Pnmcolormap User Manual(1) General Commands Manual Pnmcolormap User Manual(1)
NAME
pnmcolormap - create quantization color map for a Netpbm image
SYNOPSIS
pnmcolormap
[-center|-meancolor|-meanpixel]
[-spreadbrightness|-spreadluminosity]
[-splitpixelct|-splitcolorct|-splitspread]
[-sort]
[-square]
ncolors|all
[pnmfile]
DESCRIPTION
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, as-
sumes 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
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 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
METHOD ⟨#quant⟩ .
The output image is of the same format (PBM, PGM, PPM, PAM) as the in-
put image. Note that a colormap of a PBM image is not very interest-
ing.
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 in-
put images. All the images must have the same format, depth, and max-
val (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.
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). When you specify
all, the function is essentially the same as that of ppmhist -map.
ppmhist is much older.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pnmcolormap recognizes
the following command line options:
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.
-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 with 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.
-verbose
This option causes pnmcolormap to display messages to Standard
Error about the quantization..TP -center
-meancolor
-meanpixel
-spreadbrightness
-spreadluminosity
-splitpixelct
-splitcolorct
-splitspread
These options control the quantization algorithm. See
QUANTIZATION METHOD ⟨#quant⟩ .
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 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.
There are three ways pnmcolormap can choose the box to split in
each step:
• Split the box containing the most pixels. This is the default,
and you can select it explicitly with option -splitpix-
elct.
• Split the box containing the most colors. This appears to be
useful
for academic purposes only. Select this with option
-splitcolorct.
• Split the box containing the largest color spread. Select this
with option -splitspread. This can produce a better re-
sult for
small details with colors not found elsewhere in the im-
age.
-splitpixelct, splitcolorct, and splitspread were new
in Netpbm 10.88 (September 2019). Before that, pnmcolormap always
splits the box containing the most pixels.
When you split a box, you do it so each sub-box has the same number of
pixels (except one sub-box has more if the full box has an odd number),
with the 'greatest' pixels in one sub-box and the 'least' pixels 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 op-
tions. The default is -spreadbrightness. Where there are multiple
colors of the median magnitude, they are distributed arbitrarily among
between the subboxes. This arbitrary distribution depends upon what
the system's qsort function does with multiple equal values, so pnmcol-
ormap may produce slightly different results on different systems.
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 im-
age.
Select among these with the -center, -meancolor, and -meanpixel op-
tions. The default is -center.
Note that in all three methods, there may be colors in the output which
do not appear in the input at all.
Also note that the color chosen to represent the colors in Box A the
best may also represent a color in Box B better than the color chosen
to represent the colors in Box B the best. This is true for various
measures of goodness of representation of one color by another. In
particular, if you use pnmremap to map the colors in the very image
that you used to create the color map to the colors in that colormap,
the colors in Box B will often map to the color pnmcolormap chose to
represent some other box and in fact the color pnmcolormap chose to
represent Box B may not appear in the pnmremap output at all.
REFERENCES
"Color Image Quantization for Frame Buffer Display" by Paul Heckbert,
SIGGRAPH '82 Proceedings, page 297.
SEE ALSO
pnmremap(1), pnmquant(1), ppmquantall(1), pamgetcolor(1), pamdepth(1),
ppmdither(1), pamseq(1), ppmmake(1), pnmcat(1), ppm(1)
HISTORY
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 re-
placed pnmquant with a program that simply calls pnmcolormap and pnm-
remap.
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML
source. The master documentation is at
http://netpbm.sourceforge.net/doc/pnmcolormap.html
netpbm documentation 09 February 2019 Pnmcolormap User Manual(1)
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