Reference iquantize

    iquantize [-v] [-G] [-d type] [-n max] [-m method] inimage outimage 
    

This program takes a 32 bit raster image and quantizes the color space to use a maximum given number of colors. The program can also choose the “best” colors for the specified image.

-v Verbose output.

-G Force output to be gif format.

-d type Specify dithering method 0 = no dithering 1 = Floyd-Steinberg (default)

-n max Specify the maximum number of colors to use (256).

-m method Specify color map selection algorithm. 0 = quick method 1 = median cut algorithm (default) or specify a map file

If a map file is specified for the method, then only colors from the mapfile will be used. The mapfile is a list of colors ranging from 0 to 255 in R, G, and B. Each color should be specified on a different line. For example:

    0 0 0
    85 85 85
    170 170 170
    255 255 255
    

This mapfile would result in a monochrome image using only four colors.