This program is part of Netpbm.
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
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
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 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.
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 oth-
erwise 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 col-
ormap of every color in the input image (no quantization).
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
-sort This option causes the output colormap to be sorted by the red
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.
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 bright-
ness) 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.
'Color Image Quantization for Frame Buffer Display' by Paul Heckbert,
SIGGRAPH '82 Proceedings, page 297.
pnmremap, pnmquant, ppmquantall, pamdepth, ppmdither,
pamseq, ppmmake, pnmcat, ppm
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 pnmcol-
ormap and pnmremap.
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 23 October 2005 Pnmcolormap User Manual(0)