Interactive Sequences

           GIF Sequence Exchanges for an Interactive Environment

 The following sequences are defined for use  in  mediating  control
 between a GIF sender and GIF receiver over an interactive communications
 line.  These  sequences  do  not  apply  to  applications  that  involve
 downloading  of  static  GIF  files and are not considered part of a GIF
 file.

 GIF CAPABILITIES ENQUIRY
   The GCE sequence is issued from a host and requests an  interactive GIF
   decoder  to  return  a  response  message that defines the graphics
   parameters for the decoder.  This involves returning  information  about
   available screen sizes, number of bits/colour supported and the amount of
   colour detail supported.  The escape sequence for the GCE is defined as:

      ESC [ > 0 g     (g is lower case, spaces inserted for clarity)
                       (0x1B 0x5B 0x3E 0x30 0x67)


 GIF CAPABILITIES RESPONSE
   The GIF Capabilities Response message is returned by an interactive GIF
   decoder  and  defines  the  decoder's  display capabilities for all
   graphics modes that are supported by the software.  Note that  this  can
   also include graphics printers as well as a monitor screen.  The general
   format of this message is:


     #version;protocol{;dev, width, height, colour-bits, colour-res}... <CR>

     '#'          - GCR identifier character (Number Sign)
     version      - GIF format version number;  initially '87a'
     protocol='0' - No end-to-end protocol supported by decoder
                    Transfer as direct 8-bit data stream.
     protocol='1' - Can use an error correction protocol to transfer GIF data
                    interactively from the host directly to the display.

     dev = '0'    - Screen parameter set follows
     dev = '1'    - Printer parameter set follows

     width        - Maximum supported display width in pixels
     height       - Maximum supported display height in pixels
     colour-bits  - Number of  bits  per  pixel  supported.   The  number  of
                    supported colours is therefore 2**colour-bits.
     colour-res   - Number of bits  per  colour  component  supported  in  the
                    hardware  colour  palette.   If  colour-res  is  '0'
                    then  no hardware palette table is available.

   Note that all values in the  GCR  are  returned  as  ASCII  decimal
   numbers and the message is terminated by a Carriage Return character.

   The  following   GCR   message   describes   three   standard   EGA
   configurations  with  no  printer;  the GIF data stream can be processed
   within an error correcting protocol:

      #87a;1 ;0,320,200,4,0 ;0,640,200,2,2 ;0,640,350,4,2<CR>


 ENTER GIF GRAPHICS MODE
   Two sequences are currently defined to invoke  an  interactive  GIF
   decoder into action.  The only difference between them is that different
   output media are selected.  These sequences are:

     ESC [ > 1 g   Display GIF image on screen
                   (0x1B 0x5B 0x3E 0x31 0x67)

     ESC [ > 2 g   Display image directly to an attached graphics  printer.
                   The  image  may optionally be displayed on the screen as
                   well.
                   (0x1B 0x5B 0x3E 0x32 0x67)

   Note that the 'g' character terminating each sequence is  in  lower
   case.


 INTERACTIVE ENVIRONMENT
   The assumed environment for the transmission of GIF image data from an
   interactive  application  is  a  full 8-bit data stream from host to
   micro.  All 256 character codes must be transferrable.  The establishing
   of  an 8-bit data path for communications will normally be taken care of
   by the host application programs.  It is however  up  to  the  receiving
   communications programs supporting GIF to be able to receive and pass on
   all 256 8-bit codes to the GIF decoder software.