ppmquant

ppmquant - quantize the colors in a portable pixmap down to a specified number

ppmquant [-floyd|-fs] ncolors [ppmfile]
ppmquant [-floyd|-fs] -map mapfile [ppmfile]

Reads a portable pixmap as input. Chooses ncolors colors to best represent the image, maps the existing colors to the new ones, and writes a portable pixmap as output.

The quantization method is Heckbert’s "median cut".

Alternately, you can skip the color-choosing step by specifying your own set of colors with the -map flag. The mapfile is just a ppm file; it can be any shape, all that matters is the colors in it. For instance, to quantize down to the 8-color IBM TTL color set, you might use:

    P3
    8 1
    255
      0   0   0
    255   0   0
      0 255   0
      0   0 255
    255 255   0
    255   0 255
      0 255 255
    255 255 255

If you want to quantize one pixmap to use the colors in another one, just use the second one as the mapfile. You don’t have to reduce it down to only one pixel of each color, just use it as is.

The -floyd/-fs flag enables a Floyd-Steinberg error diffusion step. Floyd-Steinberg gives vastly better results on images where the unmodified quantization has banding or other artifacts, especially when going to a small number of colors such as the above IBM set. However, it does take substantially more CPU time, so the default is off.

All flags can be abbreviated to their shortest unique prefix.

"Color Image Quantization for Frame Buffer Display" by Paul Heckbert, SIGGRAPH ’82 Proceedings, page 297.

ppmquantall(1), pnmdepth(1), ppmdither(1), ppm(5)

Copyright (C) 1989, 1991 by Jef Poskanzer.