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

SYNOPSIS
    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
    value.

DESCRIPTION
    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
    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 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
    resolution.

    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.

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

SEE ALSO
   pnmquant(1),ppmquantall(1),pnmdepth(1),ppmdither(1), ppm(5)

AUTHOR
    Copyright (C) 1989, 1991 by Jef Poskanzer.