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pnmcolormap(1)							pnmcolormap(1)

NAME
       pnmcolormap - create quantization color map for a portable anymap

SYNOPSIS
       pnmcolormap [-center|-meancolor|-meanpixel] [-spreadbrightness|-spread‐
       luminosity] [-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 col‐
       ormap of a PBM image is not very interesting.

       The colormap generally has the same maxval as the input image, but pnm‐
       colormap	 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 pix‐
	      els 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 QUANTIZA‐
	      TION 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 hold‐
       ing 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  par‐
       ticular	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)
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