Why gamma? .............................................. 73
      12.8. Non-premultiplied alpha ................................. 75
      12.9. Filtering ............................................... 75
      12.10. Text strings ........................................... 76
      12.11. PNG file signature ..................................... 77
      12.12. Chunk layout ........................................... 77
      12.13. Chunk naming conventions ............................... 78
      12.14. Palette histograms ..................................... 80
   13. Appendix: Gamma Tutorial ..................................... 81
   14. Appendix: Color Tutorial ..................................... 89
   15. Appendix: Sample CRC Code .................................... 94
   16. Appendix: Online Resources ................................... 96
   17. Appendix: Revision History ................................... 96
   18. References ................................................... 97
   19. Credits ......................................................100











Boutell, et. al.             Informational                      [Page 3]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


1. Introduction

   The PNG format provides a portable, legally unencumbered, well-
   compressed, well-specified standard for lossless bitmapped image
   files.

   Although the initial motivation for developing PNG was to replace
   GIF, the design provides some useful new features not available in
   GIF, with minimal cost to developers.

   GIF features retained in PNG include:

       * Indexed-color images of up to 256 colors.
       * Streamability: files can be read and written serially, thus
         allowing the file format to be used as a communications
         protocol for on-the-fly generation and display of images.
       * Progressive display: a suitably prepared image file can be
         displayed as it is received over a communications link,
         yielding a low-resolution image very quickly followed by
         gradual improvement of detail.
       * Transparency: portions of the image can be marked as
         transparent, creating the effect of a non-rectangular image.
       * Ancillary information: textual comments and other data can be
         stored within the image file.
       * Complete hardware and platform independence.
       * Effective, 100% lossless compression.

   Important new features of PNG, not available in GIF, include:

       * Truecolor images of up to 48 bits per pixel.
       * Grayscale images of up to 16 bits per pixel.
       * Full alpha channel (general transparency masks).
       * Image gamma information, which supports automatic display of
         images with correct brightness/contrast regardless of the
         machines used to originate and display the image.
       * Reliable, straightforward detection of file corruption.
       * Faster initial presentation in progressive display mode.

   PNG is designed to be:

       * Simple and portable: developers should be able to implement PNG
         easily.
       * Legally unencumbered: to the best knowledge of the PNG authors,
         no algorithms under legal challenge are used.  (Some
         considerable effort has been spent to verify this.)
       * Well compressed: both indexed-color and truecolor images are
         compressed as effectively as in any other widely used lossless
         format, and in most cases more effectively.



Boutell, et. al.             Informational                      [Page 4]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


       * Interchangeable: any standard-conforming PNG decoder must read
         all conforming PNG files.
       * Flexible: the format allows for future extensions and private
         add-ons, without compromising interchangeability of basic PNG.
       * Robust: the design supports full file integrity checking as
         well as simple, quick detection of common transmission errors.

   The main part of this specification gives the definition of the file
   format and recommendations for encoder and decoder behavior.  An
   appendix gives the rationale for many design decisions.  Although the
   rationale is not part of the formal specification, reading it can
   help implementors understand the design.  Cross-references in the
   main text point to relevant parts of the rationale.  Additional
   appendixes, also not part of the formal specification, provide
   tutorials on gamma and color theory as well as other supporting
   material.

   In this specification, the word "must" indicates a mandatory
   requirement, while "should" indicates recommended behavior.

   See Rationale: Why a new file format? (Section 12.1), Why these
   features? (Section 12.2), Why not these features? (Section 12.3), Why
   not use format X? (Section 12.4).

   Pronunciation

      PNG is pronounced "ping".

2. Data Representation

   This chapter discusses basic data representations used in PNG files,
   as well as the expected representation of the image data.

   2.1. Integers and byte order

      All integers that require more than one byte must be in network
      byte order: the most significant byte comes first, then the less
      significant bytes in descending order of significance (MSB LSB for
      two-byte integers, B3 B2 B1 B0 for four-byte integers).  The
      highest bit (value 128) of a byte is numbered bit 7; the lowest
      bit (value 1) is numbered bit 0. Values are unsigned unless
      otherwise noted. Values explicitly noted as signed are represented
      in two's complement notation.

      See Rationale: Byte order (Section 12.5).






Boutell, et. al.             Informational                      [Page 5]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   2.2. Color values

      Colors can be represented by either grayscale or RGB (red, green,
      blue) sample data.  Grayscale data represents luminance; RGB data
      represents calibrated color information (if the cHRM chunk is
      present) or uncalibrated device-dependent color (if cHRM is
      absent).  All color values range from zero (representing black) to
      most intense at the maximum value for the sample depth.  Note that
      the maximum value at a given sample depth is (2^sampledepth)-1,
      not 2^sampledepth.

      Sample values are not necessarily linear; the gAMA chunk specifies
      the gamma characteristic of the source device, and viewers are
      strongly encouraged to compensate properly.  See Gamma correction
      (Section 2.7).

      Source data with a precision not directly supported in PNG (for
      example, 5 bit/sample truecolor) must be scaled up to the next
      higher supported bit depth.  This scaling is reversible with no
      loss of data, and it reduces the number of cases that decoders
      have to cope with.  See Recommendations for Encoders: Sample depth
      scaling (Section 9.1) and Recommendations for Decoders: Sample
      depth rescaling (Section 10.4).

   2.3. Image layout

      Conceptually, a PNG image is a rectangular pixel array, with
      pixels appearing left-to-right within each scanline, and scanlines
      appearing top-to-bottom.  (For progressive display purposes, the
      data may actually be transmitted in a different order; see
      Interlaced data order, Section 2.6.) The size of each pixel is
      determined by the bit depth, which is the number of bits per
      sample in the image data.

      Three types of pixel are supported:

          * An indexed-color pixel is represented by a single sample
            that is an index into a supplied palette.  The image bit
            depth determines the maximum number of palette entries, but
            not the color precision within the palette.
          * A grayscale pixel is represented by a single sample that is
            a grayscale level, where zero is black and the largest value
            for the bit depth is white.
          * A truecolor pixel is represented by three samples: red (zero
            = black, max = red) appears first, then green (zero = black,
            max = green), then blue (zero = black, max = blue).  The bit
            depth specifies the size of each sample, not the total pixel
            size.



Boutell, et. al.             Informational                      [Page 6]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Optionally, grayscale and truecolor pixels can also include an
      alpha sample, as described in the next section.

      Pixels are always packed into scanlines with no wasted bits
      between pixels.  Pixels smaller than a byte never cross byte
      boundaries; they are packed into bytes with the leftmost pixel in
      the high-order bits of a byte, the rightmost in the low-order
      bits.  Permitted bit depths and pixel types are restricted so that
      in all cases the packing is simple and efficient.

      PNG permits multi-sample pixels only with 8- and 16-bit samples,
      so multiple samples of a single pixel are never packed into one
      byte.  16-bit samples are stored in network byte order (MSB
      first).

      Scanlines always begin on byte boundaries.  When pixels have fewer
      than 8 bits and the scanline width is not evenly divisible by the
      number of pixels per byte, the low-order bits in the last byte of
      each scanline are wasted.  The contents of these wasted bits are
      unspecified.

      An additional "filter type" byte is added to the beginning of
      every scanline (see Filtering, Section 2.5).  The filter type byte
      is not considered part of the image data, but it is included in
      the datastream sent to the compression step.

   2.4. Alpha channel

      An alpha channel, representing transparency information on a per-
      pixel basis, can be included in grayscale and truecolor PNG
      images.

      An alpha value of zero represents full transparency, and a value
      of (2^bitdepth)-1 represents a fully opaque pixel.  Intermediate
      values indicate partially transparent pixels that can be combined
      with a background image to yield a composite image.  (Thus, alpha
      is really the degree of opacity of the pixel.  But most people
      refer to alpha as providing transparency information, not opacity
      information, and we continue that custom here.)

      Alpha channels can be included with images that have either 8 or
      16 bits per sample, but not with images that have fewer than 8
      bits per sample.  Alpha samples are represented with the same bit
      depth used for the image samples.  The alpha sample for each pixel
      is stored immediately following the grayscale or RGB samples of
      the pixel.





Boutell, et. al.             Informational                      [Page 7]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The color values stored for a pixel are not affected by the alpha
      value assigned to the pixel.  This rule is sometimes called
      "unassociated" or "non-premultiplied" alpha.  (Another common
      technique is to store sample values premultiplied by the alpha
      fraction; in effect, such an image is already composited against a
      black background.  PNG does not use premultiplied alpha.)

      Transparency control is also possible without the storage cost of
      a full alpha channel.  In an indexed-color image, an alpha value
      can be defined for each palette entry.  In grayscale and truecolor
      images, a single pixel value can be identified as being
      "transparent".  These techniques are controlled by the tRNS
      ancillary chunk type.

      If no alpha channel nor tRNS chunk is present, all pixels in the
      image are to be treated as fully opaque.

      Viewers can support transparency control partially, or not at all.

      See Rationale: Non-premultiplied alpha (Section 12.8),
      Recommendations for Encoders: Alpha channel creation (Section
      9.4), and Recommendations for Decoders: Alpha channel processing
      (Section 10.8).

   2.5. Filtering

      PNG allows the image data to be filtered before it is compressed.
      Filtering can improve the compressibility of the data.  The filter
      step itself does not reduce the size of the data.  All PNG filters
      are strictly lossless.

      PNG defines several different filter algorithms, including "None"
      which indicates no filtering.  The filter algorithm is specified
      for each scanline by a filter type byte that precedes the filtered
      scanline in the precompression datastream.  An intelligent encoder
      can switch filters from one scanline to the next.  The method for
      choosing which filter to employ is up to the encoder.

      See Filter Algorithms (Chapter 6) and Rationale: Filtering
      (Section 12.9).

   2.6. Interlaced data order

      A PNG image can be stored in interlaced order to allow progressive
      display.  The purpose of this feature is to allow images to "fade
      in" when they are being displayed on-the-fly.  Interlacing
      slightly expands the file size on average, but it gives the user a
      meaningful display much more rapidly.  Note that decoders are



Boutell, et. al.             Informational                      [Page 8]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      required to be able to read interlaced images, whether or not they
      actually perform progressive display.

      With interlace method 0, pixels are stored sequentially from left
      to right, and scanlines sequentially from top to bottom (no
      interlacing).

      Interlace method 1, known as Adam7 after its author, Adam M.
      Costello, consists of seven distinct passes over the image.  Each
      pass transmits a subset of the pixels in the image.  The pass in
      which each pixel is transmitted is defined by replicating the
      following 8-by-8 pattern over the entire image, starting at the
      upper left corner:

         1 6 4 6 2 6 4 6
         7 7 7 7 7 7 7 7
         5 6 5 6 5 6 5 6
         7 7 7 7 7 7 7 7
         3 6 4 6 3 6 4 6
         7 7 7 7 7 7 7 7
         5 6 5 6 5 6 5 6
         7 7 7 7 7 7 7 7

      Within each pass, the selected pixels are transmitted left to
      right within a scanline, and selected scanlines sequentially from
      top to bottom.  For example, pass 2 contains pixels 4, 12, 20,
      etc. of scanlines 0, 8, 16, etc. (numbering from 0,0 at the upper
      left corner).  The last pass contains the entirety of scanlines 1,
      3, 5, etc.

      The data within each pass is laid out as though it were a complete
      image of the appropriate dimensions.  For example, if the complete
      image is 16 by 16 pixels, then pass 3 will contain two scanlines,
      each containing four pixels.  When pixels have fewer than 8 bits,
      each such scanline is padded as needed to fill an integral number
      of bytes (see Image layout, Section 2.3).  Filtering is done on
      this reduced image in the usual way, and a filter type byte is
      transmitted before each of its scanlines (see Filter Algorithms,
      Chapter 6).  Notice that the transmission order is defined so that
      all the scanlines transmitted in a pass will have the same number
      of pixels; this is necessary for proper application of some of the
      filters.

      Caution: If the image contains fewer than five columns or fewer
      than five rows, some passes will be entirely empty.  Encoders and
      decoders must handle this case correctly.  In particular, filter
      type bytes are only associated with nonempty scanlines; no filter
      type bytes are present in an empty pass.



Boutell, et. al.             Informational                      [Page 9]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      See Rationale: Interlacing (Section 12.6) and Recommendations for
      Decoders: Progressive display (Section 10.9).

   2.7. Gamma correction

      PNG images can specify, via the gAMA chunk, the gamma
      characteristic of the image with respect to the original scene.
      Display programs are strongly encouraged to use this information,
      plus information about the display device they are using and room
      lighting, to present the image to the viewer in a way that
      reproduces what the image's original author saw as closely as
      possible.  See Gamma Tutorial (Chapter 13) if you aren't already
      familiar with gamma issues.

      Gamma correction is not applied to the alpha channel, if any.
      Alpha samples always represent a linear fraction of full opacity.

      For high-precision applications, the exact chromaticity of the RGB
      data in a PNG image can be specified via the cHRM chunk, allowing
      more accurate color matching than gamma correction alone will
      provide.  See Color Tutorial (Chapter 14) if you aren't already
      familiar with color representation issues.

      See Rationale: Why gamma? (Section 12.7), Recommendations for
      Encoders: Encoder gamma handling (Section 9.2), and
      Recommendations for Decoders: Decoder gamma handling (Section
      10.5).

   2.8. Text strings

      A PNG file can store text associated with the image, such as an
      image description or copyright notice.  Keywords are used to
      indicate what each text string represents.

      ISO 8859-1 (Latin-1) is the character set recommended for use in
      text strings [ISO-8859].  This character set is a superset of 7-
      bit ASCII.

      Character codes not defined in Latin-1 should not be used, because
      they have no platform-independent meaning.  If a non-Latin-1 code
      does appear in a PNG text string, its interpretation will vary
      across platforms and decoders.  Some systems might not even be
      able to display all the characters in Latin-1, but most modern
      systems can.

      Provision is also made for the storage of compressed text.

      See Rationale: Text strings (Section 12.10).



Boutell, et. al.             Informational                     [Page 10]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


3. File Structure

   A PNG file consists of a PNG signature followed by a series of
   chunks.  This chapter defines the signature and the basic properties
   of chunks.  Individual chunk types are discussed in the next chapter.

   3.1. PNG file signature

      The first eight bytes of a PNG file always contain the following
      (decimal) values:

         137 80 78 71 13 10 26 10

      This signature indicates that the remainder of the file contains a
      single PNG image, consisting of a series of chunks beginning with
      an IHDR chunk and ending with an IEND chunk.

      See Rationale: PNG file signature (Section 12.11).

   3.2. Chunk layout

      Each chunk consists of four parts:

      Length
         A 4-byte unsigned integer giving the number of bytes in the
         chunk's data field. The length counts only the data field, not
         itself, the chunk type code, or the CRC.  Zero is a valid
         length.  Although encoders and decoders should treat the length
         as unsigned, its value must not exceed (2^31)-1 bytes.

      Chunk Type
         A 4-byte chunk type code.  For convenience in description and
         in examining PNG files, type codes are restricted to consist of
         uppercase and lowercase ASCII letters (A-Z and a-z, or 65-90
         and 97-122 decimal).  However, encoders and decoders must treat
         the codes as fixed binary values, not character strings.  For
         example, it would not be correct to represent the type code
         IDAT by the EBCDIC equivalents of those letters.  Additional
         naming conventions for chunk types are discussed in the next
         section.

      Chunk Data
         The data bytes appropriate to the chunk type, if any.  This
         field can be of zero length.







Boutell, et. al.             Informational                     [Page 11]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      CRC
         A 4-byte CRC (Cyclic Redundancy Check) calculated on the
         preceding bytes in the chunk, including the chunk type code and
         chunk data fields, but not including the length field. The CRC
         is always present, even for chunks containing no data.  See CRC
         algorithm (Section 3.4).

      The chunk data length can be any number of bytes up to the
      maximum; therefore, implementors cannot assume that chunks are
      aligned on any boundaries larger than bytes.

      Chunks can appear in any order, subject to the restrictions placed
      on each chunk type.  (One notable restriction is that IHDR must
      appear first and IEND must appear last; thus the IEND chunk serves
      as an end-of-file marker.)  Multiple chunks of the same type can
      appear, but only if specifically permitted for that type.

      See Rationale: Chunk layout (Section 12.12).

   3.3. Chunk naming conventions

      Chunk type codes are assigned so that a decoder can determine some
      properties of a chunk even when it does not recognize the type
      code.  These rules are intended to allow safe, flexible extension
      of the PNG format, by allowing a decoder to decide what to do when
      it encounters an unknown chunk.  The naming rules are not normally
      of interest when the decoder does recognize the chunk's type.

      Four bits of the type code, namely bit 5 (value 32) of each byte,
      are used to convey chunk properties.  This choice means that a
      human can read off the assigned properties according to whether
      each letter of the type code is uppercase (bit 5 is 0) or
      lowercase (bit 5 is 1).  However, decoders should test the
      properties of an unknown chunk by numerically testing the
      specified bits; testing whether a character is uppercase or
      lowercase is inefficient, and even incorrect if a locale-specific
      case definition is used.

      It is worth noting that the property bits are an inherent part of
      the chunk name, and hence are fixed for any chunk type.  Thus,
      TEXT and Text would be unrelated chunk type codes, not the same
      chunk with different properties.  Decoders must recognize type
      codes by a simple four-byte literal comparison; it is incorrect to
      perform case conversion on type codes.







Boutell, et. al.             Informational                     [Page 12]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The semantics of the property bits are:

      Ancillary bit: bit 5 of first byte
         0 (uppercase) = critical, 1 (lowercase) = ancillary.

         Chunks that are not strictly necessary in order to meaningfully
         display the contents of the file are known as "ancillary"
         chunks.  A decoder encountering an unknown chunk in which the
         ancillary bit is 1 can safely ignore the chunk and proceed to
         display the image. The time chunk (tIME) is an example of an
         ancillary chunk.

         Chunks that are necessary for successful display of the file's
         contents are called "critical" chunks. A decoder encountering
         an unknown chunk in which the ancillary bit is 0 must indicate
         to the user that the image contains information it cannot
         safely interpret.  The image header chunk (IHDR) is an example
         of a critical chunk.

      Private bit: bit 5 of second byte
         0 (uppercase) = public, 1 (lowercase) = private.

         A public chunk is one that is part of the PNG specification or
         is registered in the list of PNG special-purpose public chunk
         types.  Applications can also define private (unregistered)
         chunks for their own purposes.  The names of private chunks
         must have a lowercase second letter, while public chunks will
         always be assigned names with uppercase second letters.  Note
         that decoders do not need to test the private-chunk property
         bit, since it has no functional significance; it is simply an
         administrative convenience to ensure that public and private
         chunk names will not conflict.  See Additional chunk types
         (Section 4.4) and Recommendations for Encoders: Use of private
         chunks (Section 9.8).

      Reserved bit: bit 5 of third byte
         Must be 0 (uppercase) in files conforming to this version of
         PNG.

         The significance of the case of the third letter of the chunk
         name is reserved for possible future expansion.  At the present
         time all chunk names must have uppercase third letters.
         (Decoders should not complain about a lowercase third letter,
         however, as some future version of the PNG specification could
         define a meaning for this bit.  It is sufficient to treat a
         chunk with a lowercase third letter in the same way as any
         other unknown chunk type.)




Boutell, et. al.             Informational                     [Page 13]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Safe-to-copy bit: bit 5 of fourth byte
         0 (uppercase) = unsafe to copy, 1 (lowercase) = safe to copy.

         This property bit is not of interest to pure decoders, but it
         is needed by PNG editors (programs that modify PNG files).
         This bit defines the proper handling of unrecognized chunks in
         a file that is being modified.

         If a chunk's safe-to-copy bit is 1, the chunk may be copied to
         a modified PNG file whether or not the software recognizes the
         chunk type, and regardless of the extent of the file
         modifications.

         If a chunk's safe-to-copy bit is 0, it indicates that the chunk
         depends on the image data.  If the program has made any changes
         to critical chunks, including addition, modification, deletion,
         or reordering of critical chunks, then unrecognized unsafe
         chunks must not be copied to the output PNG file.  (Of course,
         if the program does recognize the chunk, it can choose to
         output an appropriately modified version.)

         A PNG editor is always allowed to copy all unrecognized chunks
         if it has only added, deleted, modified, or reordered ancillary
         chunks.  This implies that it is not permissible for ancillary
         chunks to depend on other ancillary chunks.

         PNG editors that do not recognize a critical chunk must report
         an error and refuse to process that PNG file at all. The
         safe/unsafe mechanism is intended for use with ancillary
         chunks.  The safe-to-copy bit will always be 0 for critical
         chunks.

         Rules for PNG editors are discussed further in Chunk Ordering
         Rules (Chapter 7).

      For example, the hypothetical chunk type name "bLOb" has the
      property bits:

         bLOb  <-- 32 bit chunk type code represented in text form
         ||||
         |||+- Safe-to-copy bit is 1 (lower case letter; bit 5 is 1)
         ||+-- Reserved bit is 0     (upper case letter; bit 5 is 0)
         |+--- Private bit is 0      (upper case letter; bit 5 is 0)
         +---- Ancillary bit is 1    (lower case letter; bit 5 is 1)

      Therefore, this name represents an ancillary, public, safe-to-copy
      chunk.




Boutell, et. al.             Informational                     [Page 14]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      See Rationale: Chunk naming conventions (Section 12.13).

   3.4. CRC algorithm

      Chunk CRCs are calculated using standard CRC methods with pre and
      post conditioning, as defined by ISO 3309 [ISO-3309] or ITU-T V.42
      [ITU-V42].  The CRC polynomial employed is

         x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1

      The 32-bit CRC register is initialized to all 1's, and then the
      data from each byte is processed from the least significant bit
      (1) to the most significant bit (128).  After all the data bytes
      are processed, the CRC register is inverted (its ones complement
      is taken).  This value is transmitted (stored in the file) MSB
      first.  For the purpose of separating into bytes and ordering, the
      least significant bit of the 32-bit CRC is defined to be the
      coefficient of the x^31 term.

