DXT1 RGB vs DXT1 ARGB

Spur

I've been messing around with a 1k texture today and saving it as DXT1 RGB and DXT1 ARGB. The final DDS files both share the same file size of 683kb. How is that possible since one of the files is minus an alpha channel?

almighty_gir

http://www.fsdeveloper.com/wiki/index.php?title=DXT_compression_explained

in particular:

But let's start with a note about compression itself. You have probably noticed that the DXT compressed textures always have the same size. This is because they have a fixed compression ratio. Because the resulting size is fixed, it also means that the quality loss of the compression depends on the original texture you used.

Bruno Afonseca

The one with the alpha will eliminate RGB information on the pixel blocks where alpha = 0, so it evens out

Spur

fonfa wrote: »

The one with the alpha will eliminate RGB information on the pixel blocks where alpha = 0, so it evens out

Ok, so there will never be a difference in file size between the RGB and ARGB for DXT1?

Then my test with the same texture, DXT5 came in at 1,366kb. I need the clarity of the DXT5 alpha but the alpha in the texture probably only utilizes 128x64. I'm going to assume it would be more efficient to break off the alpha to something like 128x128 DXT5 and then keep the main diffuse map DXT1?

Sorry almighty, didn't notice your reply till I posted. Thanks for the response guys.

Bruno Afonseca

Hey Spur,

It really depends on the asset, and workflow also, there's no hard rule for it.
If you're talking about a single asset then might not be worth the trouble of splitting in two textures. This way you save memory, but then you add an extra texture fetch and so on. If you're talking about dozens of assets, then the memory saving might be more significant.