1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
|
Network Working Group M. Speer
Request for Comment: 2029 D. Hoffman
Category: Standards Track Sun Microsystems, Inc.
October 1996
RTP Payload Format of Sun's CellB Video Encoding
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo describes a packetization scheme for the CellB video
encoding. The scheme proposed allows applications to transport CellB
video flows over protocols used by RTP. This document is meant for
implementors of video applications that want to use RTP and CellB.
1. Introduction
The Cell image compression algorithm is a variable bit-rate video
coding scheme. It provides "high" quality, low bit-rate image
compression at low computational cost. The bytestream that is
produced by the Cell encoder consists of instructional codes and
information about the compressed image.
For futher information on Cell compression technology, refer to [1].
Currently, there are two versions of the Cell compression technology:
CellA and CellB. CellA is primarily designed for the encoding of
stored video intended for local display, and will not be discussed in
this memo.
CellB, derived from CellA, has been optimized for network-based video
applications. It is computationally symmetric in both encode and
decode. CellB utilizes a fixed colormap and vector quantization
techniques in the YUV color space to achieve compression.
Speer & Hoffman Standards Track [Page 1]
^L
RFC 2029 RTP Payload Format October 1996
2. Network Packetization and Encapsulation
2.1 RTP Usage
The RTP timestamp is in units of 90KHz. The same timestamp value is
used for all packet payloads of a frame. The RTP maker bit denotes
the end of a frame.
2.2 CellB Header
The packetization of the CellB bytestream is designed to make the
resulting packet stream robust to packet loss. To achieve this goal,
an additional header is added to each RTP packet to uniquely identify
the location of the first cell of the packet within the current
frame. In addition, the width and height of the frame in pixels is
carried in each CellB packet header. Although the size can only
change between frames, it is carried in every packet to simplify the
packet encoding.
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cell X Location | Cell Y Location |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Width of Image | Height of Image |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Compressed CellB Data |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
All fields are 16-bit unsigned integers in network byte order, and
are placed at the beginning of the payload for each RTP packet. The
Cell X and the Cell Y Location coordinates are expressed as cell
coordinates, not pixel coordinates. Since cells represent 4x4 blocks
of pixels, the X or Y dimension of the cell coordinates range in
value from 0 through 1/4 of the of the same dimension in pixel
coordinates.
2.3 Packetization Rules
A packet can be of any size chosen by the implementor, up to a full
frame. All multi-byte codes must be completely contained within a
packet. In general, the implementor should avoid packet sizes that
result in fragmentation by the network.
Speer & Hoffman Standards Track [Page 2]
^L
RFC 2029 RTP Payload Format October 1996
3. References
1. "Cell Image Compression Byte Stream Description,"
ftp://playground.sun.com:/pub/multimedia/video/
cellbytestream.ps.Z
2. Turletti, T., and C. Huitema, "RTP Payload Format
for H.261 Video Streams", RFC 2032, October 1996.
3. Schulzrinne, H., Casner, S., Frederick, R., and
V. Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
4. Schulzrinne, H., "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 1890,
January 1996.
4 Authors' Addresses
Michael F. Speer
Sun Microsystems Computer Corporation
2550 Garcia Ave MailStop UMPK14-305
Mountain View, CA 94043
Voice: +1 415 786 6368
Fax: +1 415 786 6445
EMail: michael.speer@eng.sun.com
Don Hoffman
Sun Microsystems Computer Corporation
2550 Garcia Ave MailStop UMPK14-305
Mountain View, CA 94043
Voice: +1 415 786 6370
Fax: +1 415 786 6445
EMail: don.hoffman@eng.sun.com
Speer & Hoffman Standards Track [Page 3]
^L
RFC 2029 RTP Payload Format October 1996
Appendix A - Structure of the CellB Video Stream
The CellB bytestream consists of cell codes, skip codes and
quantization-table specific codes. These are now described.
A.1 CellB Cell Code
Cell codes are 4 bytes in length, and describe a 4x4 pixel cell.
There are two possible luminance (Y) levels for each cell, but only
one pair of chrominance (UV) values. The CellB cell code is shown
below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 M M M M M M M M M M M M M M M|U V U V U V U V|Y Y Y Y Y Y Y Y|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4x4 Bitmask U/V Code Y/Y Code
The first two bytes of the cell code are a bitmask. Each bit in the
mask represents a pixel in a 16-pixel cell. Bit 0 represents the
value of the upper right-hand pixel of the cell, and subsequent bits
represent the pixels in row-major order. If a pixel's bit is set in
the 4x4 Bitmask, then the pixel will be rendered with the pixel value
<Y(1), U, V>. If the pixel's bit is not set in the bitmask, then the
pixel's value will be rendered with the value <Y(0), U, V>. The
bitmask for the cell is normalized so that the most significant bit
is always 0 (i.e., corresponding to <Y(0), U, V>).
The U/V field of the cell code represents the chrominance component.
This code is in index into a table of vectors that represents two
independent components of chrominance.
The Y/Y field of the cell code represents two luminance values (Y(0)
and Y(1)). This code is an index into a table of two-compoment
luminance vectors.
The derivation of the U/V and Y/Y tables is outside the scope of this
memo. A complete discussion can be found in [1].
Speer & Hoffman Standards Track [Page 4]
^L
RFC 2029 RTP Payload Format October 1996
A.2 CellB Skip Code
The single byte CellB skip code tells the CellB decoder to skip the
next S+1 cells in the current video frame being decoded. The maximum
number of cells that can be skipped is 32. The format of the skip
code is shown below.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 0 S S S S S|
+-+-+-+-+-+-+-+-+
A.3 CellB Y/Y Table Code
The single byte "new Y/Y table" code is used to tell the decoder that
the next 512 bytes are a new Y/Y quantization table. The code and
the representation of the table are shown below. The sample
encoder/decoder pair in this document do not implement this feature
of the CellB compression. However, future CellB codecs may implement
this feature.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 0|
+-+-+-+-+-+-+-+-+
The format of the new Y/Y table is:
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Y1_000 | Y2_000 | Y1_001 | Y2_001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Y1_254 | Y2_254 | Y1_255 | Y2_255 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Speer & Hoffman Standards Track [Page 5]
^L
RFC 2029 RTP Payload Format October 1996
A.4 CellB U/V Table Code
The single byte "new U/V table" code is used to tell the decoder that
the next 512 bytes represent a new U/V quantization table. The code
is shown below. The sample encoder/decoder pair provided in this
document do not implement this feature of the CellB compression.
However, future CellB codecs may implement this feature.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+
The bytes of the new U/V quantization table are arranged as:
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| U_000 | V_000 | U_001 | V_001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| U_254 | V_254 | U_255 | V_255 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Appendix B - Availability of CellB
It is the viewpoint of Sun Microsystems, Inc, that CellB is
publically available for use without any license.
Speer & Hoffman Standards Track [Page 6]
^L
|
