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)