      Practical calculation of the CRC always employs a precalculated
      table to greatly accelerate the computation. See Sample CRC Code
      (Chapter 15).

4. Chunk Specifications

   This chapter defines the standard types of PNG chunks.

   4.1. Critical chunks

      All implementations must understand and successfully render the
      standard critical chunks.  A valid PNG image must contain an IHDR
      chunk, one or more IDAT chunks, and an IEND chunk.

      4.1.1. IHDR Image header

         The IHDR chunk must appear FIRST.  It contains:

            Width:              4 bytes
            Height:             4 bytes
            Bit depth:          1 byte
            Color type:         1 byte
            Compression method: 1 byte
            Filter method:      1 byte
            Interlace method:   1 byte







Boutell, et. al.             Informational                     [Page 15]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         Width and height give the image dimensions in pixels.  They are
         4-byte integers. Zero is an invalid value. The maximum for each
         is (2^31)-1 in order to accommodate languages that have
         difficulty with unsigned 4-byte values.

         Bit depth is a single-byte integer giving the number of bits
         per sample or per palette index (not per pixel).  Valid values
         are 1, 2, 4, 8, and 16, although not all values are allowed for
         all color types.

         Color type is a single-byte integer that describes the
         interpretation of the image data.  Color type codes represent
         sums of the following values: 1 (palette used), 2 (color used),
         and 4 (alpha channel used). Valid values are 0, 2, 3, 4, and 6.

         Bit depth restrictions for each color type are imposed to
         simplify implementations and to prohibit combinations that do
         not compress well.  Decoders must support all legal
         combinations of bit depth and color type.  The allowed
         combinations are:

            Color    Allowed    Interpretation
            Type    Bit Depths

            0       1,2,4,8,16  Each pixel is a grayscale sample.

            2       8,16        Each pixel is an R,G,B triple.

            3       1,2,4,8     Each pixel is a palette index;
                                a PLTE chunk must appear.

            4       8,16        Each pixel is a grayscale sample,
                                followed by an alpha sample.

            6       8,16        Each pixel is an R,G,B triple,
                                followed by an alpha sample.

         The sample depth is the same as the bit depth except in the
         case of color type 3, in which the sample depth is always 8
         bits.

         Compression method is a single-byte integer that indicates the
         method used to compress the image data.  At present, only
         compression method 0 (deflate/inflate compression with a 32K
         sliding window) is defined.  All standard PNG images must be
         compressed with this scheme.  The compression method field is
         provided for possible future expansion or proprietary variants.
         Decoders must check this byte and report an error if it holds



Boutell, et. al.             Informational                     [Page 16]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         an unrecognized code.  See Deflate/Inflate Compression (Chapter
         5) for details.

         Filter method is a single-byte integer that indicates the
         preprocessing method applied to the image data before
         compression.  At present, only filter method 0 (adaptive
         filtering with five basic filter types) is defined.  As with
         the compression method field, decoders must check this byte and
         report an error if it holds an unrecognized code.  See Filter
         Algorithms (Chapter 6) for details.

         Interlace method is a single-byte integer that indicates the
         transmission order of the image data.  Two values are currently
         defined: 0 (no interlace) or 1 (Adam7 interlace).  See
         Interlaced data order (Section 2.6) for details.

      4.1.2. PLTE Palette

         The PLTE chunk contains from 1 to 256 palette entries, each a
         three-byte series of the form:

            Red:   1 byte (0 = black, 255 = red)
            Green: 1 byte (0 = black, 255 = green)
            Blue:  1 byte (0 = black, 255 = blue)

         The number of entries is determined from the chunk length.  A
         chunk length not divisible by 3 is an error.

         This chunk must appear for color type 3, and can appear for
         color types 2 and 6; it must not appear for color types 0 and
         4. If this chunk does appear, it must precede the first IDAT
         chunk.  There must not be more than one PLTE chunk.

         For color type 3 (indexed color), the PLTE chunk is required.
         The first entry in PLTE is referenced by pixel value 0, the
         second by pixel value 1, etc.  The number of palette entries
         must not exceed the range that can be represented in the image
         bit depth (for example, 2^4 = 16 for a bit depth of 4).  It is
         permissible to have fewer entries than the bit depth would
         allow.  In that case, any out-of-range pixel value found in the
         image data is an error.

         For color types 2 and 6 (truecolor and truecolor with alpha),
         the PLTE chunk is optional.  If present, it provides a
         suggested set of from 1 to 256 colors to which the truecolor
         image can be quantized if the viewer cannot display truecolor
         directly.  If PLTE is not present, such a viewer will need to
         select colors on its own, but it is often preferable for this



Boutell, et. al.             Informational                     [Page 17]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         to be done once by the encoder.  (See Recommendations for
         Encoders: Suggested palettes, Section 9.5.)

         Note that the palette uses 8 bits (1 byte) per sample
         regardless of the image bit depth specification.  In
         particular, the palette is 8 bits deep even when it is a
         suggested quantization of a 16-bit truecolor image.

         There is no requirement that the palette entries all be used by
         the image, nor that they all be different.

      4.1.3. IDAT Image data

         The IDAT chunk contains the actual image data.  To create this
         data:

             * Begin with image scanlines represented as described in
               Image layout (Section 2.3); the layout and total size of
               this raw data are determined by the fields of IHDR.
             * Filter the image data according to the filtering method
               specified by the IHDR chunk.  (Note that with filter
               method 0, the only one currently defined, this implies
               prepending a filter type byte to each scanline.)
             * Compress the filtered data using the compression method
               specified by the IHDR chunk.

         The IDAT chunk contains the output datastream of the
         compression algorithm.

         To read the image data, reverse this process.

         There can be multiple IDAT chunks; if so, they must appear
         consecutively with no other intervening chunks.  The compressed
         datastream is then the concatenation of the contents of all the
         IDAT chunks.  The encoder can divide the compressed datastream
         into IDAT chunks however it wishes.  (Multiple IDAT chunks are
         allowed so that encoders can work in a fixed amount of memory;
         typically the chunk size will correspond to the encoder's
         buffer size.) It is important to emphasize that IDAT chunk
         boundaries have no semantic significance and can occur at any
         point in the compressed datastream.  A PNG file in which each
         IDAT chunk contains only one data byte is legal, though
         remarkably wasteful of space.  (For that matter, zero-length
         IDAT chunks are legal, though even more wasteful.)

         See Filter Algorithms (Chapter 6) and Deflate/Inflate
         Compression (Chapter 5) for details.




Boutell, et. al.             Informational                     [Page 18]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      4.1.4. IEND Image trailer

         The IEND chunk must appear LAST.  It marks the end of the PNG
         datastream.  The chunk's data field is empty.

   4.2. Ancillary chunks

      All ancillary chunks are optional, in the sense that encoders need
      not write them and decoders can ignore them.  However, encoders
      are encouraged to write the standard ancillary chunks when the
      information is available, and decoders are encouraged to interpret
      these chunks when appropriate and feasible.

      The standard ancillary chunks are listed in alphabetical order.
      This is not necessarily the order in which they would appear in a
      file.

      4.2.1. bKGD Background color

         The bKGD chunk specifies a default background color to present
         the image against.  Note that viewers are not bound to honor
         this chunk; a viewer can choose to use a different background.

         For color type 3 (indexed color), the bKGD chunk contains:

            Palette index:  1 byte

         The value is the palette index of the color to be used as
         background.

         For color types 0 and 4 (grayscale, with or without alpha),
         bKGD contains:

            Gray:  2 bytes, range 0 .. (2^bitdepth)-1

         (For consistency, 2 bytes are used regardless of the image bit
         depth.)  The value is the gray level to be used as background.

         For color types 2 and 6 (truecolor, with or without alpha),
         bKGD contains:

            Red:   2 bytes, range 0 .. (2^bitdepth)-1
            Green: 2 bytes, range 0 .. (2^bitdepth)-1
            Blue:  2 bytes, range 0 .. (2^bitdepth)-1

         (For consistency, 2 bytes per sample are used regardless of the
         image bit depth.)  This is the RGB color to be used as
         background.



Boutell, et. al.             Informational                     [Page 19]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         When present, the bKGD chunk must precede the first IDAT chunk,
         and must follow the PLTE chunk, if any.

         See Recommendations for Decoders: Background color (Section
         10.7).

      4.2.2. cHRM Primary chromaticities and white point

         Applications that need device-independent specification of
         colors in a PNG file can use the cHRM chunk to specify the 1931
         CIE x,y chromaticities of the red, green, and blue primaries
         used in the image, and the referenced white point. See Color
         Tutorial (Chapter 14) for more information.

         The cHRM chunk contains:

            White Point x: 4 bytes
            White Point y: 4 bytes
            Red x:         4 bytes
            Red y:         4 bytes
            Green x:       4 bytes
            Green y:       4 bytes
            Blue x:        4 bytes
            Blue y:        4 bytes

         Each value is encoded as a 4-byte unsigned integer,
         representing the x or y value times 100000.  For example, a
         value of 0.3127 would be stored as the integer 31270.

         cHRM is allowed in all PNG files, although it is of little
         value for grayscale images.

         If the encoder does not know the chromaticity values, it should
         not write a cHRM chunk; the absence of a cHRM chunk indicates
         that the image's primary colors are device-dependent.

         If the cHRM chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

         See Recommendations for Encoders: Encoder color handling
         (Section 9.3), and Recommendations for Decoders: Decoder color
         handling (Section 10.6).









Boutell, et. al.             Informational                     [Page 20]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      4.2.3. gAMA Image gamma

         The gAMA chunk specifies the gamma of the camera (or simulated
         camera) that produced the image, and thus the gamma of the
         image with respect to the original scene.  More precisely, the
         gAMA chunk encodes the file_gamma value, as defined in Gamma
         Tutorial (Chapter 13).

         The gAMA chunk contains:

            Image gamma: 4 bytes

         The value is encoded as a 4-byte unsigned integer, representing
         gamma times 100000.  For example, a gamma of 0.45 would be
         stored as the integer 45000.

         If the encoder does not know the image's gamma value, it should
         not write a gAMA chunk; the absence of a gAMA chunk indicates
         that the gamma is unknown.

         If the gAMA chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

         See Gamma correction (Section 2.7), Recommendations for
         Encoders: Encoder gamma handling (Section 9.2), and
         Recommendations for Decoders: Decoder gamma handling (Section
         10.5).

      4.2.4. hIST Image histogram

         The hIST chunk gives the approximate usage frequency of each
         color in the color palette.  A histogram chunk can appear only
         when a palette chunk appears.  If a viewer is unable to provide
         all the colors listed in the palette, the histogram may help it
         decide how to choose a subset of the colors for display.

         The hIST chunk contains a series of 2-byte (16 bit) unsigned
         integers.  There must be exactly one entry for each entry in
         the PLTE chunk.  Each entry is proportional to the fraction of
         pixels in the image that have that palette index; the exact
         scale factor is chosen by the encoder.

         Histogram entries are approximate, with the exception that a
         zero entry specifies that the corresponding palette entry is
         not used at all in the image.  It is required that a histogram
         entry be nonzero if there are any pixels of that color.





Boutell, et. al.             Informational                     [Page 21]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         When the palette is a suggested quantization of a truecolor
         image, the histogram is necessarily approximate, since a
         decoder may map pixels to palette entries differently than the
         encoder did.  In this situation, zero entries should not
         appear.

         The hIST chunk, if it appears, must follow the PLTE chunk, and
         must precede the first IDAT chunk.

         See Rationale: Palette histograms (Section 12.14), and
         Recommendations for Decoders: Suggested-palette and histogram
         usage (Section 10.10).

      4.2.5. pHYs Physical pixel dimensions

         The pHYs chunk specifies the intended pixel size or aspect
         ratio for display of the image.  It contains:

            Pixels per unit, X axis: 4 bytes (unsigned integer)
            Pixels per unit, Y axis: 4 bytes (unsigned integer)
            Unit specifier:          1 byte

         The following values are legal for the unit specifier:

            0: unit is unknown
            1: unit is the meter

         When the unit specifier is 0, the pHYs chunk defines pixel
         aspect ratio only; the actual size of the pixels remains
         unspecified.

         Conversion note: one inch is equal to exactly 0.0254 meters.

         If this ancillary chunk is not present, pixels are assumed to
         be square, and the physical size of each pixel is unknown.

         If present, this chunk must precede the first IDAT chunk.

         See Recommendations for Decoders: Pixel dimensions (Section
         10.2).

      4.2.6. sBIT Significant bits

         To simplify decoders, PNG specifies that only certain sample
         depths can be used, and further specifies that sample values
         should be scaled to the full range of possible values at the
         sample depth.  However, the sBIT chunk is provided in order to
         store the original number of significant bits.  This allows



Boutell, et. al.             Informational                     [Page 22]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         decoders to recover the original data losslessly even if the
         data had a sample depth not directly supported by PNG.  We
         recommend that an encoder emit an sBIT chunk if it has
         converted the data from a lower sample depth.

         For color type 0 (grayscale), the sBIT chunk contains a single
         byte, indicating the number of bits that were significant in
         the source data.

         For color type 2 (truecolor), the sBIT chunk contains three
         bytes, indicating the number of bits that were significant in
         the source data for the red, green, and blue channels,
         respectively.

         For color type 3 (indexed color), the sBIT chunk contains three
         bytes, indicating the number of bits that were significant in
         the source data for the red, green, and blue components of the
         palette entries, respectively.

         For color type 4 (grayscale with alpha channel), the sBIT chunk
         contains two bytes, indicating the number of bits that were
         significant in the source grayscale data and the source alpha
         data, respectively.

         For color type 6 (truecolor with alpha channel), the sBIT chunk
         contains four bytes, indicating the number of bits that were
         significant in the source data for the red, green, blue and
         alpha channels, respectively.

         Each depth specified in sBIT must be greater than zero and less
         than or equal to the sample depth (which is 8 for indexed-color
         images, and the bit depth given in IHDR for other color types).

         A decoder need not pay attention to sBIT: the stored image is a
         valid PNG file of the sample depth indicated by IHDR.  However,
         if the decoder wishes to recover the original data at its
         original precision, this can be done by right-shifting the
         stored samples (the stored palette entries, for an indexed-
         color image).  The encoder must scale the data in such a way
         that the high-order bits match the original data.

         If the sBIT chunk appears, it must precede the first IDAT
         chunk, and it must also precede the PLTE chunk if present.

         See Recommendations for Encoders: Sample depth scaling (Section
         9.1) and Recommendations for Decoders: Sample depth rescaling
         (Section 10.4).




Boutell, et. al.             Informational                     [Page 23]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      4.2.7. tEXt Textual data

         Textual information that the encoder wishes to record with the
         image can be stored in tEXt chunks.  Each tEXt chunk contains a
         keyword and a text string, in the format:

            Keyword:        1-79 bytes (character string)
            Null separator: 1 byte
            Text:           n bytes (character string)

         The keyword and text string are separated by a zero byte (null
         character).  Neither the keyword nor the text string can
         contain a null character.  Note that the text string is not
         null-terminated (the length of the chunk is sufficient
         information to locate the ending).  The keyword must be at
         least one character and less than 80 characters long.  The text
         string can be of any length from zero bytes up to the maximum
         permissible chunk size less the length of the keyword and
         separator.

         Any number of tEXt chunks can appear, and more than one with
         the same keyword is permissible.

         The keyword indicates the type of information represented by
         the text string.  The following keywords are predefined and
         should be used where appropriate:

            Title            Short (one line) title or caption for image
            Author           Name of image's creator
            Description      Description of image (possibly long)
            Copyright        Copyright notice
            Creation Time    Time of original image creation
            Software         Software used to create the image
            Disclaimer       Legal disclaimer
            Warning          Warning of nature of content
            Source           Device used to create the image
            Comment          Miscellaneous comment; conversion from
                             GIF comment

         For the Creation Time keyword, the date format defined in
         section 5.2.14 of RFC 1123 is suggested, but not required
         [RFC-1123].  Decoders should allow for free-format text
         associated with this or any other keyword.

         Other keywords may be invented for other purposes.  Keywords of
         general interest can be registered with the maintainers of the
         PNG specification.  However, it is also permitted to use
         private unregistered keywords.  (Private keywords should be



Boutell, et. al.             Informational                     [Page 24]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         reasonably self-explanatory, in order to minimize the chance
         that the same keyword will be used for incompatible purposes by
         different people.)

         Both keyword and text are interpreted according to the ISO
         8859-1 (Latin-1) character set [ISO-8859].  The text string can
         contain any Latin-1 character.  Newlines in the text string
         should be represented by a single linefeed character (decimal
         10); use of other control characters in the text is
         discouraged.

         Keywords must contain only printable Latin-1 characters and
         spaces; that is, only character codes 32-126 and 161-255
         decimal are allowed.  To reduce the chances for human
         misreading of a keyword, leading and trailing spaces are
         forbidden, as are consecutive spaces.  Note also that the non-
         breaking space (code 160) is not permitted in keywords, since
         it is visually indistinguishable from an ordinary space.

         Keywords must be spelled exactly as registered, so that
         decoders can use simple literal comparisons when looking for
         particular keywords.  In particular, keywords are considered
         case-sensitive.

         See Recommendations for Encoders: Text chunk processing
         (Section 9.7) and Recommendations for Decoders: Text chunk
         processing (Section 10.11).

      4.2.8. tIME Image last-modification time

         The tIME chunk gives the time of the last image modification
         (not the time of initial image creation).  It contains:

            Year:   2 bytes (complete; for example, 1995, not 95)
            Month:  1 byte (1-12)
            Day:    1 byte (1-31)
            Hour:   1 byte (0-23)
            Minute: 1 byte (0-59)
            Second: 1 byte (0-60)    (yes, 60, for leap seconds; not 61,
                                      a common error)

         Universal Time (UTC, also called GMT) should be specified
         rather than local time.








Boutell, et. al.             Informational                     [Page 25]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         The tIME chunk is intended for use as an automatically-applied
         time stamp that is updated whenever the image data is changed.
         It is recommended that tIME not be changed by PNG editors that
         do not change the image data.  See also the Creation Time tEXt
         keyword, which can be used for a user-supplied time.

      4.2.9. tRNS Transparency

         The tRNS chunk specifies that the image uses simple
         transparency: either alpha values associated with palette
         entries (for indexed-color images) or a single transparent
         color (for grayscale and truecolor images).  Although simple
         transparency is not as elegant as the full alpha channel, it
         requires less storage space and is sufficient for many common
         cases.

         For color type 3 (indexed color), the tRNS chunk contains a
         series of one-byte alpha values, corresponding to entries in
         the PLTE chunk:

            Alpha for palette index 0:  1 byte
            Alpha for palette index 1:  1 byte
            ... etc ...

         Each entry indicates that pixels of the corresponding palette
         index must be treated as having the specified alpha value.
         Alpha values have the same interpretation as in an 8-bit full
         alpha channel: 0 is fully transparent, 255 is fully opaque,
         regardless of image bit depth. The tRNS chunk must not contain
         more alpha values than there are palette entries, but tRNS can
         contain fewer values than there are palette entries.  In this
         case, the alpha value for all remaining palette entries is
         assumed to be 255.  In the common case in which only palette
         index 0 need be made transparent, only a one-byte tRNS chunk is
         needed.

         For color type 0 (grayscale), the tRNS chunk contains a single
         gray level value, stored in the format:

            Gray:  2 bytes, range 0 .. (2^bitdepth)-1

         (For consistency, 2 bytes are used regardless of the image bit
         depth.) Pixels of the specified gray level are to be treated as
         transparent (equivalent to alpha value 0); all other pixels are
         to be treated as fully opaque (alpha value (2^bitdepth)-1).






Boutell, et. al.             Informational                     [Page 26]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         For color type 2 (truecolor), the tRNS chunk contains a single
         RGB color value, stored in the format:

            Red:   2 bytes, range 0 .. (2^bitdepth)-1
            Green: 2 bytes, range 0 .. (2^bitdepth)-1
            Blue:  2 bytes, range 0 .. (2^bitdepth)-1

         (For consistency, 2 bytes per sample are used regardless of the
         image bit depth.) Pixels of the specified color value are to be
         treated as transparent (equivalent to alpha value 0); all other
         pixels are to be treated as fully opaque (alpha value
         (2^bitdepth)-1).

         tRNS is prohibited for color types 4 and 6, since a full alpha
         channel is already present in those cases.

         Note: when dealing with 16-bit grayscale or truecolor data, it
         is important to compare both bytes of the sample values to
         determine whether a pixel is transparent.  Although decoders
         may drop the low-order byte of the samples for display, this
         must not occur until after the data has been tested for
         transparency.  For example, if the grayscale level 0x0001 is
         specified to be transparent, it would be incorrect to compare
         only the high-order byte and decide that 0x0002 is also
         transparent.

         When present, the tRNS chunk must precede the first IDAT chunk,
         and must follow the PLTE chunk, if any.

      4.2.10. zTXt Compressed textual data

         The zTXt chunk contains textual data, just as tEXt does;
         however, zTXt takes advantage of compression.  zTXt and tEXt
         chunks are semantically equivalent, but zTXt is recommended for
         storing large blocks of text.

         A zTXt chunk contains:

            Keyword:            1-79 bytes (character string)
            Null separator:     1 byte
            Compression method: 1 byte
            Compressed text:    n bytes

         The keyword and null separator are exactly the same as in the
         tEXt chunk.  Note that the keyword is not compressed.  The
         compression method byte identifies the compression method used
         in this zTXt chunk.  The only value presently defined for it is
         0 (deflate/inflate compression). The compression method byte is



Boutell, et. al.             Informational                     [Page 27]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         followed by a compressed datastream that makes up the remainder
         of the chunk.  For compression method 0, this datastream
         adheres to the zlib datastream format (see Deflate/Inflate
         Compression, Chapter 5).  Decompression of this datastream
         yields Latin-1 text that is identical to the text that would be
         stored in an equivalent tEXt chunk.

         Any number of zTXt and tEXt chunks can appear in the same file.
         See the preceding definition of the tEXt chunk for the
         predefined keywords and the recommended format of the text.

