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

       pnmcolormap - create quantization color map for a Netpbm image








       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 repre-
       sent 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 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 different 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  com-
       puting  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, pam-
       seq, ppmmake, and pnmcat can be useful.

       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).

       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.

	      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.

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




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

       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

       This method involves separating all the colors into 'boxes,' each holding colors that rep-
       resent 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.

       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  sepa-
       rately  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

       'Color Image Quantization for Frame Buffer Display' by Paul Heckbert,  SIGGRAPH	'82  Pro-
       ceedings, page 297.

       pnmremap(1) , pnmquant(1) , ppmquantall(1) , pamdepth(1) , ppmdither(1) , pamseq(1) , ppm-
       make(1) , pnmcat(1) , ppm(1)

       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 pnmremap.

       Copyright (C) 1989, 1991 by Jef Poskanzer.

netpbm documentation			 23 October 2005	       Pnmcolormap User Manual(0)
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