High compression ratio mode for lossless WebP about image HOT 2 OPEN

To be clear "not very high" is relative to what WebP is capable of. I just did a quick benchmark using the image from image-rs/image-webp#71. I converted the image to lossless form so everything would be using an identical baseline and then compressed with a bunch of different encoders:

Of course, using only a single image doesn't tell the whole story. image-webp's hard-coded encoding parameters make it vulnerable to edge cases and so forth, but it shows that the encoder is overall quite respectable.

from image.

The key change needed to achieve higher compression is to sweep over compression parameters and select the set that produces the best output. libwebp uses various heuristics that cut down on the number of candidate parameter options to pick from, so that's probably worth looking into. Heuristics are particularly important for the faster compression settings that don't have time to consider all the options

As far as changes to the encoder itself, the biggest is probably added support for varying the huffman codes used across the image. Since storing the huffman trees takes up space, you have to bin regions of the image that have similar entropy distributions to reduce the total number of trees used. At the moment I don't fully understand this process for deciding which regions should share the same codes

from image.