         See Recommendations for Encoders: Text chunk processing
         (Section 9.7), and Recommendations for Decoders: Text chunk
         processing (Section 10.11).

   4.3. Summary of standard chunks

      This table summarizes some properties of the standard chunk types.

         Critical chunks (must appear in this order, except PLTE
                          is optional):

                 Name  Multiple  Ordering constraints
                         OK?

                 IHDR    No      Must be first
                 PLTE    No      Before IDAT
                 IDAT    Yes     Multiple IDATs must be consecutive
                 IEND    No      Must be last

         Ancillary chunks (need not appear in this order):

                 Name  Multiple  Ordering constraints
                         OK?

                 cHRM    No      Before PLTE and IDAT
                 gAMA    No      Before PLTE and IDAT
                 sBIT    No      Before PLTE and IDAT
                 bKGD    No      After PLTE; before IDAT
                 hIST    No      After PLTE; before IDAT
                 tRNS    No      After PLTE; before IDAT
                 pHYs    No      Before IDAT
                 tIME    No      None
                 tEXt    Yes     None
                 zTXt    Yes     None






Boutell, et. al.             Informational                     [Page 28]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Standard keywords for tEXt and zTXt chunks:

         Title            Short (one line) title or caption for image
         Author           Name of image's creator
         Description      Description of image (possibly long)
         Copyright        Copyright notice
         Creation Time    Time of original image creation
         Software         Software used to create the image
         Disclaimer       Legal disclaimer
         Warning          Warning of nature of content
         Source           Device used to create the image
         Comment          Miscellaneous comment; conversion from
                          GIF comment

   4.4. Additional chunk types

      Additional public PNG chunk types are defined in the document "PNG
      Special-Purpose Public Chunks" [PNG-EXTENSIONS].  Chunks described
      there are expected to be less widely supported than those defined
      in this specification.  However, application authors are
      encouraged to use those chunk types whenever appropriate for their
      applications.  Additional chunk types can be proposed for
      inclusion in that list by contacting the PNG specification
      maintainers at png-info@uunet.uu.net or at png-group@w3.org.

      New public chunks will only be registered if they are of use to
      others and do not violate the design philosophy of PNG. Chunk
      registration is not automatic, although it is the intent of the
      authors that it be straightforward when a new chunk of potentially
      wide application is needed.  Note that the creation of new
      critical chunk types is discouraged unless absolutely necessary.

      Applications can also use private chunk types to carry data that
      is not of interest to other applications.  See Recommendations for
      Encoders: Use of private chunks (Section 9.8).

      Decoders must be prepared to encounter unrecognized public or
      private chunk type codes.  Unrecognized chunk types must be
      handled as described in Chunk naming conventions (Section 3.3).

5. Deflate/Inflate Compression

   PNG compression method 0 (the only compression method presently
   defined for PNG) specifies deflate/inflate compression with a 32K
   sliding window.  Deflate compression is an LZ77 derivative used in
   zip, gzip, pkzip and related programs.  Extensive research has been
   done supporting its patent-free status.  Portable C implementations
   are freely available.



Boutell, et. al.             Informational                     [Page 29]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Deflate-compressed datastreams within PNG are stored in the "zlib"
   format, which has the structure:

      Compression method/flags code: 1 byte
      Additional flags/check bits:   1 byte
      Compressed data blocks:        n bytes
      Check value:                   4 bytes

   Further details on this format are given in the zlib specification
   [RFC-1950].

   For PNG compression method 0, the zlib compression method/flags code
   must specify method code 8 ("deflate" compression) and an LZ77 window
   size of not more than 32K.  Note that the zlib compression method
   number is not the same as the PNG compression method number.  The
   additional flags must not specify a preset dictionary.

   The compressed data within the zlib datastream is stored as a series
   of blocks, each of which can represent raw (uncompressed) data,
   LZ77-compressed data encoded with fixed Huffman codes, or LZ77-
   compressed data encoded with custom Huffman codes.  A marker bit in
   the final block identifies it as the last block, allowing the decoder
   to recognize the end of the compressed datastream.  Further details
   on the compression algorithm and the encoding are given in the
   deflate specification [RFC-1951].

   The check value stored at the end of the zlib datastream is
   calculated on the uncompressed data represented by the datastream.
   Note that the algorithm used is not the same as the CRC calculation
   used for PNG chunk check values.  The zlib check value is useful
   mainly as a cross-check that the deflate and inflate algorithms are
   implemented correctly.  Verifying the chunk CRCs provides adequate
   confidence that the PNG file has been transmitted undamaged.

   In a PNG file, the concatenation of the contents of all the IDAT
   chunks makes up a zlib datastream as specified above.  This
   datastream decompresses to filtered image data as described elsewhere
   in this document.

   It is important to emphasize that the boundaries between IDAT chunks
   are arbitrary and can fall anywhere in the zlib datastream.  There is
   not necessarily any correlation between IDAT chunk boundaries and
   deflate block boundaries or any other feature of the zlib data.  For
   example, it is entirely possible for the terminating zlib check value
   to be split across IDAT chunks.






Boutell, et. al.             Informational                     [Page 30]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   In the same vein, there is no required correlation between the
   structure of the image data (i.e., scanline boundaries) and deflate
   block boundaries or IDAT chunk boundaries.  The complete image data
   is represented by a single zlib datastream that is stored in some
   number of IDAT chunks; a decoder that assumes any more than this is
   incorrect.  (Of course, some encoder implementations may emit files
   in which some of these structures are indeed related.  But decoders
   cannot rely on this.)

   PNG also uses zlib datastreams in zTXt chunks.  In a zTXt chunk, the
   remainder of the chunk following the compression method byte is a
   zlib datastream as specified above.  This datastream decompresses to
   the user-readable text described by the chunk's keyword.  Unlike the
   image data, such datastreams are not split across chunks; each zTXt
   chunk contains an independent zlib datastream.

   Additional documentation and portable C code for deflate and inflate
   are available from the Info-ZIP archives at
   <URL:ftp://ftp.uu.net/pub/archiving/zip/>.

6. Filter Algorithms

   This chapter describes the filter algorithms that can be applied
   before compression.  The purpose of these filters is to prepare the
   image data for optimum compression.

   6.1. Filter types

      PNG filter method 0 defines five basic filter types:

         Type    Name

         0       None
         1       Sub
         2       Up
         3       Average
         4       Paeth

      (Note that filter method 0 in IHDR specifies exactly this set of
      five filter types.  If the set of filter types is ever extended, a
      different filter method number will be assigned to the extended
      set, so that decoders need not decompress the data to discover
      that it contains unsupported filter types.)

      The encoder can choose which of these filter algorithms to apply
      on a scanline-by-scanline basis.  In the image data sent to the
      compression step, each scanline is preceded by a filter type byte
      that specifies the filter algorithm used for that scanline.



Boutell, et. al.             Informational                     [Page 31]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Filtering algorithms are applied to bytes, not to pixels,
      regardless of the bit depth or color type of the image.  The
      filtering algorithms work on the byte sequence formed by a
      scanline that has been represented as described in Image layout
      (Section 2.3).  If the image includes an alpha channel, the alpha
      data is filtered in the same way as the image data.

      When the image is interlaced, each pass of the interlace pattern
      is treated as an independent image for filtering purposes.  The
      filters work on the byte sequences formed by the pixels actually
      transmitted during a pass, and the "previous scanline" is the one
      previously transmitted in the same pass, not the one adjacent in
      the complete image.  Note that the subimage transmitted in any one
      pass is always rectangular, but is of smaller width and/or height
      than the complete image.  Filtering is not applied when this
      subimage is empty.

      For all filters, the bytes "to the left of" the first pixel in a
      scanline must be treated as being zero.  For filters that refer to
      the prior scanline, the entire prior scanline must be treated as
      being zeroes for the first scanline of an image (or of a pass of
      an interlaced image).

      To reverse the effect of a filter, the decoder must use the
      decoded values of the prior pixel on the same line, the pixel
      immediately above the current pixel on the prior line, and the
      pixel just to the left of the pixel above.  This implies that at
      least one scanline's worth of image data will have to be stored by
      the decoder at all times.  Even though some filter types do not
      refer to the prior scanline, the decoder will always need to store
      each scanline as it is decoded, since the next scanline might use
      a filter that refers to it.

      PNG imposes no restriction on which filter types can be applied to
      an image.  However, the filters are not equally effective on all
      types of data.  See Recommendations for Encoders: Filter selection
      (Section 9.6).

      See also Rationale: Filtering (Section 12.9).

   6.2. Filter type 0: None

      With the None filter, the scanline is transmitted unmodified; it
      is only necessary to insert a filter type byte before the data.







Boutell, et. al.             Informational                     [Page 32]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   6.3. Filter type 1: Sub

      The Sub filter transmits the difference between each byte and the
      value of the corresponding byte of the prior pixel.

      To compute the Sub filter, apply the following formula to each
      byte of the scanline:

         Sub(x) = Raw(x) - Raw(x-bpp)

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw(x) refers to the raw data byte at that
      byte position in the scanline, and bpp is defined as the number of
      bytes per complete pixel, rounding up to one. For example, for
      color type 2 with a bit depth of 16, bpp is equal to 6 (three
      samples, two bytes per sample); for color type 0 with a bit depth
      of 2, bpp is equal to 1 (rounding up); for color type 4 with a bit
      depth of 16, bpp is equal to 4 (two-byte grayscale sample, plus
      two-byte alpha sample).

      Note this computation is done for each byte, regardless of bit
      depth.  In a 16-bit image, each MSB is predicted from the
      preceding MSB and each LSB from the preceding LSB, because of the
      way that bpp is defined.

      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Sub values is
      transmitted as the filtered scanline.

      For all x < 0, assume Raw(x) = 0.

      To reverse the effect of the Sub filter after decompression,
      output the following value:

         Sub(x) + Raw(x-bpp)

      (computed mod 256), where Raw refers to the bytes already decoded.

   6.4. Filter type 2: Up

      The Up filter is just like the Sub filter except that the pixel
      immediately above the current pixel, rather than just to its left,
      is used as the predictor.

      To compute the Up filter, apply the following formula to each byte
      of the scanline:

         Up(x) = Raw(x) - Prior(x)



Boutell, et. al.             Informational                     [Page 33]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw(x) refers to the raw data byte at that
      byte position in the scanline, and Prior(x) refers to the
      unfiltered bytes of the prior scanline.

      Note this is done for each byte, regardless of bit depth.
      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Up values is
      transmitted as the filtered scanline.

      On the first scanline of an image (or of a pass of an interlaced
      image), assume Prior(x) = 0 for all x.

      To reverse the effect of the Up filter after decompression, output
      the following value:

         Up(x) + Prior(x)

      (computed mod 256), where Prior refers to the decoded bytes of the
      prior scanline.

   6.5. Filter type 3: Average

      The Average filter uses the average of the two neighboring pixels
      (left and above) to predict the value of a pixel.

      To compute the Average filter, apply the following formula to each
      byte of the scanline:

         Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2)

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw(x) refers to the raw data byte at that
      byte position in the scanline, Prior(x) refers to the unfiltered
      bytes of the prior scanline, and bpp is defined as for the Sub
      filter.

      Note this is done for each byte, regardless of bit depth.  The
      sequence of Average values is transmitted as the filtered
      scanline.

      The subtraction of the predicted value from the raw byte must be
      done modulo 256, so that both the inputs and outputs fit into
      bytes.  However, the sum Raw(x-bpp)+Prior(x) must be formed
      without overflow (using at least nine-bit arithmetic).  floor()
      indicates that the result of the division is rounded to the next
      lower integer if fractional; in other words, it is an integer
      division or right shift operation.



Boutell, et. al.             Informational                     [Page 34]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      For all x < 0, assume Raw(x) = 0.  On the first scanline of an
      image (or of a pass of an interlaced image), assume Prior(x) = 0
      for all x.

      To reverse the effect of the Average filter after decompression,
      output the following value:

         Average(x) + floor((Raw(x-bpp)+Prior(x))/2)

      where the result is computed mod 256, but the prediction is
      calculated in the same way as for encoding.  Raw refers to the
      bytes already decoded, and Prior refers to the decoded bytes of
      the prior scanline.

   6.6. Filter type 4: Paeth

      The Paeth filter computes a simple linear function of the three
      neighboring pixels (left, above, upper left), then chooses as
      predictor the neighboring pixel closest to the computed value.
      This technique is due to Alan W. Paeth [PAETH].

      To compute the Paeth filter, apply the following formula to each
      byte of the scanline:

         Paeth(x) = Raw(x) - PaethPredictor(Raw(x-bpp), Prior(x),
                                            Prior(x-bpp))

      where x ranges from zero to the number of bytes representing the
      scanline minus one, Raw(x) refers to the raw data byte at that
      byte position in the scanline, Prior(x) refers to the unfiltered
      bytes of the prior scanline, and bpp is defined as for the Sub
      filter.

      Note this is done for each byte, regardless of bit depth.
      Unsigned arithmetic modulo 256 is used, so that both the inputs
      and outputs fit into bytes.  The sequence of Paeth values is
      transmitted as the filtered scanline.














Boutell, et. al.             Informational                     [Page 35]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The PaethPredictor function is defined by the following
      pseudocode:

         function PaethPredictor (a, b, c)
         begin
              ; a = left, b = above, c = upper left
              p := a + b - c        ; initial estimate
              pa := abs(p - a)      ; distances to a, b, c
              pb := abs(p - b)
              pc := abs(p - c)
              ; return nearest of a,b,c,
              ; breaking ties in order a,b,c.
              if pa <= pb AND pa <= pc then return a
              else if pb <= pc then return b
              else return c
         end

      The calculations within the PaethPredictor function must be
      performed exactly, without overflow.  Arithmetic modulo 256 is to
      be used only for the final step of subtracting the function result
      from the target byte value.

      Note that the order in which ties are broken is critical and must
      not be altered.  The tie break order is: pixel to the left, pixel
      above, pixel to the upper left.  (This order differs from that
      given in Paeth's article.)

      For all x < 0, assume Raw(x) = 0 and Prior(x) = 0.  On the first
      scanline of an image (or of a pass of an interlaced image), assume
      Prior(x) = 0 for all x.

      To reverse the effect of the Paeth filter after decompression,
      output the following value:

         Paeth(x) + PaethPredictor(Raw(x-bpp), Prior(x), Prior(x-bpp))

      (computed mod 256), where Raw and Prior refer to bytes already
      decoded.  Exactly the same PaethPredictor function is used by both
      encoder and decoder.

7. Chunk Ordering Rules

   To allow new chunk types to be added to PNG, it is necessary to
   establish rules about the ordering requirements for all chunk types.
   Otherwise a PNG editing program cannot know what to do when it
   encounters an unknown chunk.





Boutell, et. al.             Informational                     [Page 36]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   We define a "PNG editor" as a program that modifies a PNG file and
   wishes to preserve as much as possible of the ancillary information
   in the file.  Two examples of PNG editors are a program that adds or
   modifies text chunks, and a program that adds a suggested palette to
   a truecolor PNG file.  Ordinary image editors are not PNG editors in
   this sense, because they usually discard all unrecognized information
   while reading in an image.  (Note: we strongly encourage programs
   handling PNG files to preserve ancillary information whenever
   possible.)

   As an example of possible problems, consider a hypothetical new
   ancillary chunk type that is safe-to-copy and is required to appear
   after PLTE if PLTE is present.  If our program to add a suggested
   PLTE does not recognize this new chunk, it may insert PLTE in the
   wrong place, namely after the new chunk.  We could prevent such
   problems by requiring PNG editors to discard all unknown chunks, but
   that is a very unattractive solution.  Instead, PNG requires
   ancillary chunks not to have ordering restrictions like this.

   To prevent this type of problem while allowing for future extension,
   we put some constraints on both the behavior of PNG editors and the
   allowed ordering requirements for chunks.

   7.1. Behavior of PNG editors

      The rules for PNG editors are:

          * When copying an unknown unsafe-to-copy ancillary chunk, a
            PNG editor must not move the chunk relative to any critical
            chunk.  It can relocate the chunk freely relative to other
            ancillary chunks that occur between the same pair of
            critical chunks.  (This is well defined since the editor
            must not add, delete, modify, or reorder critical chunks if
            it is preserving unknown unsafe-to-copy chunks.)
          * When copying an unknown safe-to-copy ancillary chunk, a PNG
            editor must not move the chunk from before IDAT to after
            IDAT or vice versa.  (This is well defined because IDAT is
            always present.)  Any other reordering is permitted.
          * When copying a known ancillary chunk type, an editor need
            only honor the specific chunk ordering rules that exist for
            that chunk type.  However, it can always choose to apply the
            above general rules instead.
          * PNG editors must give up on encountering an unknown critical
            chunk type, because there is no way to be certain that a
            valid file will result from modifying a file containing such
            a chunk.  (Note that simply discarding the chunk is not good
            enough, because it might have unknown implications for the
            interpretation of other chunks.)



Boutell, et. al.             Informational                     [Page 37]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      These rules are expressed in terms of copying chunks from an input
      file to an output file, but they apply in the obvious way if a PNG
      file is modified in place.

      See also Chunk naming conventions (Section 3.3).

   7.2. Ordering of ancillary chunks

      The ordering rules for an ancillary chunk type cannot be any
      stricter than this:

          * Unsafe-to-copy chunks can have ordering requirements
            relative to critical chunks.
          * Safe-to-copy chunks can have ordering requirements relative
            to IDAT.

      The actual ordering rules for any particular ancillary chunk type
      may be weaker.  See for example the ordering rules for the
      standard ancillary chunk types (Summary of standard chunks,
      Section 4.3).

      Decoders must not assume more about the positioning of any
      ancillary chunk than is specified by the chunk ordering rules.  In
      particular, it is never valid to assume that a specific ancillary
      chunk type occurs with any particular positioning relative to
      other ancillary chunks.  (For example, it is unsafe to assume that
      your private ancillary chunk occurs immediately before IEND.  Even
      if your application always writes it there, a PNG editor might
      have inserted some other ancillary chunk after it.  But you can
      safely assume that your chunk will remain somewhere between IDAT
      and IEND.)

   7.3. Ordering of critical chunks

      Critical chunks can have arbitrary ordering requirements, because
      PNG editors are required to give up if they encounter unknown
      critical chunks.  For example, IHDR has the special ordering rule
      that it must always appear first.  A PNG editor, or indeed any
      PNG-writing program, must know and follow the ordering rules for
      any critical chunk type that it can emit.











Boutell, et. al.             Informational                     [Page 38]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


8. Miscellaneous Topics

   8.1. File name extension

      On systems where file names customarily include an extension
      signifying file type, the extension ".png" is recommended for PNG
      files.  Lower case ".png" is preferred if file names are case-
      sensitive.

   8.2. Internet media type

      The Internet Assigned Numbers Authority (IANA) has registered
      "image/png" as the Internet Media Type for PNG [RFC-2045, RFC-
      2048].  For robustness, decoders may choose to also support the
      interim media type "image/x-png" which was in use before
      registration was complete.

   8.3. Macintosh file layout

      In the Apple Macintosh system, the following conventions are
      recommended:

          * The four-byte file type code for PNG files is "PNGf".  (This
            code has been registered with Apple for PNG files.) The
            creator code will vary depending on the creating
            application.
          * The contents of the data fork must be a PNG file exactly as
            described in the rest of this specification.
          * The contents of the resource fork are unspecified.  It may
            be empty or may contain application-dependent resources.
          * When transferring a Macintosh PNG file to a non-Macintosh
            system, only the data fork should be transferred.

   8.4. Multiple-image extension

      PNG itself is strictly a single-image format.  However, it may be
      necessary to store multiple images within one file; for example,
      this is needed to convert some GIF files.  In the future, a
      multiple-image format based on PNG may be defined.  Such a format
      will be considered a separate file format and will have a
      different signature.  PNG-supporting applications may or may not
      choose to support the multiple-image format.

      See Rationale: Why not these features? (Section 12.3).







Boutell, et. al.             Informational                     [Page 39]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   8.5. Security considerations

      A PNG file or datastream is composed of a collection of explicitly
      typed "chunks".  Chunks whose contents are defined by the
      specification could actually contain anything, including malicious
      code.  But there is no known risk that such malicious code could
      be executed on the recipient's computer as a result of decoding
      the PNG image.

      The possible security risks associated with future chunk types
      cannot be specified at this time.  Security issues will be
      considered when evaluating chunks proposed for registration as
      public chunks.  There is no additional security risk associated
      with unknown or unimplemented chunk types, because such chunks
      will be ignored, or at most be copied into another PNG file.

      The tEXt and zTXt chunks contain data that is meant to be
      displayed as plain text.  It is possible that if the decoder
      displays such text without filtering out control characters,
      especially the ESC (escape) character, certain systems or
      terminals could behave in undesirable and insecure ways.  We
      recommend that decoders filter out control characters to avoid
      this risk; see Recommendations for Decoders: Text chunk processing
      (Section 10.11).

      Because every chunk's length is available at its beginning, and
      because every chunk has a CRC trailer, there is a very robust
      defense against corrupted data and against fraudulent chunks that
      attempt to overflow the decoder's buffers.  Also, the PNG
      signature bytes provide early detection of common file
      transmission errors.

      A decoder that fails to check CRCs could be subject to data
      corruption.  The only likely consequence of such corruption is
      incorrectly displayed pixels within the image.  Worse things might
      happen if the CRC of the IHDR chunk is not checked and the width
      or height fields are corrupted.  See Recommendations for Decoders:
      Error checking (Section 10.1).

      A poorly written decoder might be subject to buffer overflow,
      because chunks can be extremely large, up to (2^31)-1 bytes long.
      But properly written decoders will handle large chunks without
      difficulty.








Boutell, et. al.             Informational                     [Page 40]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


9. Recommendations for Encoders

   This chapter gives some recommendations for encoder behavior.  The
   only absolute requirement on a PNG encoder is that it produce files
   that conform to the format specified in the preceding chapters.
   However, best results will usually be achieved by following these
   recommendations.

