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pnmcolormap(1)                         General Commands Manual                        pnmcolormap(1)



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  in‐
       stead  of  one  to designate an option.  You may use either white space or an equals sign be‐
       tween an option name and its value.


DESCRIPTION
       Reads a PNM image as input.  Chooses ncolors colors to best represent the image, maps the ex‐
       isting 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  pnmcol‐‐
       ormap 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  ppm‐‐
       colors.


PARAMETERS
       The  single parameter, which is required, is the number of colors you want in the output col‐
       ormap.  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 pro‐
              duce a PPM image that is within one row or column of being square, with multiple  pix‐
              els  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 quanti‐
              zation.

       -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 in‐
       put, 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  repre‐
       sent  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.  Se‐
       lect 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  Proceed‐
       ings, 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)