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BUILDMAP(3)           DragonFly Library Functions Manual           BUILDMAP(3)


NAME

       buildmap - create a color map array from an RLE file header.


SYNOPSIS

       #include <rle.h>

       rle_pixel ** buildmap( the_hdr, minmap, orig_gamma, new_gamma )
       rle_hdr * the_hdr;
       int minmap;
       double orig_gamma, new_gamma;


DESCRIPTION

       The color map in the rle_hdr(3) structure is not in the most easily
       used form.  The function buildmap returns a pointer to a colormap array
       with certain minimum dimensions, making it a little easier to implement
       color mapping in a program.  The color map from first argument,
       the_hdr, is used to build the result.  If no map is present in the_hdr,
       then an identity map of the minimum size will be returned.

       The returned color map will have at least minmap rows or channels, each
       of which is at least 256 entries long (so that indexing into the color
       map with an 8 bit rle_pixel value will always succeed.)

       The color map from the_hdr will be composed with a gamma compensation
       curve to account for the gamma of the display for which the input color
       map was presumably computed.  The argument orig_gamma specifies the
       gamma of the compensation curve.  It would typically be the gamma of
       the original display.

       If gamma is 0, then if a picture comment image_gamma=i_gamma is
       present, gamma will be set to 1.0/i_gamma.  Otherwise, if a comment
       display_gamma=d_gamma is present, gamma will be set to d_gamma.  The
       gamma compensation value for pixel i is 255*(i/255)^gamma.

       If this color map will be used directly for another display, the gamma
       of this new display should be passed in new_gamma.

       The returned value is a pointer to an array of pointers to arrays of
       rle_pixel values.  It may be doubly indexed in C code, so that if cmap
       is the return value, the RGB color mapping for a pixel pixval is
       (cmap[0][pixval], cmap[1][pixval], cmap[2][pixval]).


NOTES

       Generally, unless the user explicitly specifies the image or original
       display gamma (e.g., as with the -i or -I flags of getx11(1), you
       should pass 0 for orig_gamma.  This lets buildmap use the value from
       the_hdr, if it is present.

       If you are going to use the result of buildmap to generate values to be
       dithered, new_gamma should always be 1.0, and the display gamma (-g in
       getx11) should be passed to dithermap(3).  If you are not planning to
       dither, then pass the user supplied display gamma as new_gamma.

       The color map storage allocated by buildmap can be released by calling
       free( map[0] ).


SEE ALSO

       dithermap(3), rle_hdr(3), librle(3), RLE(5).


AUTHOR

       Spencer W. Thomas, University of Utah

4th Berkeley Distribution           3/6/85                         BUILDMAP(3)

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