JPEG XL saving options clarification

[HiKiyos]

Been having fun experimenting with new file formats in version 1.8.0

I find JPEG XL’s options puzzling. There is the “Compression” option with the range of values 0 to 9. Do these values correspond to the 10 encode effort levels of the libjxl encoder? If so, then how exactly?

Compression levels 1–9 perform consistently and predictably: 1 is the fastest and gives the largest file size, and 9 is the slowest with the smallest size. And then there’s level 0: its speed, resulting file size and quality is pretty much the same as level 7 in both lossy and lossless modes. How does this make sense? And where is the 10th effort level?

[Kadet]

Compression 0 is default effort=7.
Compression 1..9 is effort 1..9 like it was in older libjxl.
Now in libjxl it's 1..10, but Pierre don't changed this parameter after changing library to new 10.3 version.
Effort 1..4 sometimes give smaller file that e 7..10 but in this case quality is worse This is why I usually set e 8.

[HiKiyos]

This confirms my suspicions, thank you!