Manual: pnmcolormap

Pnmcolormap User Manual(1)			   General Commands Manual			  Pnmcolormap User Manual(1)

NAME
       pnmcolormap - create quantization color map for a Netpbm image

SYNOPSIS
       pnmcolormap

       [-center|-meancolor|-meanpixel]

       [-spreadbrightness|-spreadluminosity]

       [-splitpixelct|-splitcolorct|-splitspread]

       [-sort]

       [-square]

       [-verbose]

       [-debug]

       ncolors|all

       [pnmfile]

DESCRIPTION
       This program is part of Netpbm(1).

       pnmcolormap  reads  a  PNM or PAM image as input, chooses ncolors colors to best represent the image and writes a PNM
       color map defining them as output.  A PAM image may actually contain tuples of any kind, but pnmcolormap's concept of
       the tuple values that best represent the ones present in the image may not make sense if the tuple type isn't RGB  or
       GRAYSCALE.  The design of the program, and the rest of this manual, assumes the tuples represent colors.

       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 ones pnmcolormap generates have exactly one pixel of each color, except where padding is neces‐
       sary with the -square option.

       The quantization method is Heckbert's "median cut".  See QUANTIZATION METHOD .

       The output image is of the same format (PBM, PGM, PPM, PAM) as the input image.	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.

       pnmcolormap works on a multi-image input stream.	 In that case, it produces one colormap that applies to all  of	 the
       colors in all of the input images.  All the images must have the same format, depth, and maxval (but may have differ‐
       ent  height and width).	This is useful if you need to quantize a bunch of images that will form a movie or otherwise
       be used together -- you generally want them all to draw from the same palette, whereas  computing  a  colormap  sepa‐
       rately  from each image would make the same color in two images map to different colors.	 Before Netpbm 10.31 (Decem‐
       ber 2005), pnmcolormap ignored any image after the first.

       If you want to create a colormap without basing it on the colors in an input image, pamseq, ppmmake, and	 pamcat	 can
       be useful.

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).  When you specify all, the function is essentially the same as that of ppmhist
       -map.  ppmhist is much older.

OPTIONS
       In addition to the options common to all programs based on libnetpbm (most notably -quiet, see  Common Options ), pn‐
       mcolormap recognizes the following command line options:

       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.

       -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 with one column for each color
	      in the colormap.	This option causes pnmcolormap instead to produce a PPM image that is within one row or col‐
	      umn of being square, with the last pixel duplicated as necessary to create a number of pixels which is such an
	      almost-perfect square.

       -verbose
	      This option causes pnmcolormap to display messages to Standard Error about the quantization..TP -center

       -debug This option causes pnmcolormap to display messages to Standard Error about internal workings.

       -center

       -meancolor

       -meanpixel

       -spreadbrightness

       -spreadluminosity

       -splitpixelct

       -splitcolorct

       -splitspread
	      These options control the quantization algorithm.	 See QUANTIZATION METHOD .

QUANTIZATION METHOD
       A quantization method is a way to choose which colors, being fewer in number than in the input, you want in the	out‐
       put.  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 col‐
       ors you want in the output, and choose one color to represent each box.

       There are three ways pnmcolormap can choose the box to split in
	 each step:

       •      Split the box containing the most pixels.	 This is the default,
		    and you can select it explicitly with option -splitpixelct.

       •      Split the box containing the most colors.	 This appears to be useful
		    for academic purposes only.	 Select this with option
		    -splitcolorct.

       •      Split the box containing the largest color spread.  Select this
		    with option -splitspread.  This can produce a better result for
		    small details with colors not found elsewhere in the image.

       -splitpixelct, splitcolorct, and splitspread were new
	 in Netpbm 10.88 (September 2019).  Before that, pnmcolormap always
	 splits the box containing the most pixels.

       When  you  split	 a  box, you do it so each sub-box has the same number of pixels (except one sub-box has more if the
       full box has an odd number), with the 'greatest' pixels	in  one	 sub-box  and  the  'least'  pixels  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.	 Where there
       are multiple colors of the median magnitude, they are distributed arbitrarily among between the subboxes.

       This arbitrary distribution is repeatable, though, for a given Netpbm version -- every invocation of pnmcolormap gen‐
       erates  the  same  color	 map.  Before Netpbm 11.03 (June 2023), the distribution would depend upon what the system's
       qsort function does with multiple equal values, so pnmcolormap may produce slightly different  results  on  different
       systems.

       pnmcolormap  provides  three ways of choosing a color to represent a box: 1) the center color - the color halfway be‐
       tween 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.

       Select among these with the -center, -meancolor, and -meanpixel options.	 The default is -center.

       Note that in all three methods, there may be colors in the output which do not appear in the input at all.

       Also note that the color chosen to represent the colors in Box A the best may also represent a color in Box B  better
       than  the  color	 chosen to represent the colors in Box B the best.  This is true for various measures of goodness of
       representation of one color by another.	In particular, if you use pnmremap to map the colors in the very image	that
       you used to create the color map to the colors in that colormap, the colors in Box B will often map to the color pnm‐
       colormap	 chose to represent some other box and in fact the color pnmcolormap chose to represent Box B may not appear
       in the pnmremap output at all.

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

SEE ALSO
       pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1),  pamdepth(1),	ppmdither(1),  pamseq(1),  ppmmake(1),	pam‐
       cat(1), ppm(1)

HISTORY
       Before  Netpbm 10.15 (April 2003), pnmcolormap used a lot more memory for large images because it kept the entire in‐
       put image in memory.  Now, it processes it a row at a time, but	because	 it  sometimes	must  make  multiple  passes
       through the image, it first copies the input into a temporary seekable file if it is not already in a seekable file.

       pnmcolormap  first  appeared  in Netpbm 9.23 (January 2002).  Before that, its function was available only as part of
       the function of pnmquant (which was derived from the much older ppmquant).  Color quantization really  has  two	main
       subfunctions,  so  Netpbm 9.23 split it out into two separate programs: pnmcolormap and pnmremap and then Netpbm 9.24
       replaced pnmquant with a program that simply calls pnmcolormap and pnmremap.

AUTHOR
       Copyright (C) 1989, 1991 by Jef Poskanzer.

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master documentation is at

	      http://netpbm.sourceforge.net/doc/pnmcolormap.html

netpbm documentation				      21 February 2023				  Pnmcolormap User Manual(1)