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
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 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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