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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
       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 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.

                                12 January 1991                    ppmquant(1)