Currently only Effort 8 uses LZ77 and Effort 9+ use LZ77-Optimal, causing a large difference between 7 and 8 when -P 0 is used. This has been present since a regression in v0.11.

This PR enables RLE for Efforts 5-7, with Predictor Zero enabling RLE at Effort 1-4, LZ77 for Effort 5-8 and LZ77-Optimal for Effort 9+. ICC compression is also higher for profiles up to 16KiB in size. This results in roughly a 5% encoding speed penalty but up to 25,000x smaller filesize, when using a solid color image with cjxl -d 0 -P 0 -g 3. Test Image

Main

JPEG XL encoder v0.12.0 5e1e5530 [_AVX2_,SSE4,SSE2]
Encoding [Modular, lossless, effort: 7]
Compressed to 12583.2 kB (6.000 bpp).
4096 x 4096,  6.958 MP/s [6.96, 6.96], , 1 reps, 16 threads.

PR

JPEG XL encoder v0.12.0 [_AVX2_,SSE4,SSE2]
Encoding [Modular, lossless, effort: 7]
Compressed to 479 bytes (0.000 bpp).
4096 x 4096,  7.064 MP/s [7.06, 7.06], , 1 reps, 16 threads.