[shiandow]

Could be noise, but finding a smooth image that rounds to a good enough approximation of the original is quite useful. If you see a video player talk about debanding it is a exactly that.

I don't know if JPEG XL constrains solutions to be smooth.

[adgjlsfhk1]

I believe they constrain to piecewise smooth (i.e. don't smooth out edges but do smooth out nose)