BC7 has a weakness: There’s only a single alpha mode that supports 2 subsets (mode 7). And it assumes alpha is correlated with RGB. The other alpha modes are only 1 subset modes, so they can’t handle complex RGB blocks well.
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So if the block has uncorrelated alpha and is complex enough in RGB, there’s not a lot you can do. Either RGB suffers or alpha suffers.
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Also: Modes 4 and 5 support component rotations, allowing one of the RGB components to be encoded seperately. This results in different looking artifacts vs. all other modes.
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So as a developer using BC7, you need to be very careful what you decide to put into the alpha channel. Ideally it should be correlated (resemble strongly) RGB. It’s not really a fully separate channel like DXT5/BC3.
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(Yes modes 4,5 are like BC3. My point is basically that with BC7, the complexity of alpha will degrade RGB quality. Not so with BC3.)
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