BUILDMAP(3) 3 (3/6/85) 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
Page 1 (printed 12/1/98)
BUILDMAP(3) 3 (3/6/85) BUILDMAP(3)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
Page 2 (printed 12/1/98)
