Transparency
Additive Transparency
The flames on this burning bed are using additive transparency to keep the colors "hot."
in-engine
map
See additive color model.
Alpha Transparency
A texture using alpha transparency in RT3D.
The RGB part of the texture file.
The alpha channel of the texture file, in 8bit (256 colors).
Alpha Bit Depths
A closeup of the 8bit (256 colors) alpha channel. This is the highest bit depth used for alpha channels, because you can get a full range of grays with 256 colors. If we had a higher bit depth like 16bit (65535 colors), you would see the alpha looking a little bit smoother, but because texture filtering is so common now, it ends up softening your 8bit alpha anyway, and it looks fine.
A closeup of a 4bit (16 colors) version of the alpha channel. Still a lot of detail, but starting to break up some around the edges. This is a much smaller file than the 8bit alpha, which is good because it takes up much less memory. A good trade off.
A closeup of a 1bit (2 colors) version of the alpha channel. 1bit means only black and white, so there's no anti-aliasing. This is a very small file-- the visual quality suffers, but it saves a lot of memory. Not worth the degradation unless you really need the memory.
Subtractive Transparency
In Engine
Map
The x-rays on this light-table use subtractive transparency to make things under them darker, the way real x-rays do. The subtractive method isn't used all that often, so if you need it you should ask your programmer(s) if they can add it as a specific feature of the engine.
