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


       pnmcolormap - create quantization color map for a Netpbm image









       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


       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 .

       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-

       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
       otherwise 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.


       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).


       -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  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 QUANTIZA-
              TION 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

       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

       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.


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


       pnmremap(1),  pnmquant(1),  ppmquantall(1),  pnmdepth(1), ppmdither(1),
       pamseq(1), ppmmake(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 pnm-


       Copyright (C) 1989, 1991 by Jef Poskanzer.

netpbm documentation            23 October 2005     Pnmcolormap User Manual(0)

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