pnmcolormap(1)						      General Commands Manual						    pnmcolormap(1)

NAME

       pnmcolormap - create quantization color map for a Netpbm image

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)