There is no size difference between BC3 and BC7. The difference in size between those and BC1 are due to the alpha channel. Basically, BC3 is to be used if the engine is incapable of BC7 since BC3 is basically relabeled DXT5 from previous DirectX versions. BC7 uses more advanced, less artifacty compression.
A free easy way to get the whole gamut of the BCn compression formats is use the recent versions of Paint.net for the final DDS authoring.
I think I may have missed something maybe you could explain further. When I export using GIMP or PhotoShop I must choose one of the BC formats. When you look at Bethesda textures and run them through an optimizer or compressor it reads the bc format with something like bc1 UNFORM. I know if I choose the wrong type they don't work too well in-game or CTD. So are you saying there are a different set of rules and what about exporting?
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A free easy way to get the whole gamut of the BCn compression formats is use the recent versions of Paint.net for the final DDS authoring.
File size examples would be great info, maybe some examples.
Maybe also a comparison to the old dxt system.