Choose the right DXTC compression algorithm

By Blade9722.

Preface.

Altough most of what I'm describing should appear obvious and trivial to those skilled in the arts, I noticed there are some common mistakes which regularly appears also in the work coming from well-experienced texturers.

Brief description of most common DXTC compression formats.

DXT1C/DXT1A Channels bit depth: 8:8:8:1 RGBA 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 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 typically 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 it 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.

- To be continued (later, I'm leaving now)