   9.1. Sample depth scaling

      When encoding input samples that have a sample depth that cannot
      be directly represented in PNG, the encoder must scale the samples
      up to a sample depth that is allowed by PNG.  The most accurate
      scaling method is the linear equation

         output = ROUND(input * MAXOUTSAMPLE / MAXINSAMPLE)

      where the input samples range from 0 to MAXINSAMPLE and the
      outputs range from 0 to MAXOUTSAMPLE (which is (2^sampledepth)-1).

      A close approximation to the linear scaling method can be achieved
      by "left bit replication", which is shifting the valid bits to
      begin in the most significant bit and repeating the most
      significant bits into the open bits.  This method is often faster
      to compute than linear scaling.  As an example, assume that 5-bit
      samples are being scaled up to 8 bits.  If the source sample value
      is 27 (in the range from 0-31), then the original bits are:

         4 3 2 1 0
         ---------
         1 1 0 1 1

      Left bit replication gives a value of 222:

         7 6 5 4 3  2 1 0
         ----------------
         1 1 0 1 1  1 1 0
         |=======|  |===|
             |      Leftmost Bits Repeated to Fill Open Bits
             |
         Original Bits

      which matches the value computed by the linear equation.  Left bit
      replication usually gives the same value as linear scaling, and is
      never off by more than one.






Boutell, et. al.             Informational                     [Page 41]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      A distinctly less accurate approximation is obtained by simply
      left-shifting the input value and filling the low order bits with
      zeroes.  This scheme cannot reproduce white exactly, since it does
      not generate an all-ones maximum value; the net effect is to
      darken the image slightly.  This method is not recommended in
      general, but it does have the effect of improving compression,
      particularly when dealing with greater-than-eight-bit sample
      depths.  Since the relative error introduced by zero-fill scaling
      is small at high sample depths, some encoders may choose to use
      it.  Zero-fill must not be used for alpha channel data, however,
      since many decoders will special-case alpha values of all zeroes
      and all ones.  It is important to represent both those values
      exactly in the scaled data.

      When the encoder writes an sBIT chunk, it is required to do the
      scaling in such a way that the high-order bits of the stored
      samples match the original data.  That is, if the sBIT chunk
      specifies a sample depth of S, the high-order S bits of the stored
      data must agree with the original S-bit data values.  This allows
      decoders to recover the original data by shifting right.  The
      added low-order bits are not constrained.  Note that all the above
      scaling methods meet this restriction.

      When scaling up source data, it is recommended that the low-order
      bits be filled consistently for all samples; that is, the same
      source value should generate the same sample value at any pixel
      position.  This improves compression by reducing the number of
      distinct sample values.  However, this is not a requirement, and
      some encoders may choose not to follow it.  For example, an
      encoder might instead dither the low-order bits, improving
      displayed image quality at the price of increasing file size.

      In some applications the original source data may have a range
      that is not a power of 2.  The linear scaling equation still works
      for this case, although the shifting methods do not.  It is
      recommended that an sBIT chunk not be written for such images,
      since sBIT suggests that the original data range was exactly
      0..2^S-1.

   9.2. Encoder gamma handling

      See Gamma Tutorial (Chapter 13) if you aren't already familiar
      with gamma issues.

      Proper handling of gamma encoding and the gAMA chunk in an encoder
      depends on the prior history of the sample values and on whether
      these values have already been quantized to integers.




Boutell, et. al.             Informational                     [Page 42]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      If the encoder has access to sample intensity values in floating-
      point or high-precision integer form (perhaps from a computer
      image renderer), then it is recommended that the encoder perform
      its own gamma encoding before quantizing the data to integer
      values for storage in the file.  Applying gamma encoding at this
      stage results in images with fewer banding artifacts at a given
      sample depth, or allows smaller samples while retaining the same
      visual quality.

      A linear intensity level, expressed as a floating-point value in
      the range 0 to 1, can be converted to a gamma-encoded sample value
      by

         sample = ROUND((intensity ^ encoder_gamma) * MAXSAMPLE)

      The file_gamma value to be written in the PNG gAMA chunk is the
      same as encoder_gamma in this equation, since we are assuming the
      initial intensity value is linear (in effect, camera_gamma is
      1.0).

      If the image is being written to a file only, the encoder_gamma
      value can be selected somewhat arbitrarily.  Values of 0.45 or 0.5
      are generally good choices because they are common in video
      systems, and so most PNG decoders should do a good job displaying
      such images.

      Some image renderers may simultaneously write the image to a PNG
      file and display it on-screen.  The displayed pixels should be
      gamma corrected for the display system and viewing conditions in
      use, so that the user sees a proper representation of the intended
      scene.  An appropriate gamma correction value is

         screen_gc = viewing_gamma / display_gamma

      If the renderer wants to write the same gamma-corrected sample
      values to the PNG file, avoiding a separate gamma-encoding step
      for file output, then this screen_gc value should be written in
      the gAMA chunk.  This will allow a PNG decoder to reproduce what
      the file's originator saw on screen during rendering (provided the
      decoder properly supports arbitrary values in a gAMA chunk).

      However, it is equally reasonable for a renderer to apply gamma
      correction for screen display using a gamma appropriate to the
      viewing conditions, and to separately gamma-encode the sample
      values for file storage using a standard value of gamma such as
      0.5.  In fact, this is preferable, since some PNG decoders may not
      accurately display images with unusual gAMA values.




Boutell, et. al.             Informational                     [Page 43]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Computer graphics renderers often do not perform gamma encoding,
      instead making sample values directly proportional to scene light
      intensity.  If the PNG encoder receives sample values that have
      already been quantized into linear-light integer values, there is
      no point in doing gamma encoding on them; that would just result
      in further loss of information.  The encoder should just write the
      sample values to the PNG file.  This "linear" sample encoding is
      equivalent to gamma encoding with a gamma of 1.0, so graphics
      programs that produce linear samples should always emit a gAMA
      chunk specifying a gamma of 1.0.

      When the sample values come directly from a piece of hardware, the
      correct gAMA value is determined by the gamma characteristic of
      the hardware.  In the case of video digitizers ("frame grabbers"),
      gAMA should be 0.45 or 0.5 for NTSC (possibly less for PAL or
      SECAM) since video camera transfer functions are standardized.
      Image scanners are less predictable.  Their output samples may be
      linear (gamma 1.0) since CCD sensors themselves are linear, or the
      scanner hardware may have already applied gamma correction
      designed to compensate for dot gain in subsequent printing (gamma
      of about 0.57), or the scanner may have corrected the samples for
      display on a CRT (gamma of 0.4-0.5).  You will need to refer to
      the scanner's manual, or even scan a calibrated gray wedge, to
      determine what a particular scanner does.

      File format converters generally should not attempt to convert
      supplied images to a different gamma.  Store the data in the PNG
      file without conversion, and record the source gamma if it is
      known.  Gamma alteration at file conversion time causes re-
      quantization of the set of intensity levels that are represented,
      introducing further roundoff error with little benefit.  It's
      almost always better to just copy the sample values intact from
      the input to the output file.

      In some cases, the supplied image may be in an image format (e.g.,
      TIFF) that can describe the gamma characteristic of the image.  In
      such cases, a file format converter is strongly encouraged to
      write a PNG gAMA chunk that corresponds to the known gamma of the
      source image.  Note that some file formats specify the gamma of
      the display system, not the camera.  If the input file's gamma
      value is greater than 1.0, it is almost certainly a display system
      gamma, and you should use its reciprocal for the PNG gAMA.









Boutell, et. al.             Informational                     [Page 44]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      If the encoder or file format converter does not know how an image
      was originally created, but does know that the image has been
      displayed satisfactorily on a display with gamma display_gamma
      under lighting conditions where a particular viewing_gamma is
      appropriate, then the image can be marked as having the
      file_gamma:

         file_gamma = viewing_gamma / display_gamma

      This will allow viewers of the PNG file to see the same image that
      the person running the file format converter saw.  Although this
      may not be precisely the correct value of the image gamma, it's
      better to write a gAMA chunk with an approximately right value
      than to omit the chunk and force PNG decoders to guess at an
      appropriate gamma.

      On the other hand, if the image file is being converted as part of
      a "bulk" conversion, with no one looking at each image, then it is
      better to omit the gAMA chunk entirely.  If the image gamma has to
      be guessed at, leave it to the decoder to do the guessing.

      Gamma does not apply to alpha samples; alpha is always represented
      linearly.

      See also Recommendations for Decoders: Decoder gamma handling
      (Section 10.5).

   9.3. Encoder color handling

      See Color Tutorial (Chapter 14) if you aren't already familiar
      with color issues.

      If it is possible for the encoder to determine the chromaticities
      of the source display primaries, or to make a strong guess based
      on the origin of the image or the hardware running it, then the
      encoder is strongly encouraged to output the cHRM chunk.  If it
      does so, the gAMA chunk should also be written; decoders can do
      little with cHRM if gAMA is missing.













Boutell, et. al.             Informational                     [Page 45]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Video created with recent video equipment probably uses the CCIR
      709 primaries and D65 white point [ITU-BT709], which are:

                  R           G           B         White
         x      0.640       0.300       0.150       0.3127
         y      0.330       0.600       0.060       0.3290

      An older but still very popular video standard is SMPTE-C [SMPTE-
      170M]:

                  R           G           B         White
         x      0.630       0.310       0.155       0.3127
         y      0.340       0.595       0.070       0.3290

      The original NTSC color primaries have not been used in decades.
      Although you may still find the NTSC numbers listed in standards
      documents, you won't find any images that actually use them.

      Scanners that produce PNG files as output should insert the filter
      chromaticities into a cHRM chunk and the camera_gamma into a gAMA
      chunk.

      In the case of hand-drawn or digitally edited images, you have to
      determine what monitor they were viewed on when being produced.
      Many image editing programs allow you to specify what type of
      monitor you are using.  This is often because they are working in
      some device-independent space internally.  Such programs have
      enough information to write valid cHRM and gAMA chunks, and should
      do so automatically.

      If the encoder is compiled as a portion of a computer image
      renderer that performs full-spectral rendering, the monitor values
      that were used to convert from the internal device-independent
      color space to RGB should be written into the cHRM chunk. Any
      colors that are outside the gamut of the chosen RGB device should
      be clipped or otherwise constrained to be within the gamut; PNG
      does not store out of gamut colors.

      If the computer image renderer performs calculations directly in
      device-dependent RGB space, a cHRM chunk should not be written
      unless the scene description and rendering parameters have been
      adjusted to look good on a particular monitor.  In that case, the
      data for that monitor (if known) should be used to construct a
      cHRM chunk.







Boutell, et. al.             Informational                     [Page 46]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      There are often cases where an image's exact origins are unknown,
      particularly if it began life in some other format.  A few image
      formats store calibration information, which can be used to fill
      in the cHRM chunk.  For example, all PhotoCD images use the CCIR
      709 primaries and D65 whitepoint, so these values can be written
      into the cHRM chunk when converting a PhotoCD file.  PhotoCD also
      uses the SMPTE-170M transfer function, which is closely
      approximated by a gAMA of 0.5.  (PhotoCD can store colors outside
      the RGB gamut, so the image data will require gamut mapping before
      writing to PNG format.)  TIFF 6.0 files can optionally store
      calibration information, which if present should be used to
      construct the cHRM chunk.  GIF and most other formats do not store
      any calibration information.

      It is not recommended that file format converters attempt to
      convert supplied images to a different RGB color space.  Store the
      data in the PNG file without conversion, and record the source
      primary chromaticities if they are known.  Color space
      transformation at file conversion time is a bad idea because of
      gamut mismatches and rounding errors.  As with gamma conversions,
      it's better to store the data losslessly and incur at most one
      conversion when the image is finally displayed.

      See also Recommendations for Decoders: Decoder color handling
      (Section 10.6).

   9.4. Alpha channel creation

      The alpha channel can be regarded either as a mask that
      temporarily hides transparent parts of the image, or as a means
      for constructing a non-rectangular image.  In the first case, the
      color values of fully transparent pixels should be preserved for
      future use.  In the second case, the transparent pixels carry no
      useful data and are simply there to fill out the rectangular image
      area required by PNG.  In this case, fully transparent pixels
      should all be assigned the same color value for best compression.

      Image authors should keep in mind the possibility that a decoder
      will ignore transparency control.  Hence, the colors assigned to
      transparent pixels should be reasonable background colors whenever
      feasible.

      For applications that do not require a full alpha channel, or
      cannot afford the price in compression efficiency, the tRNS
      transparency chunk is also available.






Boutell, et. al.             Informational                     [Page 47]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      If the image has a known background color, this color should be
      written in the bKGD chunk.  Even decoders that ignore transparency
      may use the bKGD color to fill unused screen area.

      If the original image has premultiplied (also called "associated")
      alpha data, convert it to PNG's non-premultiplied format by
      dividing each sample value by the corresponding alpha value, then
      multiplying by the maximum value for the image bit depth, and
      rounding to the nearest integer.  In valid premultiplied data, the
      sample values never exceed their corresponding alpha values, so
      the result of the division should always be in the range 0 to 1.
      If the alpha value is zero, output black (zeroes).

   9.5. Suggested palettes

      A PLTE chunk can appear in truecolor PNG files.  In such files,
      the chunk is not an essential part of the image data, but simply
      represents a suggested palette that viewers may use to present the
      image on indexed-color display hardware.  A suggested palette is
      of no interest to viewers running on truecolor hardware.

      If an encoder chooses to provide a suggested palette, it is
      recommended that a hIST chunk also be written to indicate the
      relative importance of the palette entries.  The histogram values
      are most easily computed as "nearest neighbor" counts, that is,
      the approximate usage of each palette entry if no dithering is
      applied.  (These counts will often be available for free as a
      consequence of developing the suggested palette.)

      For images of color type 2 (truecolor without alpha channel), it
      is recommended that the palette and histogram be computed with
      reference to the RGB data only, ignoring any transparent-color
      specification.  If the file uses transparency (has a tRNS chunk),
      viewers can easily adapt the resulting palette for use with their
      intended background color.  They need only replace the palette
      entry closest to the tRNS color with their background color (which
      may or may not match the file's bKGD color, if any).

      For images of color type 6 (truecolor with alpha channel), it is
      recommended that a bKGD chunk appear and that the palette and
      histogram be computed with reference to the image as it would
      appear after compositing against the specified background color.
      This definition is necessary to ensure that useful palette entries
      are generated for pixels having fractional alpha values.  The
      resulting palette will probably only be useful to viewers that
      present the image against the same background color.  It is
      recommended that PNG editors delete or recompute the palette if
      they alter or remove the bKGD chunk in an image of color type 6.



Boutell, et. al.             Informational                     [Page 48]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      If PLTE appears without bKGD in an image of color type 6, the
      circumstances under which the palette was computed are
      unspecified.

   9.6. Filter selection

      For images of color type 3 (indexed color), filter type 0 (None)
      is usually the most effective.  Note that color images with 256 or
      fewer colors should almost always be stored in indexed color
      format; truecolor format is likely to be much larger.

      Filter type 0 is also recommended for images of bit depths less
      than 8.  For low-bit-depth grayscale images, it may be a net win
      to expand the image to 8-bit representation and apply filtering,
      but this is rare.

      For truecolor and grayscale images, any of the five filters may
      prove the most effective.  If an encoder uses a fixed filter, the
      Paeth filter is most likely to be the best.

      For best compression of truecolor and grayscale images, we
      recommend an adaptive filtering approach in which a filter is
      chosen for each scanline.  The following simple heuristic has
      performed well in early tests: compute the output scanline using
      all five filters, and select the filter that gives the smallest
      sum of absolute values of outputs.  (Consider the output bytes as
      signed differences for this test.)  This method usually
      outperforms any single fixed filter choice.  However, it is likely
      that much better heuristics will be found as more experience is
      gained with PNG.

      Filtering according to these recommendations is effective on
      interlaced as well as noninterlaced images.

   9.7. Text chunk processing

      A nonempty keyword must be provided for each text chunk.  The
      generic keyword "Comment" can be used if no better description of
      the text is available.  If a user-supplied keyword is used, be
      sure to check that it meets the restrictions on keywords.

      PNG text strings are expected to use the Latin-1 character set.
      Encoders should avoid storing characters that are not defined in
      Latin-1, and should provide character code remapping if the local
      system's character set is not Latin-1.

      Encoders should discourage the creation of single lines of text
      longer than 79 characters, in order to facilitate easy reading.



Boutell, et. al.             Informational                     [Page 49]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      It is recommended that text items less than 1K (1024 bytes) in
      size should be output using uncompressed tEXt chunks. In
      particular, it is recommended that the basic title and author
      keywords should always be output using uncompressed tEXt chunks.
      Lengthy disclaimers, on the other hand, are ideal candidates for
      zTXt.

      Placing large tEXt and zTXt chunks after the image data (after
      IDAT) can speed up image display in some situations, since the
      decoder won't have to read over the text to get to the image data.
      But it is recommended that small text chunks, such as the image
      title, appear before IDAT.

   9.8. Use of private chunks

      Applications can use PNG private chunks to carry information that
      need not be understood by other applications.  Such chunks must be
      given names with lowercase second letters, to ensure that they can
      never conflict with any future public chunk definition.  Note,
      however, that there is no guarantee that some other application
      will not use the same private chunk name.  If you use a private
      chunk type, it is prudent to store additional identifying
      information at the beginning of the chunk data.

      Use an ancillary chunk type (lowercase first letter), not a
      critical chunk type, for all private chunks that store information
      that is not absolutely essential to view the image.  Creation of
      private critical chunks is discouraged because they render PNG
      files unportable.  Such chunks should not be used in publicly
      available software or files.  If private critical chunks are
      essential for your application, it is recommended that one appear
      near the start of the file, so that a standard decoder need not
      read very far before discovering that it cannot handle the file.

      If you want others outside your organization to understand a chunk
      type that you invent, contact the maintainers of the PNG
      specification to submit a proposed chunk name and definition for
      addition to the list of special-purpose public chunks (see
      Additional chunk types, Section 4.4).  Note that a proposed public
      chunk name (with uppercase second letter) must not be used in
      publicly available software or files until registration has been
      approved.

      If an ancillary chunk contains textual information that might be
      of interest to a human user, you should not create a special chunk
      type for it.  Instead use a tEXt chunk and define a suitable
      keyword.  That way, the information will be available to users not
      using your software.



Boutell, et. al.             Informational                     [Page 50]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Keywords in tEXt chunks should be reasonably self-explanatory,
      since the idea is to let other users figure out what the chunk
      contains.  If of general usefulness, new keywords can be
      registered with the maintainers of the PNG specification.  But it
      is permissible to use keywords without registering them first.

   9.9. Private type and method codes

      This specification defines the meaning of only some of the
      possible values of some fields.  For example, only compression
      method 0 and filter types 0 through 4 are defined.  Numbers
      greater than 127 must be used when inventing experimental or
      private definitions of values for any of these fields.  Numbers
      below 128 are reserved for possible future public extensions of
      this specification.  Note that use of private type codes may
      render a file unreadable by standard decoders.  Such codes are
      strongly discouraged except for experimental purposes, and should
      not appear in publicly available software or files.

10. Recommendations for Decoders

   This chapter gives some recommendations for decoder behavior.  The
   only absolute requirement on a PNG decoder is that it successfully
   read any file conforming to the format specified in the preceding
   chapters.  However, best results will usually be achieved by
   following these recommendations.

   10.1. Error checking

      To ensure early detection of common file-transfer problems,
      decoders should verify that all eight bytes of the PNG file
      signature are correct.  (See Rationale: PNG file signature,
      Section 12.11.) A decoder can have additional confidence in the
      file's integrity if the next eight bytes are an IHDR chunk header
      with the correct chunk length.

      Unknown chunk types must be handled as described in Chunk naming
      conventions (Section 3.3).  An unknown chunk type is not to be
      treated as an error unless it is a critical chunk.

      It is strongly recommended that decoders should verify the CRC on
      each chunk.

      In some situations it is desirable to check chunk headers (length
      and type code) before reading the chunk data and CRC.  The chunk
      type can be checked for plausibility by seeing whether all four
      bytes are ASCII letters (codes 65-90 and 97-122); note that this
      need only be done for unrecognized type codes.  If the total file



Boutell, et. al.             Informational                     [Page 51]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      size is known (from file system information, HTTP protocol, etc),
      the chunk length can be checked for plausibility as well.

      If CRCs are not checked, dropped/added data bytes or an erroneous
      chunk length can cause the decoder to get out of step and
      misinterpret subsequent data as a chunk header.  Verifying that
      the chunk type contains letters is an inexpensive way of providing
      early error detection in this situation.

      For known-length chunks such as IHDR, decoders should treat an
      unexpected chunk length as an error.  Future extensions to this
      specification will not add new fields to existing chunks; instead,
      new chunk types will be added to carry new information.

      Unexpected values in fields of known chunks (for example, an
      unexpected compression method in the IHDR chunk) must be checked
      for and treated as errors.  However, it is recommended that
      unexpected field values be treated as fatal errors only in
      critical chunks.  An unexpected value in an ancillary chunk can be
      handled by ignoring the whole chunk as though it were an unknown
      chunk type.  (This recommendation assumes that the chunk's CRC has
      been verified.  In decoders that do not check CRCs, it is safer to
      treat any unexpected value as indicating a corrupted file.)

   10.2. Pixel dimensions

      Non-square pixels can be represented (see the pHYs chunk), but
      viewers are not required to account for them; a viewer can present
      any PNG file as though its pixels are square.

      Conversely, viewers running on display hardware with non-square
      pixels are strongly encouraged to rescale images for proper
      display.

   10.3. Truecolor image handling

      To achieve PNG's goal of universal interchangeability, decoders
      are required to accept all types of PNG image: indexed-color,
      truecolor, and grayscale.  Viewers running on indexed-color
      display hardware need to be able to reduce truecolor images to
      indexed format for viewing.  This process is usually called "color
      quantization".









