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


       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.


       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:
           8 1
             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.


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


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


       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                         12 January 1991                              ppmquant(1)