NAME

       pnmcolormap - create quantization color map for a Netpbm image

SYNOPSIS

       pnmcolormap

       [-center|-meancolor|-meanpixel]

       [-spreadbrightness|-spreadluminosity]

       [-splitpixelct|-splitcolorct|-splitspread]

       [-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  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 ⟨#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
       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).
       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 ⟨index.html#commonoptions⟩ ), 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

       -meancolor

       -meanpixel

       -spreadbrightness

       -spreadluminosity

       -splitpixelct

       -splitcolorct

       -splitspread
              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 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 depends 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), 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 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