Boutell, et. al.             Informational                     [Page 52]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      A simple, fast way of doing this is to reduce the image to a fixed
      palette.  Palettes with uniform color spacing ("color cubes") are
      usually used to minimize the per-pixel computation.  For
      photograph-like images, dithering is recommended to avoid ugly
      contours in what should be smooth gradients; however, dithering
      introduces graininess that can be objectionable.

      The quality of rendering can be improved substantially by using a
      palette chosen specifically for the image, since a color cube
      usually has numerous entries that are unused in any particular
      image.  This approach requires more work, first in choosing the
      palette, and second in mapping individual pixels to the closest
      available color.  PNG allows the encoder to supply a suggested
      palette in a PLTE chunk, but not all encoders will do so, and the
      suggested palette may be unsuitable in any case (it may have too
      many or too few colors).  High-quality viewers will therefore need
      to have a palette selection routine at hand.  A large lookup table
      is usually the most feasible way of mapping individual pixels to
      palette entries with adequate speed.

      Numerous implementations of color quantization are available.  The
      PNG reference implementation, libpng, includes code for the
      purpose.

   10.4. Sample depth rescaling

      Decoders may wish to scale PNG data to a lesser sample depth (data
      precision) for display.  For example, 16-bit data will need to be
      reduced to 8-bit depth for use on most present-day display
      hardware.  Reduction of 8-bit data to 5-bit depth is also common.

      The most accurate scaling is achieved by the linear equation

         output = ROUND(input * MAXOUTSAMPLE / MAXINSAMPLE)

      where

         MAXINSAMPLE = (2^sampledepth)-1
         MAXOUTSAMPLE = (2^desired_sampledepth)-1

      A slightly less accurate conversion is achieved by simply shifting
      right by sampledepth-desired_sampledepth places.  For example, to
      reduce 16-bit samples to 8-bit, one need only discard the low-
      order byte.  In many situations the shift method is sufficiently
      accurate for display purposes, and it is certainly much faster.
      (But if gamma correction is being done, sample rescaling can be
      merged into the gamma correction lookup table, as is illustrated
      in Decoder gamma handling, Section 10.5.)



Boutell, et. al.             Informational                     [Page 53]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      When an sBIT chunk is present, the original pre-PNG data can be
      recovered by shifting right to the sample depth specified by sBIT.
      Note that linear scaling will not necessarily reproduce the
      original data, because the encoder is not required to have used
      linear scaling to scale the data up.  However, the encoder is
      required to have used a method that preserves the high-order bits,
      so shifting always works.  This is the only case in which shifting
      might be said to be more accurate than linear scaling.

      When comparing pixel values to tRNS chunk values to detect
      transparent pixels, it is necessary to do the comparison exactly.
      Therefore, transparent pixel detection must be done before
      reducing sample precision.

   10.5. Decoder gamma handling

      See Gamma Tutorial (Chapter 13) if you aren't already familiar
      with gamma issues.

      To produce correct tone reproduction, a good image display program
      should take into account the gammas of the image file and the
      display device, as well as the viewing_gamma appropriate to the
      lighting conditions near the display.  This can be done by
      calculating

         gbright = insample / MAXINSAMPLE
         bright = gbright ^ (1.0 / file_gamma)
         vbright = bright ^ viewing_gamma
         gcvideo = vbright ^ (1.0 / display_gamma)
         fbval = ROUND(gcvideo * MAXFBVAL)

      where MAXINSAMPLE is the maximum sample value in the file (255 for
      8-bit, 65535 for 16-bit, etc), MAXFBVAL is the maximum value of a
      frame buffer sample (255 for 8-bit, 31 for 5-bit, etc), insample
      is the value of the sample in the PNG file, and fbval is the value
      to write into the frame buffer. The first line converts from
      integer samples into a normalized 0 to 1 floating point value, the
      second undoes the gamma encoding of the image file to produce a
      linear intensity value, the third adjusts for the viewing
      conditions, the fourth corrects for the display system's gamma
      value, and the fifth converts to an integer frame buffer sample.
      In practice, the second through fourth lines can be merged into

         gcvideo = gbright^(viewing_gamma / (file_gamma*display_gamma))

      so as to perform only one power calculation. For color images, the
      entire calculation is performed separately for R, G, and B values.




Boutell, et. al.             Informational                     [Page 54]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      It is not necessary to perform transcendental math for every
      pixel.  Instead, compute a lookup table that gives the correct
      output value for every possible sample value. This requires only
      256 calculations per image (for 8-bit accuracy), not one or three
      calculations per pixel.  For an indexed-color image, a one-time
      correction of the palette is sufficient, unless the image uses
      transparency and is being displayed against a nonuniform
      background.

      In some cases even the cost of computing a gamma lookup table may
      be a concern.  In these cases, viewers are encouraged to have
      precomputed gamma correction tables for file_gamma values of 1.0
      and 0.5 with some reasonable choice of viewing_gamma and
      display_gamma, and to use the table closest to the gamma indicated
      in the file. This will produce acceptable results for the majority
      of real files.

      When the incoming image has unknown gamma (no gAMA chunk), choose
      a likely default file_gamma value, but allow the user to select a
      new one if the result proves too dark or too light.

      In practice, it is often difficult to determine what value of
      display_gamma should be used. In systems with no built-in gamma
      correction, the display_gamma is determined entirely by the CRT.
      Assuming a CRT_gamma of 2.5 is recommended, unless you have
      detailed calibration measurements of this particular CRT
      available.

      However, many modern frame buffers have lookup tables that are
      used to perform gamma correction, and on these systems the
      display_gamma value should be the gamma of the lookup table and
      CRT combined. You may not be able to find out what the lookup
      table contains from within an image viewer application, so you may
      have to ask the user what the system's gamma value is.
      Unfortunately, different manufacturers use different ways of
      specifying what should go into the lookup table, so interpretation
      of the system gamma value is system-dependent.  Gamma Tutorial
      (Chapter 13) gives some examples.

      The response of real displays is actually more complex than can be
      described by a single number (display_gamma). If actual
      measurements of the monitor's light output as a function of
      voltage input are available, the fourth and fifth lines of the
      computation above can be replaced by a lookup in these
      measurements, to find the actual frame buffer value that most
      nearly gives the desired brightness.





Boutell, et. al.             Informational                     [Page 55]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The value of viewing_gamma depends on lighting conditions; see
      Gamma Tutorial (Chapter 13) for more detail.  Ideally, a viewer
      would allow the user to specify viewing_gamma, either directly
      numerically, or via selecting from "bright surround", "dim
      surround", and "dark surround" conditions.  Viewers that don't
      want to do this should just assume a value for viewing_gamma of
      1.0, since most computer displays live in brightly-lit rooms.

      When viewing images that are digitized from video, or that are
      destined to become video frames, the user might want to set the
      viewing_gamma to about 1.25 regardless of the actual level of room
      lighting.  This value of viewing_gamma is "built into" NTSC video
      practice, and displaying an image with that viewing_gamma allows
      the user to see what a TV set would show under the current room
      lighting conditions.  (This is not the same thing as trying to
      obtain the most accurate rendition of the content of the scene,
      which would require adjusting viewing_gamma to correspond to the
      room lighting level.)  This is another reason viewers might want
      to allow users to adjust viewing_gamma directly.

   10.6. Decoder color handling

      See Color Tutorial (Chapter 14) if you aren't already familiar
      with color issues.

      In many cases, decoders will treat image data in PNG files as
      device-dependent RGB data and display it without modification
      (except for appropriate gamma correction). This provides the
      fastest display of PNG images.  But unless the viewer uses exactly
      the same display hardware as the original image author used, the
      colors will not be exactly the same as the original author saw,
      particularly for darker or near-neutral colors.  The cHRM chunk
      provides information that allows closer color matching than that
      provided by gamma correction alone.

      Decoders can use the cHRM data to transform the image data from
      RGB to XYZ and thence into a perceptually linear color space such
      as CIE LAB.  They can then partition the colors to generate an
      optimal palette, because the geometric distance between two colors
      in CIE LAB is strongly related to how different those colors
      appear (unlike, for example, RGB or XYZ spaces).  The resulting
      palette of colors, once transformed back into RGB color space,
      could be used for display or written into a PLTE chunk.

      Decoders that are part of image processing applications might also
      transform image data into CIE LAB space for analysis.





Boutell, et. al.             Informational                     [Page 56]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      In applications where color fidelity is critical, such as product
      design, scientific visualization, medicine, architecture, or
      advertising, decoders can transform the image data from source_RGB
      to the display_RGB space of the monitor used to view the image.
      This involves calculating the matrix to go from source_RGB to XYZ
      and the matrix to go from XYZ to display_RGB, then combining them
      to produce the overall transformation.  The decoder is responsible
      for implementing gamut mapping.

      Decoders running on platforms that have a Color Management System
      (CMS) can pass the image data, gAMA and cHRM values to the CMS for
      display or further processing.

      Decoders that provide color printing facilities can use the
      facilities in Level 2 PostScript to specify image data in
      calibrated RGB space or in a device-independent color space such
      as XYZ.  This will provide better color fidelity than a simple RGB
      to CMYK conversion.  The PostScript Language Reference manual
      gives examples of this process [POSTSCRIPT].  Such decoders are
      responsible for implementing gamut mapping between source_RGB
      (specified in the cHRM chunk) and the target printer. The
      PostScript interpreter is then responsible for producing the
      required colors.

      Decoders can use the cHRM data to calculate an accurate grayscale
      representation of a color image.  Conversion from RGB to gray is
      simply a case of calculating the Y (luminance) component of XYZ,
      which is a weighted sum of the R G and B values.  The weights
      depend on the monitor type, i.e., the values in the cHRM chunk.
      Decoders may wish to do this for PNG files with no cHRM chunk.  In
      that case, a reasonable default would be the CCIR 709 primaries
      [ITU-BT709].  Do not use the original NTSC primaries, unless you
      really do have an image color-balanced for such a monitor.  Few
      monitors ever used the NTSC primaries, so such images are probably
      nonexistent these days.

   10.7. Background color

      The background color given by bKGD will typically be used to fill
      unused screen space around the image, as well as any transparent
      pixels within the image.  (Thus, bKGD is valid and useful even
      when the image does not use transparency.)  If no bKGD chunk is
      present, the viewer will need to make its own decision about a
      suitable background color.







Boutell, et. al.             Informational                     [Page 57]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Viewers that have a specific background against which to present
      the image (such as Web browsers) should ignore the bKGD chunk, in
      effect overriding bKGD with their preferred background color or
      background image.

      The background color given by bKGD is not to be considered
      transparent, even if it happens to match the color given by tRNS
      (or, in the case of an indexed-color image, refers to a palette
      index that is marked as transparent by tRNS).  Otherwise one would
      have to imagine something "behind the background" to composite
      against.  The background color is either used as background or
      ignored; it is not an intermediate layer between the PNG image and
      some other background.

      Indeed, it will be common that bKGD and tRNS specify the same
      color, since then a decoder that does not implement transparency
      processing will give the intended display, at least when no
      partially-transparent pixels are present.

   10.8. Alpha channel processing

      In the most general case, the alpha channel can be used to
      composite a foreground image against a background image; the PNG
      file defines the foreground image and the transparency mask, but
      not the background image.  Decoders are not required to support
      this most general case.  It is expected that most will be able to
      support compositing against a single background color, however.

      The equation for computing a composited sample value is

         output = alpha * foreground + (1-alpha) * background

      where alpha and the input and output sample values are expressed
      as fractions in the range 0 to 1.  This computation should be
      performed with linear (non-gamma-encoded) sample values.  For
      color images, the computation is done separately for R, G, and B
      samples.

      The following code illustrates the general case of compositing a
      foreground image over a background image.  It assumes that you
      have the original pixel data available for the background image,
      and that output is to a frame buffer for display.  Other variants
      are possible; see the comments below the code.  The code allows
      the sample depths and gamma values of foreground image, background
      image, and frame buffer/CRT all to be different.  Don't assume
      they are the same without checking.





Boutell, et. al.             Informational                     [Page 58]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      This code is standard C, with line numbers added for reference in
      the comments below.

         01  int foreground[4];  /* image pixel: R, G, B, A */
         02  int background[3];  /* background pixel: R, G, B */
         03  int fbpix[3];       /* frame buffer pixel */
         04  int fg_maxsample;   /* foreground max sample */
         05  int bg_maxsample;   /* background max sample */
         06  int fb_maxsample;   /* frame buffer max sample */
         07  int ialpha;
         08  float alpha, compalpha;
         09  float gamfg, linfg, gambg, linbg, comppix, gcvideo;

             /* Get max sample values in data and frame buffer */
         10  fg_maxsample = (1 << fg_sample_depth) - 1;
         11  bg_maxsample = (1 << bg_sample_depth) - 1;
         12  fb_maxsample = (1 << frame_buffer_sample_depth) - 1;
             /*
              * Get integer version of alpha.
              * Check for opaque and transparent special cases;
              * no compositing needed if so.
              *
              * We show the whole gamma decode/correct process in
              * floating point, but it would more likely be done
              * with lookup tables.
              */
         13  ialpha = foreground[3];

         14  if (ialpha == 0) {
                 /*
                  * Foreground image is transparent here.
                  * If the background image is already in the frame
                  * buffer, there is nothing to do.
                  */
         15      ;
         16  } else if (ialpha == fg_maxsample) {
                 /*
                  * Copy foreground pixel to frame buffer.
                  */
         17      for (i = 0; i < 3; i++) {
         18          gamfg = (float) foreground[i] / fg_maxsample;
         19          linfg = pow(gamfg, 1.0/fg_gamma);
         20          comppix = linfg;
         21          gcvideo = pow(comppix,viewing_gamma/display_gamma);
         22          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
         23      }





Boutell, et. al.             Informational                     [Page 59]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


         24  } else {
                 /*
                  * Compositing is necessary.
                  * Get floating-point alpha and its complement.
                  * Note: alpha is always linear; gamma does not
                  * affect it.
                  */
         25      alpha = (float) ialpha / fg_maxsample;
         26      compalpha = 1.0 - alpha;
         27      for (i = 0; i < 3; i++) {
                     /*
                      * Convert foreground and background to floating
                      * point, then linearize (undo gamma encoding).
                      */
         28          gamfg = (float) foreground[i] / fg_maxsample;
         29          linfg = pow(gamfg, 1.0/fg_gamma);
         30          gambg = (float) background[i] / bg_maxsample;
         31          linbg = pow(gambg, 1.0/bg_gamma);
                     /*
                      * Composite.
                      */
         32          comppix = linfg * alpha + linbg * compalpha;
                     /*
                      * Gamma correct for display.
                      * Convert to integer frame buffer pixel.
                      */
         33          gcvideo = pow(comppix,viewing_gamma/display_gamma);
         34          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
         35      }
         36  }

      Variations:

          * If output is to another PNG image file instead of a frame
            buffer, lines 21, 22, 33, and 34 should be changed to be
            something like

               /*
                * Gamma encode for storage in output file.
                * Convert to integer sample value.
                */
               gamout = pow(comppix, outfile_gamma);
               outpix[i] = (int) (gamout * out_maxsample + 0.5);

            Also, it becomes necessary to process background pixels when
            alpha is zero, rather than just skipping pixels.  Thus, line
            15 will need to be replaced by copies of lines 17-23, but
            processing background instead of foreground pixel values.



Boutell, et. al.             Informational                     [Page 60]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


          * If the sample depths of the output file, foreground file,
            and background file are all the same, and the three gamma
            values also match, then the no-compositing code in lines
            14-23 reduces to nothing more than copying pixel values from
            the input file to the output file if alpha is one, or
            copying pixel values from background to output file if alpha
            is zero.  Since alpha is typically either zero or one for
            the vast majority of pixels in an image, this is a great
            savings.  No gamma computations are needed for most pixels.
          * When the sample depths and gamma values all match, it may
            appear attractive to skip the gamma decoding and encoding
            (lines 28-31, 33-34) and just perform line 32 using gamma-
            encoded sample values. Although this doesn't hurt image
            quality too badly, the time savings are small if alpha
            values of zero and one are special-cased as recommended
            here.
          * If the original pixel values of the background image are no
            longer available, only processed frame buffer pixels left by
            display of the background image, then lines 30 and 31 need
            to extract intensity from the frame buffer pixel values
            using code like

               /*
                * Decode frame buffer value back into linear space.
                */
               gcvideo = (float) fbpix[i] / fb_maxsample;
               linbg = pow(gcvideo, display_gamma / viewing_gamma);

            However, some roundoff error can result, so it is better to
            have the original background pixels available if at all
            possible.
          * Note that lines 18-22 are performing exactly the same gamma
            computation that is done when no alpha channel is present.
            So, if you handle the no-alpha case with a lookup table, you
            can use the same lookup table here.  Lines 28-31 and 33-34
            can also be done with (different) lookup tables.
          * Of course, everything here can be done in integer
            arithmetic.  Just be careful to maintain sufficient
            precision all the way through.

      Note: in floating point, no overflow or underflow checks are
      needed, because the input sample values are guaranteed to be
      between 0 and 1, and compositing always yields a result that is in
      between the input values (inclusive).  With integer arithmetic,
      some roundoff-error analysis might be needed to guarantee no
      overflow or underflow.





Boutell, et. al.             Informational                     [Page 61]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      When displaying a PNG image with full alpha channel, it is
      important to be able to composite the image against some
      background, even if it's only black.  Ignoring the alpha channel
      will cause PNG images that have been converted from an
      associated-alpha representation to look wrong.  (Of course, if the
      alpha channel is a separate transparency mask, then ignoring alpha
      is a useful option: it allows the hidden parts of the image to be
      recovered.)

      Even if the decoder author does not wish to implement true
      compositing logic, it is simple to deal with images that contain
      only zero and one alpha values.  (This is implicitly true for
      grayscale and truecolor PNG files that use a tRNS chunk; for
      indexed-color PNG files, it is easy to check whether tRNS contains
      any values other than 0 and 255.)  In this simple case,
      transparent pixels are replaced by the background color, while
      others are unchanged.  If a decoder contains only this much
      transparency capability, it should deal with a full alpha channel
      by treating all nonzero alpha values as fully opaque; that is, do
      not replace partially transparent pixels by the background.  This
      approach will not yield very good results for images converted
      from associated-alpha formats, but it's better than doing nothing.

   10.9. Progressive display

      When receiving images over slow transmission links, decoders can
      improve perceived performance by displaying interlaced images
      progressively.  This means that as each pass is received, an
      approximation to the complete image is displayed based on the data
      received so far.  One simple yet pleasing effect can be obtained
      by expanding each received pixel to fill a rectangle covering the
      yet-to-be-transmitted pixel positions below and to the right of
      the received pixel.  This process can be described by the
      following pseudocode:

         Starting_Row [1..7] =  { 0, 0, 4, 0, 2, 0, 1 }
         Starting_Col [1..7] =  { 0, 4, 0, 2, 0, 1, 0 }
         Row_Increment [1..7] = { 8, 8, 8, 4, 4, 2, 2 }
         Col_Increment [1..7] = { 8, 8, 4, 4, 2, 2, 1 }
         Block_Height [1..7] =  { 8, 8, 4, 4, 2, 2, 1 }
         Block_Width [1..7] =   { 8, 4, 4, 2, 2, 1, 1 }

         pass := 1
         while pass <= 7
         begin
             row := Starting_Row[pass]

             while row < height



Boutell, et. al.             Informational                     [Page 62]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


             begin
                 col := Starting_Col[pass]

                 while col < width
                 begin
                     visit (row, col,
                            min (Block_Height[pass], height - row),
                            min (Block_Width[pass], width - col))
                     col := col + Col_Increment[pass]
                 end
                 row := row + Row_Increment[pass]
             end

             pass := pass + 1
         end

      Here, the function "visit(row,column,height,width)" obtains the
      next transmitted pixel and paints a rectangle of the specified
      height and width, whose upper-left corner is at the specified row
      and column, using the color indicated by the pixel.  Note that row
      and column are measured from 0,0 at the upper left corner.

      If the decoder is merging the received image with a background
      image, it may be more convenient just to paint the received pixel
      positions; that is, the "visit()" function sets only the pixel at
      the specified row and column, not the whole rectangle.  This
      produces a "fade-in" effect as the new image gradually replaces
      the old.  An advantage of this approach is that proper alpha or
      transparency processing can be done as each pixel is replaced.
      Painting a rectangle as described above will overwrite
      background-image pixels that may be needed later, if the pixels
      eventually received for those positions turn out to be wholly or
      partially transparent.  Of course, this is only a problem if the
      background image is not stored anywhere offscreen.

   10.10. Suggested-palette and histogram usage

      In truecolor PNG files, the encoder may have provided a suggested
      PLTE chunk for use by viewers running on indexed-color hardware.

      If the image has a tRNS chunk, the viewer will need to adapt the
      suggested palette for use with its desired background color.  To
      do this, replace the palette entry closest to the tRNS color with
      the desired background color; or just add a palette entry for the
      background color, if the viewer can handle more colors than there
      are PLTE entries.





Boutell, et. al.             Informational                     [Page 63]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      For images of color type 6 (truecolor with alpha channel), any
      suggested palette should have been designed for display of the
      image against a uniform background of the color specified by bKGD.
      Viewers should probably ignore the palette if they intend to use a
      different background, or if the bKGD chunk is missing.  Viewers
      can use a suggested palette for display against a different
      background than it was intended for, but the results may not be
      very good.

      If the viewer presents a transparent truecolor image against a
      background that is more complex than a single color, it is
      unlikely that the suggested palette will be optimal for the
      composite image.  In this case it is best to perform a truecolor
      compositing step on the truecolor PNG image and background image,
      then color-quantize the resulting image.

      The histogram chunk is useful when the viewer cannot provide as
      many colors as are used in the image's palette.  If the viewer is
      only short a few colors, it is usually adequate to drop the
      least-used colors from the palette.  To reduce the number of
      colors substantially, it's best to choose entirely new
      representative colors, rather than trying to use a subset of the
      existing palette.  This amounts to performing a new color
      quantization step; however, the existing palette and histogram can
      be used as the input data, thus avoiding a scan of the image data.

