I've been stumped for the last 2 days trying to figure out why the size of an ATF when it is loaded into GPU memory is bigger than the size of the ATF on disk.
I have a 2Kx2K ATF texture for PVRTC 4bpp, no mipmaps, so size on disk is about 2 Megs. That makes sense since if I do the math I get: (4x2048x2048)/8 = 2 Megs. My understanding is that when I upload this texture to the GPU, it should be uploaded directly and should occupy 2 Megs as well, thus an 8 times improvement over using RGBA. When I tested it, I noticed in Adobe scout that the GPU Memory (Base Compressed section of Average GPU Memory in Scout), is reporting 6 Megs being used, which is 3 times bigger than what I would have expected. I am baffled by these findings, and wonder if anyone here has any clue or hint as to why this is happening. Any ideas, and anyone else experience that?
A 2048x2048 PVRTC texture should take about 3 MB of VRAM.
I'm guessing you have transparency in your file, since those will take double (6 MB).
Your math is right, assuming that no mip maps we're considered. Mip maps require 33% more memory (2 x 1.33 = 2.66 MB). AIR/Flash annoyingly takes 50% more. So 2 MB x 1.5 = 3 MB. This will occur whether or not the ATF has mip maps, as the memory will be allocated anyway.