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ppmquant(1)                            General Commands Manual                           ppmquant(1)



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 in‐
       stead 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.  pp‐‐
       mquant may be faster, though.

       Reads a PPM image as input.  Chooses ncolors colors to best represent the image, maps the ex‐
       isting 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  mapfile.   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 unmodified quantization has banding or other arti‐
       facts, 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 Proceed‐
       ings, page 297.


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


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



                                           12 January 1991                               ppmquant(1)