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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)