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+                         NETWORK WORKING GROUP
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+                        REQUEST FOR COMMENTS 181
+
+                               NIC #7124
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+                             JUNE 27, 1971
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+                            JOHN Mc CONNELL
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+                          AMES RESEARCH CENTER
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+                        MOFFET FIELD, CALIFORNIA
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+                       MODIFICATIONS TO RFC #177
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+CATEGORY  D.6
+RFC's Obsoleted  none
+RFC's Updated    177
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+                                                                [Page 1]
+
+                      MODIFICATIONS TO RFC #177
+
+    This RFC is intended to modifiy the proposal for a device
+independent graphical display description discussed in RFC #177.  Thanks
+are due to Stan Mantiply of Ames for his ideas.  The main changes are in
+the defini- tion of coordinate areas to avoid one problem encountered
+with the old definition and to provide more flexibility.  Terms used are
+those defined in RFC #177.
+
+    The problem encountered in the previous proposal was that the areas
+of the NGS (network standard graphics screen), which were supposed to be
+absolute boundaries, could be crossed when a list was executed within
+that space.  Scaling of lists was also awkwardly done with specific
+commands in the network standard graphic display list (NGDL).
+
+    Coordinates will still be defined with 16 bit unsigned normalized
+fractions.  This will leave the resolution of the picture up to the de-
+vice which eventually receives the NGDL.  The values will not now be
+tied to the NGS or an area of the screen, but will be fractional
+displace- ments from the lower left corner of the screen, an area, or an
+image space.
+
+    Areas will still be rectangular and will be allowed to overlap, but
+not contain another area completely.  The attributes of an area are: its
+16 bit integer name, by which it may be manipulated, two (x,y) pairs
+which will define its lower left and upper right corners with respect to
+the NGS.  No list executed in an area may cross its boundaries.  Within
+an area may be any number of image spaces which are dynamically defined
+with the execution of a list and define its relationship to the contain-
+ing area.
+
+    The list item types 7 and 8 shall be changed from EXECUTE LIST
+(Name,x,y) and COPY LIST (Name,x,y) to EXECUTE LIST
+(Name,(Xl,Yl),(Xr,Yr)) and COPY LIST (Name,(Xl,Yl),(Xr,Yr))
+respectively.  The coordinate pairs (Xl,Yl) and (Xr,Yr) define the image
+area of that list as displacements from the lower left corner of the
+containing area.  If the list executes other lists within it, their
+image areas are defined as displacements from the image area of the list
+containing the execute command.  Thus each list can be scaled
+differently each time it is executed and may even be distorted if the
+image area defined has different ratios of X and Y lengths.  Lists
+executed within other lists will have the same relationship to their
+containing lists regardless of that lists's rela- tionship to the NGS
+area.  Since the initial image area is defined rela- tive to the NGSD
+area, no overlaps can occur.
+
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+                                                                [Page 2]
+
+If a table (item type = 9) is executed, the coordinate pairs it
+contains are relative to whatever area is valid at the time of its exe-
+cution.
+
+    A change to the NGLI is implied.  Whenever an EXECUTE LIST command
+is encountered, the current state is saved, a new current origin is com-
+puted, NGLP is reset to zero, and new X and Y scale factors must be cal-
+culated to transform the coordinates in the new list to actual screen
+coor- dinates.
+
+    Another construct for graphics processing will be added.  This is
+the mode stack for each graphics mode.  It will be used for saving and
+restoring character and vector mode values as lists are recursively exe-
+cuted.  Four new list items will be added to handle these operations.
+
+   PUSH CHARACTER MODE    (item type = 14)
+   PUSH VECTOR MODE       (item type = 15)
+   POP CHARACTER MODE     (item type = 16)
+   POP VECTOR MODE        (item type = 17)
+
+    The DEFINE LIST command (type = 1) shall have its parameters
+changed.  It shall have two parameters, a 16 bit integer name, by which
+it may be manipulated and referenced, and a 16 bit integer defining the
+length of the list in bytes rather than items.  This change is intended
+to make processing the NGDS easier and will eliminate the TABLE item
+(type = 11).
+
+    The first three list items should have the WORD IMAGE eliminated
+from them, since they are defining NGS areas now.
+
+    The list command DEFINE IMAGE ARE (type = 2) shall be changed to
+DEFINE AREA for the same reasons.
+
+
+       [ This RFC was put into machine readable form for entry ]
+         [ into the online RFC archives by Gert Doering 6/97 ]
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+                                                                [Page 3]
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