V-Ray 3.6 -> 4.3 Significant GI Bounce Difference

[thwalker]

Noticing some significant differences in the way GI bounces between V-Ray 3.6 and 4.3. Our primary concern with this is we want to have parity between 3.6 and 4.3 as much as possible when transitioning older scenes rendered now in 4.3. Hoping someone can shed some light (haha)!

Our workflow involves pretty heavy usage of mimicking photo studio "cards", like black/white/grey foamcore, paper, etc. materials in order to bounce more or less light onto a specific product for a render. We'll typically use materials with extreme diffuse values (0, or 255) in order to have a more drastic effect. It seems like in V-Ray 4.0+, the contribution of bounce from more extreme diffuse value (closer to 0 or 255) materials is less intense? As far as we can tell, this has to do with the updates to Light Cache.

Happy to share scenes if necessary. If it helps, other specs below:

All renders with BF + LC
V-Ray 3.60.04 and Max 2018.4
V-Ray 4.30.00 and Max 2020.3

[^Lele^]

As far as we can tell, this has to do with the updates to Light Cache.

Meaning that if you revert to the old LC type, it behaves as in 3.x?
Or something else entirely?

[squintnic]

I would reassess your workflow rather than trying to match 3.6, seems weird to me.

[thwalker]

Meaning that if you revert to the old LC type, it behaves as in 3.x?
Or something else entirely?

If we open a scene in 4.X, made in 3.X, change nothing and just hit render, it appears different (sometimes significantly). We aren't changing from KDTree to Hashmap, and it also doesn't appear to be affected by the Adaptive Dome (although scenes with it obv. look different).

One thing we did notice is that objects with Cast Shadow turned Off in 3.X still cast GI, however in 4.X they appear to not bounce anything (which seems more accurate)? Can you confirm this Lele?

In a lot of these cases apparently our artists have used a thin box with a black or white material, but the obj properties have Cast Shadow turned Off, as well as Visible to Camera turned Off. This is in order to have a "bounce card" in the camera view, but not visible, and also not cast any direct light shadows but still cast indirect light shadows. I personally don't like this workflow, but it is what it is, and I'm trying to find a solve on how to emulate this in 4.X.

I would reassess your workflow rather than trying to match 3.6, seems weird to me.

Unfortunately we don't have the luxury of changing out workflow for these template scenes... They drive a lot of our consistent stream of work and we have nearly 50,000 of them developed over the course of a few years by ~100 artists. We need to make the transition to 4.X, but need to be able to still retain some amount of our old library. Eventually it will all be properly migrated to 4.X (or we'll just get rid of older scenes).

[^Lele^]

One thing we did notice is that objects with Cast Shadow turned Off in 3.X still cast GI, however in 4.X they appear to not bounce anything (which seems more accurate)? Can you confirm this Lele?

This is correct, yes.

In a lot of these cases apparently our artists have used a thin box with a black or white material, but the obj properties have Cast Shadow turned Off, as well as Visible to Camera turned Off. This is in order to have a "bounce card" in the camera view, but not visible, and also not cast any direct light shadows but still cast indirect light shadows. I personally don't like this workflow, but it is what it is, and I'm trying to find a solve on how to emulate this in 4.X.

I can't think of anything off the top of my head, and after a few simple tests.
But i'll ask and let you know for sure.