      If no palette or histogram chunk is provided, a decoder can
      develop its own, at the cost of an extra pass over the image data.
      Alternatively, a default palette (probably a color cube) can be
      used.

      See also Recommendations for Encoders: Suggested palettes (Section
      9.5).

   10.11. Text chunk processing

      If practical, decoders should have a way to display to the user
      all tEXt and zTXt chunks found in the file.  Even if the decoder
      does not recognize a particular text keyword, the user might be
      able to understand it.

      PNG text is not supposed to contain any characters outside the ISO
      8859-1 "Latin-1" character set (that is, no codes 0-31 or 127-
      159), except for the newline character (decimal 10).  But decoders
      might encounter such characters anyway.  Some of these characters
      can be safely displayed (e.g., TAB, FF, and CR, decimal 9, 12, and
      13, respectively), but others, especially the ESC character
      (decimal 27), could pose a security hazard because unexpected



Boutell, et. al.             Informational                     [Page 64]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      actions may be taken by display hardware or software.  To prevent
      such hazards, decoders should not attempt to directly display any
      non-Latin-1 characters (except for newline and perhaps TAB, FF,
      CR) encountered in a tEXt or zTXt chunk.  Instead, ignore them or
      display them in a visible notation such as "\nnn".  See Security
      considerations (Section 8.5).

      Even though encoders are supposed to represent newlines as LF, it
      is recommended that decoders not rely on this; it's best to
      recognize all the common newline combinations (CR, LF, and CR-LF)
      and display each as a single newline.  TAB can be expanded to the
      proper number of spaces needed to arrive at a column multiple of
      8.

      Decoders running on systems with non-Latin-1 character set
      encoding should provide character code remapping so that Latin-1
      characters are displayed correctly.  Some systems may not provide
      all the characters defined in Latin-1.  Mapping unavailable
      characters to a visible notation such as "\nnn" is a good
      fallback.  In particular, character codes 127-255 should be
      displayed only if they are printable characters on the decoding
      system.  Some systems may interpret such codes as control
      characters; for security, decoders running on such systems should
      not display such characters literally.

      Decoders should be prepared to display text chunks that contain
      any number of printing characters between newline characters, even
      though encoders are encouraged to avoid creating lines in excess
      of 79 characters.

11. Glossary

   a^b
      Exponentiation; a raised to the power b.  C programmers should be
      careful not to misread this notation as exclusive-or.  Note that
      in gamma-related calculations, zero raised to any power is valid
      and must give a zero result.

   Alpha
      A value representing a pixel's degree of transparency.  The more
      transparent a pixel, the less it hides the background against
      which the image is presented.  In PNG, alpha is really the degree
      of opacity: zero alpha represents a completely transparent pixel,
      maximum alpha represents a completely opaque pixel.  But most
      people refer to alpha as providing transparency information, not
      opacity information, and we continue that custom here.





Boutell, et. al.             Informational                     [Page 65]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Ancillary chunk
      A chunk that provides additional information.  A decoder can still
      produce a meaningful image, though not necessarily the best
      possible image, without processing the chunk.

   Bit depth
      The number of bits per palette index (in indexed-color PNGs) or
      per sample (in other color types).  This is the same value that
      appears in IHDR.

   Byte
      Eight bits; also called an octet.

   Channel
      The set of all samples of the same kind within an image; for
      example, all the blue samples in a truecolor image.  (The term
      "component" is also used, but not in this specification.)  A
      sample is the intersection of a channel and a pixel.

   Chromaticity
      A pair of values x,y that precisely specify the hue, though not
      the absolute brightness, of a perceived color.

   Chunk
      A section of a PNG file.  Each chunk has a type indicated by its
      chunk type name.  Most types of chunks also include some data.
      The format and meaning of the data within the chunk are determined
      by the type name.

   Composite
      As a verb, to form an image by merging a foreground image and a
      background image, using transparency information to determine
      where the background should be visible.  The foreground image is
      said to be "composited against" the background.

   CRC
      Cyclic Redundancy Check.  A CRC is a type of check value designed
      to catch most transmission errors.  A decoder calculates the CRC
      for the received data and compares it to the CRC that the encoder
      calculated, which is appended to the data.  A mismatch indicates
      that the data was corrupted in transit.

   Critical chunk
      A chunk that must be understood and processed by the decoder in
      order to produce a meaningful image from a PNG file.

   CRT
      Cathode Ray Tube: a common type of computer display hardware.



Boutell, et. al.             Informational                     [Page 66]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Datastream
      A sequence of bytes.  This term is used rather than "file" to
      describe a byte sequence that is only a portion of a file.  We
      also use it to emphasize that a PNG image might be generated and
      consumed "on the fly", never appearing in a stored file at all.

   Deflate
      The name of the compression algorithm used in standard PNG files,
      as well as in zip, gzip, pkzip, and other compression programs.
      Deflate is a member of the LZ77 family of compression methods.

   Filter
      A transformation applied to image data in hopes of improving its
      compressibility.  PNG uses only lossless (reversible) filter
      algorithms.

   Frame buffer
      The final digital storage area for the image shown by a computer
      display.  Software causes an image to appear onscreen by loading
      it into the frame buffer.

   Gamma
      The brightness of mid-level tones in an image.  More precisely, a
      parameter that describes the shape of the transfer function for
      one or more stages in an imaging pipeline.  The transfer function
      is given by the expression

         output = input ^ gamma

      where both input and output are scaled to the range 0 to 1.

   Grayscale
      An image representation in which each pixel is represented by a
      single sample value representing overall luminance (on a scale
      from black to white).  PNG also permits an alpha sample to be
      stored for each pixel of a grayscale image.

   Indexed color
      An image representation in which each pixel is represented by a
      single sample that is an index into a palette or lookup table.
      The selected palette entry defines the actual color of the pixel.

   Lossless compression
      Any method of data compression that guarantees the original data
      can be reconstructed exactly, bit-for-bit.






Boutell, et. al.             Informational                     [Page 67]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Lossy compression
      Any method of data compression that reconstructs the original data
      approximately, rather than exactly.

   LSB
      Least Significant Byte of a multi-byte value.

   Luminance
      Perceived brightness, or grayscale level, of a color.  Luminance
      and chromaticity together fully define a perceived color.

   LUT
      Look Up Table.  In general, a table used to transform data.  In
      frame buffer hardware, a LUT can be used to map indexed-color
      pixels into a selected set of truecolor values, or to perform
      gamma correction.  In software, a LUT can be used as a fast way of
      implementing any one-variable mathematical function.

   MSB
      Most Significant Byte of a multi-byte value.

   Palette
      The set of colors available in an indexed-color image.  In PNG, a
      palette is an array of colors defined by red, green, and blue
      samples.  (Alpha values can also be defined for palette entries,
      via the tRNS chunk.)

   Pixel
      The information stored for a single grid point in the image.  The
      complete image is a rectangular array of pixels.

   PNG editor
      A program that modifies a PNG file and preserves ancillary
      information, including chunks that it does not recognize.  Such a
      program must obey the rules given in Chunk Ordering Rules (Chapter
      7).

   Sample
      A single number in the image data; for example, the red value of a
      pixel.  A pixel is composed of one or more samples.  When
      discussing physical data layout (in particular, in Image layout,
      Section 2.3), we use "sample" to mean a number stored in the image
      array.  It would be more precise but much less readable to say
      "sample or palette index" in that context.  Elsewhere in the
      specification, "sample" means a color value or alpha value.  In
      the indexed-color case, these are palette entries not palette
      indexes.




Boutell, et. al.             Informational                     [Page 68]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Sample depth
      The precision, in bits, of color values and alpha values.  In
      indexed-color PNGs the sample depth is always 8 by definition of
      the PLTE chunk.  In other color types it is the same as the bit
      depth.

   Scanline
      One horizontal row of pixels within an image.

   Truecolor
      An image representation in which pixel colors are defined by
      storing three samples for each pixel, representing red, green, and
      blue intensities respectively.  PNG also permits an alpha sample
      to be stored for each pixel of a truecolor image.

   White point
      The chromaticity of a computer display's nominal white value.

   zlib
      A particular format for data that has been compressed using
      deflate-style compression.  Also the name of a library
      implementing this method.  PNG implementations need not use the
      zlib library, but they must conform to its format for compressed
      data.

12. Appendix: Rationale

   (This appendix is not part of the formal PNG specification.)

   This appendix gives the reasoning behind some of the design decisions
   in PNG.  Many of these decisions were the subject of considerable
   debate.  The authors freely admit that another group might have made
   different decisions; however, we believe that our choices are
   defensible and consistent.

   12.1. Why a new file format?

      Does the world really need yet another graphics format?  We
      believe so.  GIF is no longer freely usable, but no other commonly
      used format can directly replace it, as is discussed in more
      detail below.  We might have used an adaptation of an existing
      format, for example GIF with an unpatented compression scheme.
      But this would require new code anyway; it would not be all that
      much easier to implement than a whole new file format.  (PNG is
      designed to be simple to implement, with the exception of the
      compression engine, which would be needed in any case.)  We feel
      that this is an excellent opportunity to design a new format that
      fixes some of the known limitations of GIF.



Boutell, et. al.             Informational                     [Page 69]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   12.2. Why these features?

      The features chosen for PNG are intended to address the needs of
      applications that previously used the special strengths of GIF.
      In particular, GIF is well adapted for online communications
      because of its streamability and progressive display capability.
      PNG shares those attributes.

      We have also addressed some of the widely known shortcomings of
      GIF.  In particular, PNG supports truecolor images.  We know of no
      widely used image format that losslessly compresses truecolor
      images as effectively as PNG does.  We hope that PNG will make use
      of truecolor images more practical and widespread.

      Some form of transparency control is desirable for applications in
      which images are displayed against a background or together with
      other images.  GIF provided a simple transparent-color
      specification for this purpose.  PNG supports a full alpha channel
      as well as transparent-color specifications.  This allows both
      highly flexible transparency and compression efficiency.

      Robustness against transmission errors has been an important
      consideration.  For example, images transferred across Internet
      are often mistakenly processed as text, leading to file
      corruption.  PNG is designed so that such errors can be detected
      quickly and reliably.

      PNG has been expressly designed not to be completely dependent on
      a single compression technique. Although deflate/inflate
      compression is mentioned in this document, PNG would still exist
      without it.

   12.3. Why not these features?

      Some features have been deliberately omitted from PNG.  These
      choices were made to simplify implementation of PNG, promote
      portability and interchangeability, and make the format as simple
      and foolproof as possible for users.  In particular:













Boutell, et. al.             Informational                     [Page 70]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


          * There is no uncompressed variant of PNG.  It is possible to
            store uncompressed data by using only uncompressed deflate
            blocks (a feature normally used to guarantee that deflate
            does not make incompressible data much larger).  However,
            PNG software must support full deflate/inflate; any software
            that does not is not compliant with the PNG standard. The
            two most important features of PNG---portability and
            compression---are absolute requirements for online
            applications, and users demand them. Failure to support full
            deflate/inflate compromises both of these objectives.
          * There is no lossy compression in PNG.  Existing formats such
            as JFIF already handle lossy compression well.  Furthermore,
            available lossy compression methods (e.g., JPEG) are far
            from foolproof --- a poor choice of quality level can ruin
            an image.  To avoid user confusion and unintentional loss of
            information, we feel it is best to keep lossy and lossless
            formats strictly separate.  Also, lossy compression is
            complex to implement.  Adding JPEG support to a PNG decoder
            might increase its size by an order of magnitude.  This
            would certainly cause some decoders to omit support for the
            feature, which would destroy our goal of interchangeability.
          * There is no support for CMYK or other unusual color spaces.
            Again, this is in the name of promoting portability.  CMYK,
            in particular, is far too device-dependent to be useful as a
            portable image representation.
          * There is no standard chunk for thumbnail views of images.
            In discussions with software vendors who use thumbnails in
            their products, it has become clear that most would not use
            a "standard" thumbnail chunk.  For one thing, every vendor
            has a different idea of what the dimensions and
            characteristics of a thumbnail ought to be.  Also, some
            vendors keep thumbnails in separate files to accommodate
            varied image formats; they are not going to stop doing that
            simply because of a thumbnail chunk in one new format.
            Proprietary chunks containing vendor-specific thumbnails
            appear to be more practical than a common thumbnail format.

      It is worth noting that private extensions to PNG could easily add
      these features.  We will not, however, include them as part of the
      basic PNG standard.











Boutell, et. al.             Informational                     [Page 71]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      PNG also does not support multiple images in one file.  This
      restriction is a reflection of the reality that many applications
      do not need and will not support multiple images per file.  In any
      case, single images are a fundamentally different sort of object
      from sequences of images.  Rather than make false promises of
      interchangeability, we have drawn a clear distinction between
      single-image and multi-image formats.  PNG is a single-image
      format.  (But see Multiple-image extension, Section 8.4.)

   12.4. Why not use format X?

      Numerous existing formats were considered before deciding to
      develop PNG.  None could meet the requirements we felt were
      important for PNG.

      GIF is no longer suitable as a universal standard because of legal
      entanglements.  Although just replacing GIF's compression method
      would avoid that problem, GIF does not support truecolor images,
      alpha channels, or gamma correction.  The spec has more subtle
      problems too.  Only a small subset of the GIF89 spec is actually
      portable across a variety of implementations, but there is no
      codification of the most portable part of the spec.

      TIFF is far too complex to meet our goals of simplicity and
      interchangeability.  Defining a TIFF subset would meet that
      objection, but would frustrate users making the reasonable
      assumption that a file saved as TIFF from their existing software
      would load into a program supporting our flavor of TIFF.
      Furthermore, TIFF is not designed for stream processing, has no
      provision for progressive display, and does not currently provide
      any good, legally unencumbered, lossless compression method.

      IFF has also been suggested, but is not suitable in detail:
      available image representations are too machine-specific or not
      adequately compressed.  The overall chunk structure of IFF is a
      useful concept that PNG has liberally borrowed from, but we did
      not attempt to be bit-for-bit compatible with IFF chunk structure.
      Again this is due to detailed issues, notably the fact that IFF
      FORMs are not designed to be serially writable.

      Lossless JPEG is not suitable because it does not provide for the
      storage of indexed-color images.  Furthermore, its lossless
      truecolor compression is often inferior to that of PNG.








Boutell, et. al.             Informational                     [Page 72]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   12.5. Byte order

      It has been asked why PNG uses network byte order.  We have
      selected one byte ordering and used it consistently. Which order
      in particular is of little relevance, but network byte order has
      the advantage that routines to convert to and from it are already
      available on any platform that supports TCP/IP networking,
      including all PC platforms.  The functions are trivial and will be
      included in the reference implementation.

   12.6. Interlacing

      PNG's two-dimensional interlacing scheme is more complex to
      implement than GIF's line-wise interlacing.  It also costs a
      little more in file size.  However, it yields an initial image
      eight times faster than GIF (the first pass transmits only 1/64th
      of the pixels, compared to 1/8th for GIF).  Although this initial
      image is coarse, it is useful in many situations.  For example, if
      the image is a World Wide Web imagemap that the user has seen
      before, PNG's first pass is often enough to determine where to
      click.  The PNG scheme also looks better than GIF's, because
      horizontal and vertical resolution never differ by more than a
      factor of two; this avoids the odd "stretched" look seen when
      interlaced GIFs are filled in by replicating scanlines.
      Preliminary results show that small text in an interlaced PNG
      image is typically readable about twice as fast as in an
      equivalent GIF, i.e., after PNG's fifth pass or 25% of the image
      data, instead of after GIF's third pass or 50%.  This is again due
      to PNG's more balanced increase in resolution.

   12.7. Why gamma?

      It might seem natural to standardize on storing sample values that
      are linearly proportional to light intensity (that is, have gamma
      of 1.0).  But in fact, it is common for images to have a gamma of
      less than 1.  There are three good reasons for this:

          * For reasons detailed in Gamma Tutorial (Chapter 13), all
            video cameras apply a "gamma correction" function to the
            intensity information.  This causes the video signal to have
            a gamma of about 0.5 relative to the light intensity in the
            original scene.  Thus, images obtained by frame-grabbing
            video already have a gamma of about 0.5.
          * The human eye has a nonlinear response to intensity, so
            linear encoding of samples either wastes sample codes in
            bright areas of the image, or provides too few sample codes
            to avoid banding artifacts in dark areas of the image, or
            both.  At least 12 bits per sample are needed to avoid



Boutell, et. al.             Informational                     [Page 73]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


            visible artifacts in linear encoding with a 100:1 image
            intensity range.  An image gamma in the range 0.3 to 0.5
            allocates sample values in a way that roughly corresponds to
            the eye's response, so that 8 bits/sample are enough to
            avoid artifacts caused by insufficient sample precision in
            almost all images.  This makes "gamma encoding" a much
            better way of storing digital images than the simpler linear
            encoding.
          * Many images are created on PCs or workstations with no gamma
            correction hardware and no software willing to provide gamma
            correction either.  In these cases, the images have had
            their lighting and color chosen to look best on this
            platform --- they can be thought of as having "manual" gamma
            correction built in.  To see what the image author intended,
            it is necessary to treat such images as having a file_gamma
            value in the range 0.4-0.6, depending on the room lighting
            level that the author was working in.

      In practice, image gamma values around 1.0 and around 0.5 are both
      widely found.  Older image standards such as GIF often do not
      account for this fact.  The JFIF standard specifies that images in
      that format should use linear samples, but many JFIF images found
      on the Internet actually have a gamma somewhere near 0.4 or 0.5.
      The variety of images found and the variety of systems that people
      display them on have led to widespread problems with images
      appearing "too dark" or "too light".

      PNG expects viewers to compensate for image gamma at the time that
      the image is displayed. Another possible approach is to expect
      encoders to convert all images to a uniform gamma at encoding
      time. While that method would speed viewers slightly, it has
      fundamental flaws:

          * Gamma correction is inherently lossy due to quantization and
            roundoff error.  Requiring conversion at encoding time thus
            causes irreversible loss. Since PNG is intended to be a
            lossless storage format, this is undesirable; we should
            store unmodified source data.
          * The encoder might not know the source gamma value. If the
            decoder does gamma correction at viewing time, it can adjust
            the gamma (change the displayed brightness) in response to
            feedback from a human user. The encoder has no such
            recourse.
          * Whatever "standard" gamma we settled on would be wrong for
            some displays. Hence viewers would still need gamma
            correction capability.





Boutell, et. al.             Informational                     [Page 74]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Since there will always be images with no gamma or an incorrect
      recorded gamma, good viewers will need to incorporate gamma
      adjustment code anyway. Gamma correction at viewing time is thus
      the right way to go.

      See Gamma Tutorial (Chapter 13) for more information.

   12.8. Non-premultiplied alpha

      PNG uses "unassociated" or "non-premultiplied" alpha so that
      images with separate transparency masks can be stored losslessly.
      Another common technique, "premultiplied alpha", stores pixel
      values premultiplied by the alpha fraction; in effect, the image
      is already composited against a black background.  Any image data
      hidden by the transparency mask is irretrievably lost by that
      method, since multiplying by a zero alpha value always produces
      zero.

      Some image rendering techniques generate images with premultiplied
      alpha (the alpha value actually represents how much of the pixel
      is covered by the image).  This representation can be converted to
      PNG by dividing the sample values by alpha, except where alpha is
      zero.  The result will look good if displayed by a viewer that
      handles alpha properly, but will not look very good if the viewer
      ignores the alpha channel.

      Although each form of alpha storage has its advantages, we did not
      want to require all PNG viewers to handle both forms.  We
      standardized on non-premultiplied alpha as being the lossless and
      more general case.

   12.9. Filtering

      PNG includes filtering capability because filtering can
      significantly reduce the compressed size of truecolor and
      grayscale images.  Filtering is also sometimes of value on
      indexed-color images, although this is less common.

      The filter algorithms are defined to operate on bytes, rather than
      pixels; this gains simplicity and speed with very little cost in
      compression performance.  Tests have shown that filtering is
      usually ineffective for images with fewer than 8 bits per sample,
      so providing pixelwise filtering for such images would be
      pointless.  For 16 bit/sample data, bytewise filtering is nearly
      as effective as pixelwise filtering, because MSBs are predicted
      from adjacent MSBs, and LSBs are predicted from adjacent LSBs.





Boutell, et. al.             Informational                     [Page 75]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The encoder is allowed to change filters for each new scanline.
      This creates no additional complexity for decoders, since a
      decoder is required to contain defiltering logic for every filter
      type anyway.  The only cost is an extra byte per scanline in the
      pre-compression datastream.  Our tests showed that when the same
      filter is selected for all scanlines, this extra byte compresses
      away to almost nothing, so there is little storage cost compared
      to a fixed filter specified for the whole image.  And the
      potential benefits of adaptive filtering are too great to ignore.
      Even with the simplistic filter-choice heuristics so far
      discovered, adaptive filtering usually outperforms fixed filters.
      In particular, an adaptive filter can change behavior for
      successive passes of an interlaced image; a fixed filter cannot.

   12.10. Text strings

      Most graphics file formats include the ability to store some
      textual information along with the image.  But many applications
      need more than that: they want to be able to store several
      identifiable pieces of text.  For example, a database using PNG
      files to store medical X-rays would likely want to include
      patient's name, doctor's name, etc.  A simple way to do this in
      PNG would be to invent new private chunks holding text.  The
      disadvantage of such an approach is that other applications would
      have no idea what was in those chunks, and would simply ignore
      them.  Instead, we recommend that textual information be stored in
      standard tEXt chunks with suitable keywords.  Use of tEXt tells
      any PNG viewer that the chunk contains text that might be of
      interest to a human user.  Thus, a person looking at the file with
      another viewer will still be able to see the text, and even
      understand what it is if the keywords are reasonably self-
      explanatory.  (To this end, we recommend spelled-out keywords, not
      abbreviations that will be hard for a person to understand.
      Saving a few bytes on a keyword is false economy.)

