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 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 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 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
       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 ), pnmcolormap 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 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

       -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 output.  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 colors 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 generates 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 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.

       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 pnmcolormap 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), pamcat(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
       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)