ray nudge -4
Just a note: ray nudge distance
can not be negative, and default applies because wrong values can MESS with renders.
This said Meelis you mentioned one possible explanation for this "bug", let me explain how I understand this. I want to be corrected by experts but there's some logic behind the reasoning.
You said that scenes were going brighter while converging; this is always
true because more light as been cast. Now, what is SSS doing in the scene ? Basically, each ray entering the medium can bounce into
the material an arbitrary number of times.
In that case of internal scattering, obviously each computation is not
contributing to the scene lighting, and that's correct in regard with a finite computing power
, as opposed to in reality. Furthermore, each bounce weakens the ray.
Non uniform SSS is the slowest to render and you know that already
Does that make sense ?
This is somewhat comparable with how Oren-Nayar is looking darker with higher sigma, because energy distribution has changed over the surface
; with SSS, energy distribution is changed over the scene
As Meelis suggested it should converge as expected, it's not a bug but the result of a finite computing power
Acting on this would induce some bias/issues for sure.
e: Best work-around I think: compensate paths not contributing to the scene lighting with film sensivity.
* in unbiased renderers, when you change one material you are changing the energy balance of the whole scene.