      The ISO 8859-1 (Latin-1) character set was chosen as a compromise
      between functionality and portability.  Some platforms cannot
      display anything more than 7-bit ASCII characters, while others
      can handle characters beyond the Latin-1 set.  We felt that
      Latin-1 represents a widely useful and reasonably portable
      character set.  Latin-1 is a direct subset of character sets
      commonly used on popular platforms such as Microsoft Windows and X
      Windows.  It can also be handled on Macintosh systems with a
      simple remapping of characters.

      There is presently no provision for text employing character sets
      other than Latin-1. We recognize that the need for other character
      sets will increase.  However, PNG already requires that



Boutell, et. al.             Informational                     [Page 76]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      programmers implement a number of new and unfamiliar features, and
      text representation is not PNG's primary purpose. Since PNG
      provides for the creation and public registration of new ancillary
      chunks of general interest, we expect that text chunks for other
      character sets, such as Unicode, eventually will be registered and
      increase gradually in popularity.

   12.11. PNG file signature

      The first eight bytes of a PNG file always contain the following
      values:

         (decimal)              137  80  78  71  13  10  26  10
         (hexadecimal)           89  50  4e  47  0d  0a  1a  0a
         (ASCII C notation)    \211   P   N   G  \r  \n \032 \n

      This signature both identifies the file as a PNG file and provides
      for immediate detection of common file-transfer problems.  The
      first two bytes distinguish PNG files on systems that expect the
      first two bytes to identify the file type uniquely.  The first
      byte is chosen as a non-ASCII value to reduce the probability that
      a text file may be misrecognized as a PNG file; also, it catches
      bad file transfers that clear bit 7.  Bytes two through four name
      the format.  The CR-LF sequence catches bad file transfers that
      alter newline sequences.  The control-Z character stops file
      display under MS-DOS.  The final line feed checks for the inverse
      of the CR-LF translation problem.

      A decoder may further verify that the next eight bytes contain an
      IHDR chunk header with the correct chunk length; this will catch
      bad transfers that drop or alter null (zero) bytes.

      Note that there is no version number in the signature, nor indeed
      anywhere in the file.  This is intentional: the chunk mechanism
      provides a better, more flexible way to handle format extensions,
      as explained in Chunk naming conventions (Section 12.13).

   12.12. Chunk layout

      The chunk design allows decoders to skip unrecognized or
      uninteresting chunks: it is simply necessary to skip the
      appropriate number of bytes, as determined from the length field.

      Limiting chunk length to (2^31)-1 bytes avoids possible problems
      for implementations that cannot conveniently handle 4-byte
      unsigned values.  In practice, chunks will usually be much shorter
      than that anyway.




Boutell, et. al.             Informational                     [Page 77]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      A separate CRC is provided for each chunk in order to detect
      badly-transferred images as quickly as possible.  In particular,
      critical data such as the image dimensions can be validated before
      being used.

      The chunk length is excluded from the CRC so that the CRC can be
      calculated as the data is generated; this avoids a second pass
      over the data in cases where the chunk length is not known in
      advance.  Excluding the length from the CRC does not create any
      extra risk of failing to discover file corruption, since if the
      length is wrong, the CRC check will fail: the CRC will be computed
      on the wrong set of bytes and then be tested against the wrong
      value from the file.

   12.13. Chunk naming conventions

      The chunk naming conventions allow safe, flexible extension of the
      PNG format.  This mechanism is much better than a format version
      number, because it works on a feature-by-feature basis rather than
      being an overall indicator.  Decoders can process newer files if
      and only if the files use no unknown critical features (as
      indicated by finding unknown critical chunks).  Unknown ancillary
      chunks can be safely ignored.  We decided against having an
      overall format version number because experience has shown that
      format version numbers hurt portability as much as they help.
      Version numbers tend to be set unnecessarily high, leading to
      older decoders rejecting files that they could have processed
      (this was a serious problem for several years after the GIF89 spec
      came out, for example).  Furthermore, private extensions can be
      made either critical or ancillary, and standard decoders should
      react appropriately; overall version numbers are no help for
      private extensions.

      A hypothetical chunk for vector graphics would be a critical
      chunk, since if ignored, important parts of the intended image
      would be missing.  A chunk carrying the Mandelbrot set coordinates
      for a fractal image would be ancillary, since other applications
      could display the image without understanding what the image
      represents.  In general, a chunk type should be made critical only
      if it is impossible to display a reasonable representation of the
      intended image without interpreting that chunk.










Boutell, et. al.             Informational                     [Page 78]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The public/private property bit ensures that any newly defined
      public chunk type name cannot conflict with proprietary chunks
      that could be in use somewhere.  However, this does not protect
      users of private chunk names from the possibility that someone
      else may use the same chunk name for a different purpose.  It is a
      good idea to put additional identifying information at the start
      of the data for any private chunk type.

      When a PNG file is modified, certain ancillary chunks may need to
      be changed to reflect changes in other chunks. For example, a
      histogram chunk needs to be changed if the image data changes.  If
      the file editor does not recognize histogram chunks, copying them
      blindly to a new output file is incorrect; such chunks should be
      dropped.  The safe/unsafe property bit allows ancillary chunks to
      be marked appropriately.

      Not all possible modification scenarios are covered by the
      safe/unsafe semantics.  In particular, chunks that are dependent
      on the total file contents are not supported.  (An example of such
      a chunk is an index of IDAT chunk locations within the file:
      adding a comment chunk would inadvertently break the index.)
      Definition of such chunks is discouraged.  If absolutely necessary
      for a particular application, such chunks can be made critical
      chunks, with consequent loss of portability to other applications.
      In general, ancillary chunks can depend on critical chunks but not
      on other ancillary chunks.  It is expected that mutually dependent
      information should be put into a single chunk.

      In some situations it may be unavoidable to make one ancillary
      chunk dependent on another.  Although the chunk property bits are
      insufficient to represent this case, a simple solution is
      available: in the dependent chunk, record the CRC of the chunk
      depended on.  It can then be determined whether that chunk has
      been changed by some other program.

      The same technique can be useful for other purposes.  For example,
      if a program relies on the palette being in a particular order, it
      can store a private chunk containing the CRC of the PLTE chunk.
      If this value matches when the file is again read in, then it
      provides high confidence that the palette has not been tampered
      with.  Note that it is not necessary to mark the private chunk
      unsafe-to-copy when this technique is used; thus, such a private
      chunk can survive other editing of the file.








Boutell, et. al.             Informational                     [Page 79]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   12.14. Palette histograms

      A viewer may not be able to provide as many colors as are listed
      in the image's palette.  (For example, some colors could be
      reserved by a window system.)  To produce the best results in this
      situation, it is helpful to have information about the frequency
      with which each palette index actually appears, in order to choose
      the best palette for dithering or to drop the least-used colors.
      Since images are often created once and viewed many times, it
      makes sense to calculate this information in the encoder, although
      it is not mandatory for the encoder to provide it.

      Other image formats have usually addressed this problem by
      specifying that the palette entries should appear in order of
      frequency of use.  That is an inferior solution, because it
      doesn't give the viewer nearly as much information: the viewer
      can't determine how much damage will be done by dropping the last
      few colors.  Nor does a sorted palette give enough information to
      choose a target palette for dithering, in the case that the viewer
      needs to reduce the number of colors substantially.  A palette
      histogram provides the information needed to choose such a target
      palette without making a pass over the image data.





























Boutell, et. al.             Informational                     [Page 80]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


13. Appendix: Gamma Tutorial

   (This appendix is not part of the formal PNG specification.)

   It would be convenient for graphics programmers if all of the
   components of an imaging system were linear.  The voltage coming from
   an electronic camera would be directly proportional to the intensity
   (power) of light in the scene, the light emitted by a CRT would be
   directly proportional to its input voltage, and so on.  However,
   real-world devices do not behave in this way.  All CRT displays,
   almost all photographic film, and many electronic cameras have
   nonlinear signal-to-light-intensity or intensity-to-signal
   characteristics.

   Fortunately, all of these nonlinear devices have a transfer function
   that is approximated fairly well by a single type of mathematical
   function: a power function.  This power function has the general
   equation

      output = input ^ gamma

   where ^ denotes exponentiation, and "gamma" (often printed using the
   Greek letter gamma, thus the name) is simply the exponent of the
   power function.

   By convention, "input" and "output" are both scaled to the range
   0..1, with 0 representing black and 1 representing maximum white (or
   red, etc).  Normalized in this way, the power function is completely
   described by a single number, the exponent "gamma".

   So, given a particular device, we can measure its output as a
   function of its input, fit a power function to this measured transfer
   function, extract the exponent, and call it gamma.  We often say
   "this device has a gamma of 2.5" as a shorthand for "this device has
   a power-law response with an exponent of 2.5".  We can also talk
   about the gamma of a mathematical transform, or of a lookup table in
   a frame buffer, so long as the input and output of the thing are
   related by the power-law expression above.

   How do gammas combine?

      Real imaging systems will have several components, and more than
      one of these can be nonlinear.  If all of the components have
      transfer characteristics that are power functions, then the
      transfer function of the entire system is also a power function.
      The exponent (gamma) of the whole system's transfer function is
      just the product of all of the individual exponents (gammas) of
      the separate stages in the system.



Boutell, et. al.             Informational                     [Page 81]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Also, stages that are linear pose no problem, since a power
      function with an exponent of 1.0 is really a linear function.  So
      a linear transfer function is just a special case of a power
      function, with a gamma of 1.0.

      Thus, as long as our imaging system contains only stages with
      linear and power-law transfer functions, we can meaningfully talk
      about the gamma of the entire system.  This is indeed the case
      with most real imaging systems.

   What should overall gamma be?

      If the overall gamma of an imaging system is 1.0, its output is
      linearly proportional to its input.  This means that the ratio
      between the intensities of any two areas in the reproduced image
      will be the same as it was in the original scene.  It might seem
      that this should always be the goal of an imaging system: to
      accurately reproduce the tones of the original scene.  Alas, that
      is not the case.

      When the reproduced image is to be viewed in "bright surround"
      conditions, where other white objects nearby in the room have
      about the same brightness as white in the image, then an overall
      gamma of 1.0 does indeed give real-looking reproduction of a
      natural scene.  Photographic prints viewed under room light and
      computer displays in bright room light are typical "bright
      surround" viewing conditions.

      However, sometimes images are intended to be viewed in "dark
      surround" conditions, where the room is substantially black except
      for the image.  This is typical of the way movies and slides
      (transparencies) are viewed by projection.  Under these
      circumstances, an accurate reproduction of the original scene
      results in an image that human viewers judge as "flat" and lacking
      in contrast.  It turns out that the projected image needs to have
      a gamma of about 1.5 relative to the original scene for viewers to
      judge it "natural".  Thus, slide film is designed to have a gamma
      of about 1.5, not 1.0.

      There is also an intermediate condition called "dim surround",
      where the rest of the room is still visible to the viewer, but is
      noticeably darker than the reproduced image itself.  This is
      typical of television viewing, at least in the evening, as well as
      subdued-light computer work areas.  In dim surround conditions,
      the reproduced image needs to have a gamma of about 1.25 relative
      to the original scene in order to look natural.





Boutell, et. al.             Informational                     [Page 82]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The requirement for boosted contrast (gamma) in dark surround
      conditions is due to the way the human visual system works, and
      applies equally well to computer monitors.  Thus, a PNG viewer
      trying to achieve the maximum realism for the images it displays
      really needs to know what the room lighting conditions are, and
      adjust the gamma of the displayed image accordingly.

      If asking the user about room lighting conditions is inappropriate
      or too difficult, just assume that the overall gamma
      (viewing_gamma as defined below) should be 1.0 or 1.25.  That's
      all that most systems that implement gamma correction do.

   What is a CRT's gamma?

      All CRT displays have a power-law transfer characteristic with a
      gamma of about 2.5.  This is due to the physical processes
      involved in controlling the electron beam in the electron gun, and
      has nothing to do with the phosphor.

      An exception to this rule is fancy "calibrated" CRTs that have
      internal electronics to alter their transfer function.  If you
      have one of these, you probably should believe what the
      manufacturer tells you its gamma is.  But in all other cases,
      assuming 2.5 is likely to be pretty accurate.

      There are various images around that purport to measure gamma,
      usually by comparing the intensity of an area containing
      alternating white and black with a series of areas of continuous
      gray of different intensity.  These are usually not reliable.
      Test images that use a "checkerboard" pattern of black and white
      are the worst, because a single white pixel will be reproduced
      considerably darker than a large area of white.  An image that
      uses alternating black and white horizontal lines (such as the
      "gamma.png" test image at
      ftp://ftp.uu.net/graphics/png/images/suite/gamma.png) is much
      better, but even it may be inaccurate at high "picture" settings
      on some CRTs.

      If you have a good photometer, you can measure the actual light
      output of a CRT as a function of input voltage and fit a power
      function to the measurements.  However, note that this procedure
      is very sensitive to the CRT's black level adjustment, somewhat
      sensitive to its picture adjustment, and also affected by ambient
      light.  Furthermore, CRTs spread some light from bright areas of
      an image into nearby darker areas; a single bright spot against a
      black background may be seen to have a "halo".  Your measuring
      technique will need to minimize the effects of this.




Boutell, et. al.             Informational                     [Page 83]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Because of the difficulty of measuring gamma, using either test
      images or measuring equipment, you're usually better off just
      assuming gamma is 2.5 rather than trying to measure it.

   What is gamma correction?

      A CRT has a gamma of 2.5, and we can't change that.  To get an
      overall gamma of 1.0 (or somewhere near that) for an imaging
      system, we need to have at least one other component of the "image
      pipeline" that is nonlinear.  If, in fact, there is only one
      nonlinear stage in addition to the CRT, then it's traditional to
      say that the CRT has a certain gamma, and that the other nonlinear
      stage provides "gamma correction" to compensate for the CRT.
      However, exactly where the "correction" is done depends on
      circumstance.

      In all broadcast video systems, gamma correction is done in the
      camera.  This choice was made in the days when television
      electronics were all analog, and a good gamma-correction circuit
      was expensive to build.  The original NTSC video standard required
      cameras to have a transfer function with a gamma of 1/2.2, or
      about 0.45.  Recently, a more complex two-part transfer function
      has been adopted [SMPTE-170M], but its behavior can be well
      approximated by a power function with a gamma of 0.5.  When the
      resulting image is displayed on a CRT with a gamma of 2.5, the
      image on screen ends up with a gamma of about 1.25 relative to the
      original scene, which is appropriate for "dim surround" viewing.

      These days, video signals are often digitized and stored in
      computer frame buffers.  This works fine, but remember that gamma
      correction is "built into" the video signal, and so the digitized
      video has a gamma of about 0.5 relative to the original scene.

      Computer rendering programs often produce linear samples.  To
      display these correctly, intensity on the CRT needs to be directly
      proportional to the sample values in the frame buffer.  This can
      be done with a special hardware lookup table between the frame
      buffer and the CRT hardware.  The lookup table (often called LUT)
      is loaded with a mapping that implements a power function with a
      gamma of 0.4, thus providing "gamma correction" for the CRT gamma.

      Thus, gamma correction sometimes happens before the frame buffer,
      sometimes after.  As long as images created in a particular
      environment are always displayed in that environment, everything
      is fine.  But when people try to exchange images, differences in
      gamma correction conventions often result in images that seem far
      too bright and washed out, or far too dark and contrasty.




Boutell, et. al.             Informational                     [Page 84]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Gamma-encoded samples are good

      So, is it better to do gamma correction before or after the frame
      buffer?

      In an ideal world, sample values would be stored in floating
      point, there would be lots of precision, and it wouldn't really
      matter much.  But in reality, we're always trying to store images
      in as few bits as we can.

      If we decide to use samples that are linearly proportional to
      intensity, and do the gamma correction in the frame buffer LUT, it
      turns out that we need to use at least 12 bits for each of red,
      green, and blue to have enough precision in intensity.  With any
      less than that, we will sometimes see "contour bands" or "Mach
      bands" in the darker areas of the image, where two adjacent sample
      values are still far enough apart in intensity for the difference
      to be visible.

      However, through an interesting coincidence, the human eye's
      subjective perception of brightness is related to the physical
      stimulation of light intensity in a manner that is very much like
      the power function used for gamma correction.  If we apply gamma
      correction to measured (or calculated) light intensity before
      quantizing to an integer for storage in a frame buffer, we can get
      away with using many fewer bits to store the image.  In fact, 8
      bits per color is almost always sufficient to avoid contouring
      artifacts.  This is because, since gamma correction is so closely
      related to human perception, we are assigning our 256 available
      sample codes to intensity values in a manner that approximates how
      visible those intensity changes are to the eye.  Compared to a
      linear-sample image, we allocate fewer sample values to brighter
      parts of the tonal range and more sample values to the darker
      portions of the tonal range.

      Thus, for the same apparent image quality, images using gamma-
      encoded sample values need only about two-thirds as many bits of
      storage as images using linear samples.













Boutell, et. al.             Informational                     [Page 85]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   General gamma handling

      When more than two nonlinear transfer functions are involved in
      the image pipeline, the term "gamma correction" becomes too vague.
      If we consider a pipeline that involves capturing (or calculating)
      an image, storing it in an image file, reading the file, and
      displaying the image on some sort of display screen, there are at
      least 5 places in the pipeline that could have nonlinear transfer
      functions.  Let's give each a specific name for their
      characteristic gamma:

      camera_gamma
         the characteristic of the image sensor

      encoding_gamma
         the gamma of any transformation performed by the software
         writing the image file

      decoding_gamma
         the gamma of any transformation performed by the software
         reading the image file

      LUT_gamma
         the gamma of the frame buffer LUT, if present

      CRT_gamma
         the gamma of the CRT, generally 2.5

      In addition, let's add a few other names:

      file_gamma
         the gamma of the image in the file, relative to the original
         scene.  This is

            file_gamma = camera_gamma * encoding_gamma

      display_gamma
         the gamma of the "display system" downstream of the frame
         buffer.  This is

            display_gamma = LUT_gamma * CRT_gamma

      viewing_gamma
         the overall gamma that we want to obtain to produce pleasing
         images --- generally 1.0 to 1.5.






Boutell, et. al.             Informational                     [Page 86]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The file_gamma value, as defined above, is what goes in the gAMA
      chunk in a PNG file.  If file_gamma is not 1.0, we know that gamma
      correction has been done on the sample values in the file, and we
      could call them "gamma corrected" samples.  However, since there
      can be so many different values of gamma in the image display
      chain, and some of them are not known at the time the image is
      written, the samples are not really being "corrected" for a
      specific display condition.  We are really using a power function
      in the process of encoding an intensity range into a small integer
      field, and so it is more correct to say "gamma encoded" samples
      instead of "gamma corrected" samples.

      When displaying an image file, the image decoding program is
      responsible for making the overall gamma of the system equal to
      the desired viewing_gamma, by selecting the decoding_gamma
      appropriately.  When displaying a PNG file, the gAMA chunk
      provides the file_gamma value.  The display_gamma may be known for
      this machine, or it might be obtained from the system software, or
      the user might have to be asked what it is.  The correct
      viewing_gamma depends on lighting conditions, and that will
      generally have to come from the user.

      Ultimately, you should have

         file_gamma * decoding_gamma * display_gamma = viewing_gamma

   Some specific examples

      In digital video systems, camera_gamma is about 0.5 by declaration
      of the various video standards documents.  CRT_gamma is 2.5 as
      usual, while encoding_gamma, decoding_gamma, and LUT_gamma are all
      1.0.  As a result, viewing_gamma ends up being about 1.25.

      On frame buffers that have hardware gamma correction tables, and
      that are calibrated to display linear samples correctly,
      display_gamma is 1.0.

      Many workstations and X terminals and PC displays lack gamma
      correction lookup tables.  Here, LUT_gamma is always 1.0, so
      display_gamma is 2.5.











Boutell, et. al.             Informational                     [Page 87]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      On the Macintosh, there is a LUT.  By default, it is loaded with a
      table whose gamma is about 0.72, giving a display_gamma (LUT and
      CRT combined) of about 1.8.  Some Macs have a "Gamma" control
      panel that allows gamma to be changed to 1.0, 1.2, 1.4, 1.8, or
      2.2.  These settings load alternate LUTs that are designed to give
      a display_gamma that is equal to the label on the selected button.
      Thus, the "Gamma" control panel setting can be used directly as
      display_gamma in decoder calculations.

      On recent SGI systems, there is a hardware gamma-correction table
      whose contents are controlled by the (privileged) "gamma" program.
      The gamma of the table is actually the reciprocal of the number
      that "gamma" prints, and it does not include the CRT gamma. To
      obtain the display_gamma, you need to find the SGI system gamma
      (either by looking in a file, or asking the user) and then
      calculating

         display_gamma = 2.5 / SGI_system_gamma

      You will find SGI systems with the system gamma set to 1.0 and 2.2
      (or higher), but the default when machines are shipped is 1.7.

   A note about video gamma

      The original NTSC video standards specified a simple power-law
      camera transfer function with a gamma of 1/2.2 or 0.45.  This is
      not possible to implement exactly in analog hardware because the
      function has infinite slope at x=0, so all cameras deviated to
      some degree from this ideal.  More recently, a new camera transfer
      function that is physically realizable has been accepted as a
      standard [SMPTE-170M].  It is

         Vout = 4.5 * Vin                    if Vin < 0.018
         Vout = 1.099 * (Vin^0.45) - 0.099   if Vin >= 0.018

      where Vin and Vout are measured on a scale of 0 to 1.  Although
      the exponent remains 0.45, the multiplication and subtraction
      change the shape of the transfer function, so it is no longer a
      pure power function.  If you want to perform extremely precise
      calculations on video signals, you should use the expression above
      (or its inverse, as required).

      However, PNG does not provide a way to specify that an image uses
      this exact transfer function; the gAMA chunk always assumes a pure
      power-law function. If we plot the two-part transfer function
      above along with the family of pure power functions, we find that
      a power function with a gamma of about 0.5 to 0.52 (not 0.45) most
      closely approximates the transfer function.  Thus, when writing a



Boutell, et. al.             Informational                     [Page 88]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      PNG file with data obtained from digitizing the output of a modern
      video camera, the gAMA chunk should contain 0.5 or 0.52, not 0.45.
      The remaining difference between the true transfer function and
      the power function is insignificant for almost all purposes.  (In
      fact, the alignment errors in most cameras are likely to be larger
      than the difference between these functions.)  The designers of
      PNG deemed the simplicity and flexibility of a power-law
      definition of gAMA to be more important than being able to
      describe the SMPTE-170M transfer curve exactly.

