pnmcolormap - create quantization color map for a Netpbm image
pnmcolormap [-center|-meancolor|-meanpixel] [-spreadbrightness|-spreadluminosity] [-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.
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 colormap of a PBM image is not very inter-
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.
If you want to create a colormap without basing it on the colors in an input image, see
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 multiple
pixels of the same color as necessary to create a number of pixels which is a per-
This option causes pnmcolormap to display messages to Standard Error about the
These options control the quantization algorithm. See QUANTIZATION METHOD below.
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.
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 defi-
nition 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
"Color Image Quantization for Frame Buffer Display" by Paul Heckbert, SIGGRAPH '82 Pro-
ceedings, page 297.
pnmremap(1), pnmquant(1), ppmquantall(1), pnmdepth(1), ppmdither(1), ppmquant(1), ppm(5)
Copyright (C) 1989, 1991 by Jef Poskanzer. Copyright (C) 2001 by Bryan Henderson.
12 December 2001 pnmcolormap(1)