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)