Quote:

I'm using ATI's Compressinator, without much luck -- I seem to get major artifacting.



"DXT1C/DXT1A.
Compression ratio: 8:1.

The DXT1A is often described as "RGB texture with one-bit-depth alpha". Actually, for each pixel there are two available configurations: if the alpha channel is set to 1 (white), RGB channels store usual color info. Instead if the alpha channel is set to 0 (black), upon saving the texture RGB channel are set to 0 as well. In other words, each pixel could contain color info only if the alpha channel is set to white. This makes a sense, as a black alpha channel usually means that the pixel is completely transparent (invisible), so there's no need to store color info. This algorithm is tipically used for icons, decals, blood, tatoos, i.e. texture with a completely transparent part.

Instead, DXT1C is often described as "RGB texture withouth alpha channel". This is not true: the alpha channel is still present, but is completely white. Actually, there's no physical distinction between this and DXT1A, simply upon choosing DXT1C as compression algorithm the alpha channel is set to be completely white just before saving. This is tipically used for common completely opaque textures.

(...)

DXT5
Compression ratio: 4:1.

This is often described as "RGB texture with interpolated alpha". The application field is the same of DXT3, theoretically this should be chosen if alpha channel is featuring slow gradients. Again, from my personal experience this algorithm is better than DXT3 in most cases, so I recommend using DXT5 when a comparison is not available.

Notice that DXT1 compression ratio is double than DXT3,DXT5, and so it generates half size dds files. The three algorithms are different only for the alpha channel compression, the RGB part is exactly the same for all three. So if you don't need alpha blending you should use DXT1, as using DXT3 or DXT5 will only double dds file size without any improvement in visual quality. Instead, I regularly find out in texture replacement lots of DXT5 compressed textures with the default white alpha channel.

The most common mistake is this: some developers probably think that DXT5 is increasing RGB visual quality, so they systematically use it without caring whether alpha channel is used or not. (...)"

(source)

Have a good one,
Christian

Last edited by HeelX; 02/17/07 08:07.