pnmcolormap



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.

       -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,


                               12 December 2001                 pnmcolormap(1)
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