8-Bit Indexed Color Translucency
by Travis Bemann

This article is on how to use 256 indexed color translucency. It does not work on 16-bit or 32-bit color due to memory constraints. It involves building a 256 by 256 unsigned char table, which contains the result of every color combination. The table would be extremely large with 16-bit or 32-bit color. The first index for the table (which is in the form of a 2D array) is the translucency color. The second index for the table is the color behind the translucent pixel. This is a function that shows how to use this translucency technique. (The translucency actually should not be in a function or method, even an inline method.) DON'T ACTUALLY USE THIS FUNCTION! IF A FUNCTION CALL WAS USED TO DRAW EACH SINGLE PIXEL IN A SINGLE SOLIDCOLOR POLYGON, IT WOULD BE HORRENDOUSLY SLOW!

Variables

x: the pixel's x coordinate
y: the pixel's y coordinate
c: the pixel's translucent color
b: the target graphic's base address
r: the target graphic's row bytes (includes padding, if there is any)
t: a pointer to the mixing table

Implementation


void Polygon3D::DrawTranslucentPixel(unsigned long x, unsigned long y, 
                                     unsigned char c, unsigned char *b, 
                                     unsigned long r, unsigned char *t)
{
    *(unsigned char*)(b+x+y*r) = t[c][*(unsigned char*)(b+x+y*r)];
}

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Date this article was posted to GameDev.net: 7/5/2000
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Alpha Blending