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INV_CMAP(3)           DragonFly Library Functions Manual           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

4th Berkeley Distribution       Month DD, YYYY                     INV_CMAP(3)

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