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                         NETWORK WORKING GROUP                        REQUEST FOR COMMENTS 181                               NIC #7124                             JUNE 27, 1971JOHN Mc CONNELL                          AMES RESEARCH CENTER                        MOFFET FIELD, CALIFORNIA                       MODIFICATIONS TO RFC #177CATEGORY  D.6RFC's Obsoleted  noneRFC's Updated    177                                                                [Page 1]

                      MODIFICATIONS TO RFC #177    This RFC is intended to modifiy the proposal for a deviceindependent graphical display description discussed in RFC #177.  Thanksare due to Stan Mantiply of Ames for his ideas.  The main changes are inthe defini- tion of coordinate areas to avoid one problem encounteredwith the old definition and to provide more flexibility.  Terms used arethose defined in RFC #177.    The problem encountered in the previous proposal was that the areasof the NGS (network standard graphics screen), which were supposed to beabsolute boundaries, could be crossed when a list was executed withinthat space.  Scaling of lists was also awkwardly done with specificcommands in the network standard graphic display list (NGDL).    Coordinates will still be defined with 16 bit unsigned normalizedfractions.  This will leave the resolution of the picture up to the de-vice which eventually receives the NGDL.  The values will not now betied to the NGS or an area of the screen, but will be fractionaldisplace- ments from the lower left corner of the screen, an area, or animage space.    Areas will still be rectangular and will be allowed to overlap, butnot contain another area completely.  The attributes of an area are: its16 bit integer name, by which it may be manipulated, two (x,y) pairswhich will define its lower left and upper right corners with respect tothe NGS.  No list executed in an area may cross its boundaries.  Withinan area may be any number of image spaces which are dynamically definedwith 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 imagearea of that list as displacements from the lower left corner of thecontaining area.  If the list executes other lists within it, theirimage areas are defined as displacements from the image area of the listcontaining the execute command.  Thus each list can be scaleddifferently each time it is executed and may even be distorted if theimage area defined has different ratios of X and Y lengths.  Listsexecuted within other lists will have the same relationship to theircontaining lists regardless of that lists's rela- tionship to the NGSarea.  Since the initial image area is defined rela- tive to the NGSDarea, no overlaps can occur.                                                                [Page 2]

If a table (item type = 9) is executed, the coordinate pairs itcontains 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 commandis 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 screencoor- dinates.    Another construct for graphics processing will be added.  This isthe mode stack for each graphics mode.  It will be used for saving andrestoring 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 parameterschanged.  It shall have two parameters, a 16 bit integer name, by whichit may be manipulated and referenced, and a 16 bit integer defining thelength of the list in bytes rather than items.  This change is intendedto make processing the NGDS easier and will eliminate the TABLE item(type = 11).    The first three list items should have the WORD IMAGE eliminatedfrom them, since they are defining NGS areas now.    The list command DEFINE IMAGE ARE (type = 2) shall be changed toDEFINE 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 ]                                                                [Page 3]