Texture Format Comparison:

Information:

• All decals were generated from the same 512 x 512 source art, including the alpha channels.
• All files were made with Photoshop 7, using the file format plugin that allows TGA alpha channels (Info and Download).
• TGAs were saved at 32 bit, as 24 and 16 don't allow alpha maps (More info on that here).
• Map in test shot is 128 x 128.
• Blue and red checker detail texture is DXT1, 256 * 256.

Resolution and Format Comparisons:

Firstly, here is a scale comparison.

Here is a reference shot taken in the Mission Editor, showing all the decals and what formats they are in.

As you can see, the mipmaps of the DDS files are evident in comparison to the TGA decals, especially on the higher resolution decals.

The file sizes of my test decals:

• 512 * 512:
• DXT: 341K
• TGA: 1.0Mb

• 256 * 256:
• DXT: 85.4K
• TGA: 256K

• 128 * 128:
• DXT: 21.4K
• TGA: 64K

DDS files saved with DXT 5 and 3 give identical file sizes.

The major advantage of DDS files is their lower use of VRAM (source), though a few tests seem to indicate that as long as a map is not especially decal intensive, the performance hit caused by using TGAs exclusively is negligible compared to the performance hit caused by the models and effects of the game. DDS decals and textures might enable a map to run on lower spec systems, but the other art assets could soon scupper that :)

Looking through the Relic files, it seems that most decals and detail textures are DXT5 or DXT1 DDS files, with only a few being TGAs.

DDS Alpha Transparency Comparison:

Though DDS files at DXT5 and DXT3 give identical filesizes, DXT alpha maps are clearly stepped, especially on larger decals and detail textures. TGA files and DXT5 DDS files give smooth alphas.

I can't give you empirical answers, but judging by the Relic artwork and a lot of custom content, it seems that 256*256 is the preferable average size for decals and detail textures, with DXT5 being the best way to record transparency. TGAs and 512*512 files seem to be useful for items that are repeated or used rarely and benefit from high detail, such as some of the crater decals.

With thanks to dr2sheds and monoRAIL