Glossary

 Pixel
   The smallest picture element of a  graphics  image.   This  usually
   corresponds  to  a single dot on a graphics screen.  Image resolution is
   typically given in units of  pixels.   For  example  a  fairly  standard
   graphics  screen  format  is  one 320 pixels across and 200 pixels high.
   Each pixel can  appear  as  one  of  several  colours  depending  on  the
   capabilities of the graphics hardware.

 Raster
   A horizontal row of pixels representing one line of an  image.   A
   typical method of working with images since most hardware is oriented to
   work most efficiently in this manner.

 LSB
   Least Significant Byte.  Refers to a convention for two byte  numeric
   values in which the less significant byte of the value preceeds the more
   significant byte.  This convention is typical on many microcomputers.

 Colour Map
   The list of definitions of each colour  used  in  a  GIF  image. These
   desired  colours are converted to available colours through a table which
   is derived by assigning an incoming colour index (from  the  image) to
   an  output  colour  index  (of  the  hardware).   While the colour map
   definitons are specified in a GIF image, the output  pixel  colours  will
   vary  based  on  the  hardware used and its ability to match the defined
   colour.

 Interlace
   The method of displaying a GIF image in which  multiple  passes are
   made,  outputting  raster  lines  spaced  apart to provide a way of
   visualizing the general content of an entire image  before  all  of  the
   data has been processed.

 B Protocol
   A CompuServe-developed error-correcting file transfer protocol available
   in  the  public  domain  and implemented in CompuServe VIDTEX products.
   This error checking mechanism will be used  in  transfers  of GIF images
   for interactive applications.

 LZW
   A sophisticated data compression algorithm  based  on  work  done  by
   Lempel-Ziv  &  Welch  which  has  the feature of very efficient one-pass
   encoding and decoding.  This allows the image  to  be  decompressed  and
   displayed  at  the  same  time.   The  original  article from which this
   technique was adapted is:

          Terry  A.   Welch,  "A  Technique  for  High   Performance   Data
          Compression", IEEE Computer, vol 17 no 6 (June 1984)

   This basic algorithm is also used in the  public  domain  ARC  file
   compression  utilities.   The  CompuServe  adaptation  of LZW for GIF is
   described in Appendix C.