INV_CMAP(3)INV_CMAP(3)NAMEinv_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 256.
colormap
The input colormap. The ith color is (Colormap[0][i], Col‐
ormap[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 quan‐
tized 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) | quan‐
tize(b) ]
Predicted performance is O(2^(3*bits)*log(colors)). The measured per‐
formance 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).)
SEE ALSOcolorquant(3).
AUTHOR
Spencer W. Thomas
4th Berkeley Distribution Month DD, YYYY INV_CMAP(3)
