ppmquant – quantize the colors in a portable pixmap down to a specified
ppmquant [-floyd|-fs] ncolors [ppmfile] ppmquant [-floyd|-fs] [-nofloyd|-nofs] -mapfile mapfile [ppmfile]
All options can be abbreviated to their shortest unique prefix. You
may use two hyphens instead of one to designate an option. You may use
either white space or equals signs between an option name and its
pnmquant is a newer, more general program that is backward compatible
with ppmquant. ppmquant may be faster, though.
Reads a PPM image as input. Chooses ncolors colors to best represent
the image, maps the existing colors to the new ones, and writes a PPM
image 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 -mapfile option. 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
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 image 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.
If you use a mapfile, the output image has the same maxval as the map‐
file. Otherwise, the output maxval is the same as the input maxval, or
less in some cases where the quantization process reduces the necessary
The -floyd/-fs option enables a Floyd-Steinberg error diffusion step.
Floyd-Steinberg gives vastly better results on images where the unmodi‐
fied 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.
-nofloyd/-nofs means not to use the Floyd-Steinberg error diffusion.
This is the default.
“Color Image Quantization for Frame Buffer Display” by Paul Heckbert,
SIGGRAPH ’82 Proceedings, page 297.
Copyright (C) 1989, 1991 by Jef Poskanzer.
12 January 1991 ppmquant