Creating graphical effects with color mapping tables.
This program demonstrates how to create custom graphic effects
with the create_color_table function. Allegro drawing routines
are affected by any color table you might have set up. In
the first part of this example, a greyscale color table is
set. The result is that a simple rectfill call, instead of
drawing a rectangle with color zero, uses the already drawn
pixels to determine the pixel to be drawn (read the comment
of return_grey_color() for a precise description of the
algorithm). In the second part of the test, the color table
is changed to be an inverse table, meaning that any pixel
drawn will be shown as its color values had been inverted.