pnmcolormap - create quantization color map for a portable anymap


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

   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


   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

   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.


   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).


   -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.

          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.

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





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


   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

   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.


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


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


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

                           12 December 2001                 pnmcolormap(1)


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