JXL "Speed" clarification

[kremzli]

Is your feature request related to a problem? Please describe.
I dont understand what "Speed" slider means under Effort, lower effort would mean faster, and how do you make it faster and have it try harder at the same time? I also tested it with JXL Lossy, speed 1 and 4 compressed better than speed 2, i dont think thats right?

Describe the solution you'd like
Explaining my question

Describe alternatives you've considered
None

[T8RIN]

Idk, it's comes from jxl-coder

[kremzli]

awxkee/jxl-coder#14 it is for decoding speed, specifying that would be much more clear

[T8RIN]

He just said to refer to libjxl specifications, if you would find this explanation, i will add this info ofc 👀

[kremzli]

Well i cant find it but i know its just for decoding speed (useful for large images, but even those are already fast), and as with my original comment, it is a known bug in libjxl that speed 1 and 4 compress better than 2. Adding decoding speed shouldnt be necessary and left to the user as it would add a bit more size to the image

[kremzli]

So maybe make it by default 0 and specify that it will make the image a bit bigger, also how mostly unnecessary it is but people hopefully would know that already

[T8RIN]

Okay, somewhat understood

[kremzli]

It can also affect quality for lossy

[kremzli]

It does not affect quality for lossy. I was just reading the help command from libjxl, what it means is higher file sizes for the same quality, that translates to lower quality at the same file sizes.
I asked on the discord for some clarification, heres what i got:
There is no documentation on what libjxl's faster decoding flag does internally. "faster decoding just disables various stuff that will cause decoding to be slower, sacrificing some compression for faster decoding. Should generally not be needed for most use cases."