What are the DXT compression levels?

[stfas]

as of lately i've been wondering whats the difference between the different dxts?

[IBBoard]

Wikipedia has more detail (and the maths) but in summary:

DXT1 allows 1-bit alpha and uses 8 bytes to store data about a 4x4 block (16 bits each for two colours plus two bits for each pixel where they map to colour one and colour two with the other two values being either 50:50 mix and transparent or 1:2 and 2:1 mix)

DXT3 has 16 bytes per 4x4 block (4 bytes for alpha channel - 4 bits per pixel, or 16 levels - and then the colour is stored the same as the 1:2/2:1 mix DXT1)

DXT5 also has 16 bytes per 4x4 block (two 8-bit alpha levels and a lookup table for one of eight levels of alpha, including a fixed 0 and 255, plus colour stored the same as DXT3).

Basically, DXT1 is good if you have no transparency or on/off transparency and want tiny file sizes, DXT3 is good for when you have sudden transparency transitions and DXT5 is good for gradient transparency. Most of the DoW (nearly all) RSH files use DXT1.

[stfas]

i remember you saying in the texture tool that DoW don't use dxt3, well the victory/defeat screens uses dxt3

[IBBoard]

I said in the texture tool not to use the TGA->DDS conversion, but that was back when it was experimental. It is now using the Squish compression library and so it should be almost as good quality as Photoshop.

I don't know why the victory/defeat screens would use DXT3, though. AFAIK they don't include any transparency and so DXT1 would halve the file size.

[stfas]

actually, alot of the screen is black, as theres two parts of the screens

[IBBoard]

Okay, I hadn't actually looked at them. I guess if they wanted some gradient to the transparency but it wasn't a subtle gradient then DXT3 would be the better choice than DXT5.

[stfas]

i got on it was dxt3 by looking on the size when i converted it back from editing it a little... and it was wrong size with dxt1 so it needed to be dxt3