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 Raster Data

 The format of the actual image is defined as the  series  of  pixel colour
 index  values that make up the image.  The pixels are stored left to right
 sequentially for an image row.  By default each  image  row  is written
 sequentially, top to bottom.  In the case that the Interlace or 'I' bit is
 set in byte 10 of the Image Descriptor then the row order  of the  image
 display  follows  a  four-pass process in which the image is filled in by
 widely spaced rows.  The first pass writes every  8th  row, starting  with
 the top row of the image window.  The second pass writes every 8th row
 starting at the fifth row from the top.   The  third  pass writes every
 4th row starting at the third row from the top.  The fourth pass completes
 the image, writing  every  other  row,  starting  at  the second row from
 the top.  A graphic description of this process follows:


   Image
   Row  Pass 1  Pass 2  Pass 3  Pass 4          Result
  -----------------------------------------------------
     0  **1a**                                  **1a**
     1                          **4a**          **4a**
     2                  **3a**                  **3a**
     3                          **4b**          **4b**
     4          **2a**                          **2a**
     5                          **4c**          **4c**
     6                  **3b**                  **3b**
     7                          **4d**          **4d**
     8  **1b**                                  **1b**
     9                          **4e**          **4e**
    10                  **3c**                  **3c**
    11                          **4f**          **4f**
    12          **2b**                          **2b**
   . . .



 The image pixel values are processed as a series of  colour  indices which
 map  into the existing colour map.  The resulting colour value from the map
 is what is actually displayed.  This series  of  pixel  indices, the
 number  of  which  is equal to image-width*image-height pixels, are passed
 to the GIF image data stream one value per pixel, compressed  and packaged
 according  to  a  version  of the LZW compression algorithm as defined in
 a later entry.