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] -map mapfile [ppmfile]

DESCRIPTION

       Reads a portable pixmap as input.  Chooses ncolors colors to best  rep-
       resent  the image, maps the existing colors to the new ones, and writes
       a portable pixmap 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 -map flag.  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 pixmap 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.

       The -floyd/-fs flag 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.

       All flags can be abbreviated to their shortest unique prefix.

REFERENCES

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

SEE ALSO

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

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                12 January 1991                    ppmquant(1)