Saturday, February 6, 2021

Simple and fast ways to reduce BC7's output entropy (and increase LZ matches)

 It's relatively easy to reduce the output entropy of BC7 by around 5-10%, without slowing down encoding or even speeding it up. I'll be adding this stuff to the bc7e ispc encoder soon. I've been testing these tricks in bc7enc_rdo:

- Weight the mode errors: For example weight mode 1 and 6's errors way lower than the other modes. This shifts the encoder to use modes 1 and 6 more often, which reduces the output data's entropy. This requires the other modes to make a truly significant difference in reducing distortion before the encoder switches to using them.

- Biased p-bits: When deciding which p-bits to use (0 vs. 1), weight the error from using p-bit 1 slightly lower (or the opposite). This will cause the encoder to favor one of the p-bits more than the other, reducing the block output data's entropy.

- Partition pattern weighting: Weight the error from using the lower frequency partitions [0,15] or [0,33] slightly lower vs. the other patterns. This reduces the output entropy of the first or second byte of BC7 modes with partitions.

- Quantize mode 6's endpoints and force its p-bits to [0,1]: Mode 6 uses 7-bit endpoint components. Use 6-bits instead, with fixed [0,1] p-bits. You'll need to do this in combination with reducing mode 6's error weight, or a multi-mode encoder won't use mode 6 as much. 

- Don't use mode 4/5 component rotations, or the index flag. 

In practice these options aren't particularly useful, and just increase the output entropy. The component rotation feature can also cause odd looking color artifacts.

- Don't use mode 0,2,3, possibly 4: These modes are less useful, at least on albedo/specular/etc. maps, sRGB content, and photos/images. Almost all BC7 encoders, including ispc_texcomp's, can't even handle mode 0 correctly anyway.

Mode 4 is useful on decorrelated alpha. If your content doesn't have much of that, just always use mode 5.

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