COLORQUANT(3)COLORQUANT(3)NAMEcolorquant - variance-based color quantization
SYNOPSIS
#include <colorquant.h>
int colorquant(red, green, blue, npix, colormap, colors, bits, rgbmap,
flags, accum_hist)
unsigned char *red, *green, *blue;
unsigned long npix;
unsigned char *colormap[3];
int colors, bits;
unsigned char *rgbmap;
int flags;
int accum_hist;
DESCRIPTION
Colorquant performs variance-based color quantization on a given image.
A representative colormap and a table for performing RGB to colormap
index mapping are computed. The number of colors to which the image
was quantized (the total number of colormap entries computed) is
returned. The arguments to colorquant are:
red, green, blue
The red, green and blue channels of the image. The ith pixel is
represented as the RGB triple (red[i], green[i], blue[i]).
These arrays usually contain values that have been 'prequan‐
tized' (see below).
npix The length, in bytes, of the red, green and blue arrays. Equal
to the total number of pixels in the image.
colormap
Points to a pre-allocated, three-channel colormap. These arrays
will be filled with the colormap values computed by the vari‐
ance-based color quantization algorithm. colormap[0][i], col‐
ormap[1][i], and colormap[2][i] are, respectively, the red,
green and blue components of the ith colormap entry.
colors The number of pre-allocated colormap entries. The image will be
quantized to at most this many colors.
bits The number of significant bits in each entry of the red, green
and blue arrays. Normally, the red, green and blue arrays con‐
tain values that have been prequantized to fewer than eight sig‐
nificant bits (see flags below). Five significant bits usually
represents a good tradeoff between image quality and running
time. Anything above six significant bits will likely lead to
excessive paging, as the size of rgbmap and the internal his‐
togram are proportional to (2^bits)^3.
rgbmap A pointer to an array of unsigned chars of size (2^bits)^3.
This array is used to map from pixels to colormap entries. The
prequantized red, green and blue components of a pixel are used
as an index into this array to retrieve the colormap index that
should be used to represent the pixel. The array is indexed as:
colorindex = rgbmap[(((r << bits) | g) << bits) | b];
where r, g, and b are the prequantized red, green and blue com‐
ponents of the pixel in question.
flags A collection of bit-flags that modify the operation of col‐
orquant. Currently defined values are CQ_FAST, CQ_QUANTIZE, and
CQ_NO_RGBMAP.
If CQ_FAST is set, the construction of rgbmap will be relatively
fast. If not, rgbmap will be built slowly but more accurately.
In most cases, the error introduced by the 'fast' approximation
is barely noticeable.
If CQ_QUANTIZE is set, the values in red, green, and blue are
taken as 8-bit values and will be quantized to bits significant
bits by colorquant. If not set, these values are assumed to be
prequantized.
If CQ_NO_RGBMAP is set, rgbmap will not be built.
accum_hist
This argument provides a facility to accumulate multiple images
into a single colormap. If accum_hist is zero, the routine
works normally. To build a colormap for several images,
accum_hist should have the value 1 for the first image, and 2
for subsequent images. Finally, after all the images have been
processed, a value of 3 for accum_hist will compute the colormap
and rgbmap. The values of colors and bits should not change
during this process. The arguments colormap, rgbmap, and fast
are ignored if accum_hist is 1 or 2, and red, green, blue, and
npix are ignored if accum_hist is 3.
AUTHOR
Craig Kolb, Yale University.
Martin Friedmann, MIT Media Lab did the accum_hist changes.
REFERENCE
Wan, Wong, and Prusinkiewicz, An Algorithm for Multidimensional Data
Clustering, Transactions on Mathematical Software, Vol. 14 #2 (June,
1988), pp. 153-162.
SEE ALSOrlequant(1), inv_cmap(3).
4th Berkeley Distribution August 14, 1989 COLORQUANT(3)
