I believe the change from the current World Ambient Occlusion panel to the new Fast GI panel committed here: rB75704091fccb introduces a confusing workflow when working with the Ambient Occlusion pass.
As of now in 2.93, to tweak the AO render pass, we can preview it in the rendered viewport, and change the distance under: World - Ambient Occlusion - Distance.
This change is confusing in my opinion. How can an user assume that to change the distance of the Ambient occlusion pass, he has to go under: light paths - fast GI approximation and change the value of a slider that is greyed out?
Yes, I agree that in this case, the AO pass should have it’s own radius control, separate from Fast GI Approximation. Using AO as a pass and using AO to bias the GI to speed it up are two separate things.
Why would you wan to do that? Fast GI is biasing the GI to get faster rendering at the expense of accuracy, AO is also biasing the GI to get faster rendering at the expense of accuracy. The difference is that Fast GI is a bit less crappy bias since it at least uses environment color in the outgoing ray direction whereas AO just uses constant white color.
So you have two mechanism supposed to do the same thing, one to do it worse, one to do it better. Why would you want to combine them to get something between worse and better?
That’s not what I mean. In order to do extreme optimization to have very fast interior renders (for video) I’d like to be able to do something like: Fast GI to have a decent GI from 2nd or 3rd diffuse bounce. This usually leads to an overall lower lighting. Then AO could kick in to “fill” the overshadowed areas.