INV_CMAP(3)

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 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).)

SEE ALSO
colorquant(3).

AUTHOR
Spencer W. Thomas