INV_CMAP

Section: C Library Functions (3)
Updated: Month DD, YYYY
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NAME

inv_cmap - efficiently compute an inverse colormap

SYNOPSIS

: void inv_cmap( colors, colormap, bits, dist_buf, rgbmap )

int colors, bits;
unsigned char *colormap[3], *rgbmap;
unsigned long *dist_buf;

DESCRIPTION

Inv_cmap computes an inverse colormap to translate an RGB color to the nearest color in the given colormap. The arguments are

colors: The number of colors in the input colormap. Must be le 256.
colormap: The input colormap. The ith color is (Colormap[0][i], Colormap[1][i], Colormap[2][i]).
bits: Controls the size and precision of the inverse colormap. The resulting colormap will be a cube 2^bits on a side, and will therefore contain 2^(3*bits) entries. RGB colors must be quantized to bits bits before using the inverse colormap.
dist_buf: Temporary storage used by inv_cmap. It should contain at least 2^(3*bits) elements.
rgbmap: The inverse colormap. Should be allocated with at least 2^(3*bits) elements. After calling inv_cmap, an RGB color (r,g,b) can be mapped to its closest representative in colormap by evaluating
#define quantize(p) ((p)>>(8-bits))
rgbmap[ (((quantize(r) << bits) | quantize(g)) << bits) | quantize(b) ]

Predicted performance is O(2^(3*bits)*log(colors)). The measured performance is sublinear (but not as good as log) in the number of input colors and also in the size of the output inverse colormap. (I.e., it goes up more slowly than 2^(3*bits).)

AUTHOR

Spencer W. Thomas