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Manual Reference Pages  -  COLORQUANT (3)


colorquant - variance-based color quantization


See Also


#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;


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 ’prequantized’ (see below).
npix The length, in bytes, of the red, green and blue arrays. Equal to the total number of pixels in the image.
  Points to a pre-allocated, three-channel colormap. These arrays will be filled with the colormap values computed by the variance-based color quantization algorithm. colormap[0][i], colormap[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 contain values that have been prequantized to fewer than eight significant 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 histogram 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 components of the pixel in question.
flags A collection of bit-flags that modify the operation of colorquant. 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.
  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.


Craig Kolb, Yale University.
Martin Friedmann, MIT Media Lab did the accum_hist changes.


Wan, Wong, and Prusinkiewicz, An Algorithm for Multidimensional Data Clustering, Transactions on Mathematical Software, Vol. 14 #2 (June, 1988), pp. 153-162.


rlequant(1), inv_cmap(3).
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--> COLORQUANT (3) August 14, 1989

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