1) Is it possible to store mipmaps of a single texture in a different format? For example storing level 0 as GL_COMPRESSED_RGB, the 1st level as RGB4, and the 2nd as GL_LUMINANCE and a 3rd as BGRA? Does a mipmap have to be smaller than the previous level? Can each mipmap be considered independent of other mipmaps in this regard?
What are the restrictions and could this affect performance or texture compression efficiency?
2) I noticed that mipmaps generated with GL_SGIS_generate_mipmap are slightly different (darker and more greenish) from my own generated mipmaps. I take the average of each 2x2 pixels.
Could this be due to rounding errors in my code, or perhaps driver optimizations? I doubt it has anything to do with texture sharpening and other driver tweaks (all disabled). Any suggestions?