pnmcolormap
create quantization color map for a portable anymap
see also :
pnmremap - pnmquant - ppmquantall - pnmdepth - ppmdither - ppmquant
Synopsis
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.
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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
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 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.
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 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 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" 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 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
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.
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 also
pnmremap ,
pnmquant , ppmquant all"> ppmquantall ,
pnmdepth , ppmdither , ppmquant,
ppm
author
Copyright (C)
1989, 1991 by Jef Poskanzer. Copyright (C) 2001 by Bryan
Henderson.