

 
Manual Reference Pages  INV_CMAP (3)
NAME
inv_cmap  efficiently compute an inverse colormap
CONTENTS
Synopsis
Description
See Also
Author
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)>>(8bits))
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
1  INV_CMAP (3)  Month DD, YYYY 
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