ppmquant(1)             UNIX System V(12 January 1991)              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
      represent 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
      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.

      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.



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