NAME

       pnmcolormap - create quantization color map for a portable anymap

SYNOPSIS

       pnmcolormap  [-center|-meancolor|-meanpixel] [-spreadbrightness|-spreadluminosity] [-sort]
       [-square] ncolors|all [pnmfile]

       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 an equals sign
       between an option name and its value.

DESCRIPTION

       Reads a PNM image as input.  Chooses ncolors colors to best represent the image, maps  the
       existing colors to the new ones, and writes a PNM color map defining them as output.

       You  can  use this map as input to pnmremap on the same input image to quantize the colors
       in that image, I.e.  produce a similar image with fewer colors.  pnmquant  does  both  the
       pnmcolormap and pnmremap steps for you.

       A  PNM  colormap is a PNM image of any dimensions that contains at least one pixel of each
       color in the set of colors it represents.

       The quantization method is Heckbert's "median cut".  See the section QUANTIZATION METHOD.

       If the input image is a PPM, the output image is a PPM.  If the input image is  a  PBM  or
       PGM,  the  output  colormap  is  a  PGM.   Note that a colormap of a PBM image is not very
       interesting.

       The colormap generally has the same maxval as the input image, but pnmcolormap may  reduce
       it if there are too many colors in the input, as part of its quantization algorithm.

       If  you  want  to create a colormap without basing it on the colors in an input image, see
       ppmcolors.

PARAMETERS

       The single parameter, which is required, is the number of colors you want  in  the  output
       colormap.   pnmcolormap may produce a color map with slightly fewer colors than that.  You
       may specify all to get a colormap of every color in the input image (no quantization).

       OPTIONS

       -sort  This option causes the output colormap to be sorted by the red component intensity,
              then the green, then the blue in ascending order.  This is an insertion sort, so it
              is not very fast on large colormaps.  Sorting is useful because it  allows  you  to
              compare two sets of colors.

       -square
              By  default, pnmcolormap produces as the color map a PPM image with one row and one
              column for each color in the colormap.  This option causes pnmcolormap  instead  to
              produce a PPM image that is within one row or column of being square, with multiple
              pixels of the same color as necessary to create a  number  of  pixels  which  is  a
              perfect square.

       -verbose
              This  option  causes  pnmcolormap  to  display messages to Standard Error about the
              quantization.

       -center

       -meancolor

       -meanpixel

       -spreadbrightness

       -spreadluminosity
              These options control the quantization algorithm.  See QUANTIZATION METHOD below.

QUANTIZATION METHOD

       A quantization method is a way to choose which colors, being fewer in number than  in  the
       input,  you  want  in  the  output.  pnmcolormap uses Heckbert's "median cut" quantization
       method.

       This method involves separating all the colors into  "boxes,"  each  holding  colors  that
       represent  about the same number of pixels.  You start with one box and split boxes in two
       until the number of boxes is the same as the number of colors you want in the output,  and
       choose one color to represent each box.

       When  you  split a box, you do it so that all the colors in one sub-box are "greater" than
       all the colors in the other.  "Greater," for a particular box, means it is brighter in the
       color  component (red, green, blue) which has the largest spread in that box.  pnmcolormap
       gives you two ways to define "largest spread.":   1)  largest  spread  of  brightness;  2)
       largest  spread of contribution to the luminosity of the color.  E.g. red is weighted much
       more than blue.  Select among  these  with  the  -spreadbrightness  and  -spreadluminosity
       options.  The default is -spreadbrightness.

       pnmcut  provides  three ways of choosing a color to represent a box: 1) the center color -
       the color halfway between the greatest and least  colors  in  the  box,  using  the  above
       definition  of "greater"; 2) the mean of the colors (each component averaged separately by
       brightness) in the box; 3) the mean weighted by the number of pixels of  a  color  in  the
       image.

       Note  that  in all three methods, there may be colors in the output which do not appear in
       the input at all.

       Select among these with the -center, -meancolor, and -meanpixel options.  The  default  is
       -center.

REFERENCES

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

SEE ALSO

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

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.  Copyright (C) 2001 by Bryan Henderson.

                                         12 December 2001                          pnmcolormap(1)