The two types of RDO BC7 encoders

There are two main high-level categories of RDO BC7 encoders:
1. The first type is optimized for highest PSNR per LZ compressed bit, but they are significantly slower vs. ispc_texcomp/bc7e.

2. The second type is optimized for highest PSNR per LZ compressed bit per encoding time. They have the same speed, or are almost as fast as ispc_texcomp/bc7e. Some may even be faster than non-RDO encoders because they entirely ignore less useful modes (like mode 0).

To optimize for PSNR per LZ compressed bit, you can create the usual rate distortion graph (bitrate on X, quality on Y), then choose the encoder with the highest PSNR at specific bitrates (the highest/leftmost curve) that meets your encoder performance needs.

Other thoughts:

- When comparing category 2 encoders, encoding time is nearly everything.

- Category 2 encoders don't need to win against category 1 encoders. They compete against non-RDO encoders. Given two encoders, one category 2 RDO and the other non-RDO, if all other things are equal the RDO encoder will win.

- Modifying bc7e to place it into category #2 will be easy.

- Category 1 is PSNR/bitrate (where bitrate is in LZ bits/texel). Or SSIM/bitrate, but I've found SSIM to be nearly useless for texture encoding.

- Category 2 is (PSNR/bitrate)/encode_time (where encode_time is in seconds).