Pnmcolormap User Manual(0)			    Pnmcolormap User Manual(0)



NAME
       pnmcolormap - create quantization color map for a Netpbm image


SYNOPSIS
       pnmcolormap

       [-center|-meancolor|-meanpixel]

       [-spreadbrightness|-spreadluminosity]

       [-sort]

       [-square]

       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 pnm-
       colormap'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 necessary with the -square option.

       The  quantization  method is Heckbert's 'median cut'.  See QUANTIZATION
       METHOD <#quant> .

       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 interest-
       ing.

       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.

       pnmcolormap works on a multi-image input stream.	 In that case, it pro-
       duces  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 different height and width).  This is useful if
       you need to quantize a bunch of images that will form a movie or other-
       wise  be	 used together -- you generally want them all to draw from the
       same palette, whereas computing a colormap separately from  each	 image
       would  make  the	 same  color  in  two  images map to different colors.
       Before Netpbm 10.31 (December  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 pnmcat 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).


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 column 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


       -meancolor


       -meanpixel


       -spreadbrightness


       -spreadluminosity
	      These  options  control	the   quantization   algorithm.	   See
	      QUANTIZATION METHOD <#quant> .





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.

       pnmcolormap 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),  pamdepth(1), ppmdither(1),
       pamseq(1), ppmmake(1), pnmcat(1), ppm(1)


HISTORY
       Before Netpbm 10.15 (April 2003), pnmcolormap used a  lot  more	memory
       for  large  images  because  it	kept the entire input 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 pnm-
       remap.


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



netpbm documentation		23 October 2005	    Pnmcolormap User Manual(0)