pnmcolormap(1)pnmcolormap(1)NAMEpnmcolormap - create quantization color map for a Netpbm
image
SYNOPSISpnmcolormap [-center|-meancolor|-meanpixel] [-spread
brightness|-spreadluminosity] [-sort] [-square] ncol_
ors|all [pnmfile]
All options can be abbreviated to their shortest unique
prefix. You may use two hyphens instead of one to desig
nate an option. You may use either white space or an
equals sign between an option name and its value.
DESCRIPTION
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 out
put.
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 con
tains at least one pixel of each color in the set of col
ors 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
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 algo
rithm.
If you want to create a colormap without basing it on the
colors in an input image, see ppmcolors.
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).
OPTIONS
-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 one column for each
color in the colormap. This option causes pnmcol
ormap 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 perfect square.
-verbose
This option causes pnmcolormap to display messages
to Standard Error about the quantization.
-center
-meancolor
-meanpixel
-spreadbrightness
-spreadluminosity
These options control the quantization algorithm.
See QUANTIZATION METHOD below.
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" quanti
zation 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 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 -spread
brightness.
pnmcut provides three ways of choosing a color to repre
sent 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 col
ors (each component averaged separately by brightness) 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.
REFERENCES
"Color Image Quantization for Frame Buffer Display" by
Paul Heckbert, SIGGRAPH '82 Proceedings, page 297.
SEE ALSOpnmremap(1), pnmquant(1), ppmquantall(1), pnmdepth(1),
ppmdither(1), ppmquant(1), ppm(5)AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer. Copyright (C)
2001 by Bryan Henderson.
12 December 2001 pnmcolormap(1)
