A color map is a 256×3-byte translation table for
color values.
In a monochrome picture, pixel values index
the color map to yield red, green and blue, like this:
uchar cmap[256][3];
red = cmap[pixel][0];
green = cmap[pixel][1];
blue = cmap[pixel][2];
In a full-color picture,
the color map is, in effect,
three intensity-compensation
tables:
cmap[red][0]
maps red channels,
cmap[green][1]
maps green channels
and
cmap[blue][2]
maps blue channels.
A color map file is just a 768-byte file containing the color map,
stored in the order implied by the declaration of
cmap
above.