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Article #15972: Color reduction techniques for graphics.

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FAQ972D.txt   Color reduction techniques for graphics.
Category   :Windows API
Platform    :All
Product    :All 32 bit  

How do I accurately (and automatically) display HiColor and 
True Color (16 and 24 bit) images on a 256 color display?

Generally, you cannot. This requires sophisticated color 
reduction techniques, and not all will work well on all images. 
Most all color reduction methods work by weighing the average 
of similar colored pixels in a given are of the image, and 
combining them. Consider a image of a person standing in a park 
with green grass, flowers, trees and a blue sky. You would need 
to remap potentially millions of colors to fit into the Windows 
palette (236 colors). While this can usually be done in a 
professional paint program with some trial and error, a single 
function would have much trouble remapping the colors to 
faithfully reproduce all images. This is the reason why paint 
programs that can accurately reduce colors give so many choices 
in the method used to reduce the color.

In the above example, you could combine many shades of blue and 
green, and dedicate more colors to faithfully reproduce the 
persons flesh tones. This method would probably produce an 
acceptable image, since the shades of the sky, grass, and trees 
are far less important than the shades necessary to reproduce 
flesh tones. On the other hand, if the purpose of the image was 
to show a spectacular panorama of a canyon, you might want to put 
less emphasis on the exact color of a house that might be in the 
picture, and instead concentrate on the colors of the canyon. As 
you can see, where one color reduction function may work well on 
one image, it would most likely have trouble with others.

Some paint programs use a "Color Wash" palette, that sometimes can 
display with fair accuracy a colorful 24 bit photograph. This is 
done by creating a palette that contains a wash of colors that are 
generally useful for displaying most images. Again, pixels are 
compared by area, and remapped to a color that is reasonably close 
to the original if possible.

7/16/98 4:31:28 PM

Last Modified: 01-SEP-99