PGI and reflections

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PGI and reflections

Post by lacilaci » Thu Sep 26, 2019 6:31 am

I'm running into this issue more and more, and in some recent work I rather disabled photonGI and rendered longer cause reflections were looking too weird.

the problem is that in many cases pgi doesn't just help with gi but also alters a LOT how reflections look and sometimes you don't even need a reference to be able to tell that there is something wrong (at one point a client even pointed out that some reflections are wrong)

I tried to replicate this issue in a testscene here:
(1.75 MiB) Downloaded 43 times
no photongi
with photongi
It isn't the best example but you can already look at the reflections and see they are very flat looking on the ceiling and the reflective sphere for example, in interiors my biggest problems are highly reflective floors or even wooden floors, but I spotted problems with reflections even on smaller objects. Sometimes it goes pretty crazy and it looks like reflections are getting cut off or even as if different glossiness values were used.

The only observation I was able to make as to why this happens is that more complex objects and especially textured materials (which usually all are) are prone to this problem. If you try it on a scene where materials have no inputs it mostly looks pretty much ok. In this testscene I used noise input for bump only, so it begins to change how photongi looks in reflections but it's not as bad as when you use material that has diffuse, roughness, bump maps and on top of that complex geometry.

The issue is mostly noticeable when viewing objects at glancing angles (obviously) cause there is a lot of reflections going on.

This is why I was hoping that maybe if we had an option to use PGI only after certain amount of bounces it could help, like use 2 bounces brute force and then fill in with PGI. So that very obvious light and reflection interaction is handled properly but stuff that's in the shadows would be handled with pgi to boost performance there. I'm not sure if it is good idea or even helpful but like I said, I already had to just disable the thing and render longer cause no settings were helping.

here as another example, the difference in reflection contrast is somehow giving the impression almost as if different glossiness is used

no pgi

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