      The PAL and SECAM video standards specify a power-law camera
      transfer function with a gamma of 1/2.8 or 0.36 --- not the 1/2.2
      of NTSC.  However, this is too low in practice, so real cameras
      are likely to have their gamma set close to NTSC practice.  Just
      guessing 0.45 or 0.5 is likely to give you viewable results, but
      if you want precise values you'll probably have to measure the
      particular camera.

   Further reading

      If you have access to the World Wide Web, read Charles Poynton's
      excellent "Gamma FAQ" [GAMMA-FAQ] for more information about
      gamma.

14. Appendix: Color Tutorial

   (This appendix is not part of the formal PNG specification.)

   About chromaticity

      The cHRM chunk is used, together with the gAMA chunk, to convey
      precise color information so that a PNG image can be displayed or
      printed with better color fidelity than is possible without this
      information.  The preceding chapters state how this information is
      encoded in a PNG image.  This tutorial briefly outlines the
      underlying color theory for those who might not be familiar with
      it.

      Note that displaying an image with incorrect gamma will produce
      much larger color errors than failing to use the chromaticity
      data.  First be sure the monitor set-up and gamma correction are
      right, then worry about chromaticity.

   The problem

      The color of an object depends not only on the precise spectrum of
      light emitted or reflected from it, but also on the observer ---
      their species, what else they can see at the same time, even what



Boutell, et. al.             Informational                     [Page 89]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      they have recently looked at!  Furthermore, two very different
      spectra can produce exactly the same color sensation.  Color is
      not an objective property of real-world objects; it is a
      subjective, biological sensation.  However, by making some
      simplifying assumptions (such as: we are talking about human
      vision) it is possible to produce a mathematical model of color
      and thereby obtain good color accuracy.

   Device-dependent color

      Display the same RGB data on three different monitors, side by
      side, and you will get a noticeably different color balance on
      each display.  This is because each monitor emits a slightly
      different shade and intensity of red, green, and blue light.  RGB
      is an example of a device-dependent color model --- the color you
      get depends on the device.  This also means that a particular
      color --- represented as say RGB 87, 146, 116 on one monitor ---
      might have to be specified as RGB 98, 123, 104 on another to
      produce the same color.

   Device-independent color

      A full physical description of a color would require specifying
      the exact spectral power distribution of the light source.
      Fortunately, the human eye and brain are not so sensitive as to
      require exact reproduction of a spectrum.  Mathematical, device-
      independent color models exist that describe fairly well how a
      particular color will be seen by humans.  The most important
      device-independent color model, to which all others can be
      related, was developed by the International Lighting Committee
      (CIE, in French) and is called XYZ.

      In XYZ, X is the sum of a weighted power distribution over the
      whole visible spectrum.  So are Y and Z, each with different
      weights.  Thus any arbitrary spectral power distribution is
      condensed down to just three floating point numbers.  The weights
      were derived from color matching experiments done on human
      subjects in the 1920s.  CIE XYZ has been an International Standard
      since 1931, and it has a number of useful properties:

          * two colors with the same XYZ values will look the same to
            humans
          * two colors with different XYZ values will not look the same
          * the Y value represents all the brightness information
            (luminance)
          * the XYZ color of any object can be objectively measured





Boutell, et. al.             Informational                     [Page 90]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Color models based on XYZ have been used for many years by people
      who need accurate control of color --- lighting engineers for film
      and TV, paint and dyestuffs manufacturers, and so on.  They are
      thus proven in industrial use.  Accurate, device-independent color
      started to spread from high-end, specialized areas into the
      mainstream during the late 1980s and early 1990s, and PNG takes
      notice of that trend.

   Calibrated, device-dependent color

      Traditionally, image file formats have used uncalibrated, device-
      dependent color.  If the precise details of the original display
      device are known, it becomes possible to convert the device-
      dependent colors of a particular image to device-independent ones.
      Making simplifying assumptions, such as working with CRTs (which
      are much easier than printers), all we need to know are the XYZ
      values of each primary color and the CRT_gamma.

      So why does PNG not store images in XYZ instead of RGB?  Well, two
      reasons.  First, storing images in XYZ would require more bits of
      precision, which would make the files bigger.  Second, all
      programs would have to convert the image data before viewing it.
      Whether calibrated or not, all variants of RGB are close enough
      that undemanding viewers can get by with simply displaying the
      data without color correction.  By storing calibrated RGB, PNG
      retains compatibility with existing programs that expect RGB data,
      yet provides enough information for conversion to XYZ in
      applications that need precise colors.  Thus, we get the best of
      both worlds.

   What are chromaticity and luminance?

      Chromaticity is an objective measurement of the color of an
      object, leaving aside the brightness information.  Chromaticity
      uses two parameters x and y, which are readily calculated from
      XYZ:

         x = X / (X + Y + Z)
         y = Y / (X + Y + Z)

      XYZ colors having the same chromaticity values will appear to have
      the same hue but can vary in absolute brightness.  Notice that x,y
      are dimensionless ratios, so they have the same values no matter
      what units we've used for X,Y,Z.







Boutell, et. al.             Informational                     [Page 91]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The Y value of an XYZ color is directly proportional to its
      absolute brightness and is called the luminance of the color.  We
      can describe a color either by XYZ coordinates or by chromaticity
      x,y plus luminance Y.  The XYZ form has the advantage that it is
      linearly related to (linear, gamma=1.0) RGB color spaces.

   How are computer monitor colors described?

      The "white point" of a monitor is the chromaticity x,y of the
      monitor's nominal white, that is, the color produced when
      R=G=B=maximum.

      It's customary to specify monitor colors by giving the
      chromaticities of the individual phosphors R, G, and B, plus the
      white point.  The white point allows one to infer the relative
      brightnesses of the three phosphors, which isn't determined by
      their chromaticities alone.

      Note that the absolute brightness of the monitor is not specified.
      For computer graphics work, we generally don't care very much
      about absolute brightness levels.  Instead of dealing with
      absolute XYZ values (in which X,Y,Z are expressed in physical
      units of radiated power, such as candelas per square meter), it is
      convenient to work in "relative XYZ" units, where the monitor's
      nominal white is taken to have a luminance (Y) of 1.0.  Given this
      assumption, it's simple to compute XYZ coordinates for the
      monitor's white, red, green, and blue from their chromaticity
      values.

      Why does cHRM use x,y rather than XYZ?  Simply because that is how
      manufacturers print the information in their spec sheets!
      Usually, the first thing a program will do is convert the cHRM
      chromaticities into relative XYZ space.

   What can I do with it?

      If a PNG file has the gAMA and cHRM chunks, the source_RGB values
      can be converted to XYZ.  This lets you:

          * do accurate grayscale conversion (just use the Y component)
          * convert to RGB for your own monitor (to see the original
            colors)
          * print the image in Level 2 PostScript with better color
            fidelity than a simple RGB to CMYK conversion could provide
          * calculate an optimal color palette
          * pass the image data to a color management system
          * etc.




Boutell, et. al.             Informational                     [Page 92]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   How do I convert from source_RGB to XYZ?

      Make a few simplifying assumptions first, like the monitor really
      is jet black with no input and the guns don't interfere with one
      another.  Then, given that you know the CIE XYZ values for each of
      red, green, and blue for a particular monitor, you put them into a
      matrix m:

                 Xr Xg Xb
            m =  Yr Yg Yb
                 Zr Zg Zb

      Here we assume we are working with linear RGB floating point data
      in the range 0..1.  If the gamma is not 1.0, make it so on the
      floating point data.  Then convert source_RGB to XYZ by matrix
      multiplication:

            X     R
            Y = m G
            Z     B

      In other words, X = Xr*R + Xg*G + Xb*B, and similarly for Y and Z.
      You can go the other way too:

            R      X
            G = im Y
            B      Z

      where im is the inverse of the matrix m.

   What is a gamut?

      The gamut of a device is the subset of visible colors which that
      device can display.  (It has nothing to do with gamma.)  The gamut
      of an RGB device can be visualized as a polyhedron in XYZ space;
      the vertices correspond to the device's black, blue, red, green,
      magenta, cyan, yellow and white.

      Different devices have different gamuts, in other words one device
      will be able to display certain colors (usually highly saturated
      ones) that another device cannot.  The gamut of a particular RGB
      device can be determined from its R, G, and B chromaticities and
      white point (the same values given in the cHRM chunk).  The gamut
      of a color printer is more complex and can only be determined by
      measurement.  However, printer gamuts are typically smaller than
      monitor gamuts, meaning that there can be many colors in a
      displayable image that cannot physically be printed.




Boutell, et. al.             Informational                     [Page 93]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      Converting image data from one device to another generally results
      in gamut mismatches --- colors that cannot be represented exactly
      on the destination device.  The process of making the colors fit,
      which can range from a simple clip to elaborate nonlinear scaling
      transformations, is termed gamut mapping.  The aim is to produce a
      reasonable visual representation of the original image.

   Further reading

      References [COLOR-1] through [COLOR-5] provide more detail about
      color theory.

15. Appendix: Sample CRC Code

   The following sample code represents a practical implementation of
   the CRC (Cyclic Redundancy Check) employed in PNG chunks.  (See also
   ISO 3309 [ISO-3309] or ITU-T V.42 [ITU-V42] for a formal
   specification.)

   The sample code is in the ANSI C programming language.  Non C users
   may find it easier to read with these hints:

   &
      Bitwise AND operator.

   ^
      Bitwise exclusive-OR operator.  (Caution: elsewhere in this
      document, ^ represents exponentiation.)

   >>
      Bitwise right shift operator.  When applied to an unsigned
      quantity, as here, right shift inserts zeroes at the left.

   !
      Logical NOT operator.

   ++
      "n++" increments the variable n.

   0xNNN
      0x introduces a hexadecimal (base 16) constant.  Suffix L
      indicates a long value (at least 32 bits).

      /* Table of CRCs of all 8-bit messages. */
      unsigned long crc_table[256];

      /* Flag: has the table been computed? Initially false. */
      int crc_table_computed = 0;



Boutell, et. al.             Informational                     [Page 94]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      /* Make the table for a fast CRC. */
      void make_crc_table(void)
      {
        unsigned long c;
        int n, k;
        for (n = 0; n < 256; n++) {
          c = (unsigned long) n;
          for (k = 0; k < 8; k++) {
            if (c & 1)
              c = 0xedb88320L ^ (c >> 1);
            else
              c = c >> 1;
          }
          crc_table[n] = c;
        }
        crc_table_computed = 1;
      }

      /* Update a running CRC with the bytes buf[0..len-1]--the CRC
         should be initialized to all 1's, and the transmitted value
         is the 1's complement of the final running CRC (see the
         crc() routine below)). */

      unsigned long update_crc(unsigned long crc, unsigned char *buf,
                               int len)
      {
        unsigned long c = crc;
        int n;

        if (!crc_table_computed)
          make_crc_table();
        for (n = 0; n < len; n++) {
          c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
        }
        return c;
      }

      /* Return the CRC of the bytes buf[0..len-1]. */
      unsigned long crc(unsigned char *buf, int len)
      {
        return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
      }









Boutell, et. al.             Informational                     [Page 95]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


16. Appendix: Online Resources

   (This appendix is not part of the formal PNG specification.)

   This appendix gives the locations of some Internet resources for PNG
   software developers.  By the nature of the Internet, the list is
   incomplete and subject to change.

   Archive sites

      The latest released versions of this document and related
      information can always be found at the PNG FTP archive site,
      ftp://ftp.uu.net/graphics/png/.  The PNG specification is
      available in several formats, including HTML, plain text, and
      PostScript.

   Reference implementation and test images

      A reference implementation in portable C is available from the PNG
      FTP archive site, ftp://ftp.uu.net/graphics/png/src/.  The
      reference implementation is freely usable in all applications,
      including commercial applications.

      Test images are available from
      ftp://ftp.uu.net/graphics/png/images/.

   Electronic mail

      The maintainers of the PNG specification can be contacted by e-
      mail at png-info@uunet.uu.net or at png-group@w3.org.

   PNG home page

      There is a World Wide Web home page for PNG at
      http://quest.jpl.nasa.gov/PNG/.  This page is a central location
      for current information about PNG and PNG-related tools.

17. Appendix: Revision History

   (This appendix is not part of the formal PNG specification.)

   The PNG format has been frozen since the Ninth Draft of March 7,
   1995, and all future changes are intended to be backwards compatible.
   The revisions since the Ninth Draft are simply clarifications,
   improvements in presentation, and additions of supporting material.

   On 1 October 1996, the PNG specification was approved as a W3C (World
   Wide Web Consortium) Recommendation.



Boutell, et. al.             Informational                     [Page 96]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   Changes since the Tenth Draft of 5 May, 1995

          * Clarified meaning of a suggested-palette PLTE chunk in a
            truecolor image that uses transparency
          * Clarified exact semantics of sBIT and allowed sample depth
            scaling procedures
          * Clarified status of spaces in tEXt chunk keywords
          * Distinguished private and public extension values in type
            and method fields
          * Added a "Creation Time" tEXt keyword
          * Macintosh representation of PNG specified
          * Added discussion of security issues
          * Added more extensive discussion of gamma and chromaticity
            handling, including tutorial appendixes
          * Clarified terminology, notably sample depth vs. bit depth
          * Added a glossary
          * Editing and reformatting

18. References

   [COLOR-1]
      Hall, Roy, Illumination and Color in Computer Generated Imagery.
      Springer-Verlag, New York, 1989.  ISBN 0-387-96774-5.

   [COLOR-2]
      Kasson, J., and W. Plouffe, "An Analysis of Selected Computer
      Interchange Color Spaces", ACM Transactions on Graphics, vol 11 no
      4 (1992), pp 373-405.

   [COLOR-3]
      Lilley, C., F. Lin, W.T. Hewitt, and T.L.J. Howard, Colour in
      Computer Graphics. CVCP, Sheffield, 1993.  ISBN 1-85889-022-5.
      Also available from
      <URL:http://info.mcc.ac.uk/CGU/ITTI/Col/colour_announce.html>

   [COLOR-4]
      Stone, M.C., W.B. Cowan, and J.C. Beatty, "Color gamut mapping and
      the printing of digital images", ACM Transactions on Graphics, vol
      7 no 3 (1988), pp 249-292.

   [COLOR-5]
      Travis, David, Effective Color Displays --- Theory and Practice.
      Academic Press, London, 1991.  ISBN 0-12-697690-2.

   [GAMMA-FAQ]
      Poynton, C., "Gamma FAQ".
      <URL:http://www.inforamp.net/%7Epoynton/Poynton-colour.html>




Boutell, et. al.             Informational                     [Page 97]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   [ISO-3309]
      International Organization for Standardization, "Information
      Processing Systems --- Data Communication High-Level Data Link
      Control Procedure --- Frame Structure", IS 3309, October 1984, 3rd
      Edition.

   [ISO-8859]
      International Organization for Standardization, "Information
      Processing --- 8-bit Single-Byte Coded Graphic Character Sets ---
      Part 1: Latin Alphabet No. 1", IS 8859-1, 1987.
      Also see sample files at
      ftp://ftp.uu.net/graphics/png/documents/iso_8859-1.*

   [ITU-BT709]
      International Telecommunications Union, "Basic Parameter Values
      for the HDTV Standard for the Studio and for International
      Programme Exchange", ITU-R Recommendation BT.709 (formerly CCIR
      Rec. 709), 1990.

   [ITU-V42]
      International Telecommunications Union, "Error-correcting
      Procedures for DCEs Using Asynchronous-to-Synchronous Conversion",
      ITU-T Recommendation V.42, 1994, Rev. 1.

   [PAETH]
      Paeth, A.W., "Image File Compression Made Easy", in Graphics Gems
      II, James Arvo, editor.  Academic Press, San Diego, 1991.  ISBN
      0-12-064480-0.

   [POSTSCRIPT]
      Adobe Systems Incorporated, PostScript Language Reference Manual,
      2nd edition. Addison-Wesley, Reading, 1990.  ISBN 0-201-18127-4.

   [PNG-EXTENSIONS]
      PNG Group, "PNG Special-Purpose Public Chunks".  Available in
      several formats from
      ftp://ftp.uu.net/graphics/png/documents/pngextensions.*

   [RFC-1123]
      Braden, R., Editor, "Requirements for Internet Hosts ---
      Application and Support", STD 3, RFC 1123, USC/Information
      Sciences Institute, October 1989.
      <URL:ftp://ds.internic.net/rfc/rfc1123.txt>








Boutell, et. al.             Informational                     [Page 98]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   [RFC-2045]
      Freed, N., and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, Innosoft, First Virtual, November 1996.
      <URL:ftp://ds.internic.net/rfc/rfc2045.txt>

   [RFC-2048]
      Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail
      Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
      Innosoft, MCI, USC/Information Sciences Institute, November 1996.
      <URL:ftp://ds.internic.net/rfc/rfc2048.txt>

   [RFC-1950]
      Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
      Specification version 3.3", RFC 1950, Aladdin Enterprises, May
      1996.
      <URL:ftp://ds.internic.net/rfc/rfc1950.txt>

   [RFC-1951]
      Deutsch, P., "DEFLATE Compressed Data Format Specification version
      1.3", RFC 1951, Aladdin Enterprises, May 1996.
      <URL:ftp://ds.internic.net/rfc/rfc1951.txt>

   [SMPTE-170M]
      Society of Motion Picture and Television Engineers, "Television
      --- Composite Analog Video Signal --- NTSC for Studio
      Applications", SMPTE-170M, 1994.
























Boutell, et. al.             Informational                     [Page 99]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


19. Credits

   Editor

      Thomas Boutell, boutell@boutell.com

   Contributing Editor

      Tom Lane, tgl@sss.pgh.pa.us

   Authors

      Authors' names are presented in alphabetical order.

          * Mark Adler, madler@alumni.caltech.edu
          * Thomas Boutell, boutell@boutell.com
          * Christian Brunschen, cb@df.lth.se
          * Adam M. Costello, amc@cs.berkeley.edu
          * Lee Daniel Crocker, lee@piclab.com
          * Andreas Dilger, adilger@enel.ucalgary.ca
          * Oliver Fromme, fromme@rz.tu-clausthal.de
          * Jean-loup Gailly, gzip@prep.ai.mit.edu
          * Chris Herborth, chrish@qnx.com
          * Alex Jakulin, Aleks.Jakulin@snet.fri.uni-lj.si
          * Neal Kettler, kettler@cs.colostate.edu
          * Tom Lane, tgl@sss.pgh.pa.us
          * Alexander Lehmann, alex@hal.rhein-main.de
          * Chris Lilley, chris@w3.org
          * Dave Martindale, davem@cs.ubc.ca
          * Owen Mortensen, 104707.650@compuserve.com
          * Keith S. Pickens, ksp@swri.edu
          * Robert P. Poole, lionboy@primenet.com
          * Glenn Randers-Pehrson, glennrp@arl.mil or
            randeg@alumni.rpi.edu
          * Greg Roelofs, newt@pobox.com
          * Willem van Schaik, willem@gintic.gov.sg
          * Guy Schalnat
          * Paul Schmidt, pschmidt@photodex.com
          * Tim Wegner, 71320.675@compuserve.com
          * Jeremy Wohl, jeremyw@anders.com

      The authors wish to acknowledge the contributions of the Portable
      Network Graphics mailing list, the readers of comp.graphics, and
      the members of the World Wide Web Consortium (W3C).







Boutell, et. al.             Informational                    [Page 100]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


      The Adam7 interlacing scheme is not patented and it is not the
      intention of the originator of that scheme to patent it. The
      scheme may be freely used by all PNG implementations. The name
      "Adam7" may be freely used to describe interlace method 1 of the
      PNG specification.

   Trademarks

      GIF is a service mark of CompuServe Incorporated.  IBM PC is a
      trademark of International Business Machines Corporation.
      Macintosh is a trademark of Apple Computer, Inc.  Microsoft and
      MS-DOS are trademarks of Microsoft Corporation.  PhotoCD is a
      trademark of Eastman Kodak Company.  PostScript and TIFF are
      trademarks of Adobe Systems Incorporated.  SGI is a trademark of
      Silicon Graphics, Inc.  X Window System is a trademark of the
      Massachusetts Institute of Technology.

COPYRIGHT NOTICE

   Copyright (c) 1996 by: Massachusetts Institute of Technology (MIT)

   This W3C specification is being provided by the copyright holders
   under the following license. By obtaining, using and/or copying this
   specification, you agree that you have read, understood, and will
   comply with the following terms and conditions:

   Permission to use, copy, and distribute this specification for any
   purpose and without fee or royalty is hereby granted, provided that
   the full text of this NOTICE appears on ALL copies of the
   specification or portions thereof, including modifications, that you
   make.

   THIS SPECIFICATION IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE NO
   REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED.  BY WAY OF
   EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO
   REPRESENTATIONS OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY
   PARTICULAR PURPOSE OR THAT THE USE OF THE SPECIFICATION WILL NOT
   INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER
   RIGHTS.  COPYRIGHT HOLDERS WILL BEAR NO LIABILITY FOR ANY USE OF THIS
   SPECIFICATION.











Boutell, et. al.             Informational                    [Page 101]
^L
RFC 2083            PNG: Portable Network Graphics            March 1997


   The name and trademarks of copyright holders may NOT be used in
   advertising or publicity pertaining to the specification without
   specific, written prior permission.  Title to copyright in this
   specification and any associated documentation will at all times
   remain with copyright holders.

Security Considerations

   Security issues are discussed in Security considerations (Section
   8.5).

Author's Address

   Thomas Boutell
   PO Box 20837
   Seattle, WA  98102

   Phone: (206) 329-4969
   EMail: boutell@boutell.com
































Boutell, et. al.             Informational                    [Page 102]
^L