FastSSS2 drastically different between light cache preview and final render

[dgruwier]

I was trying to create murky water with volumetrics, so I figured I could try using FastSSS2, starting with the orange juice preset and just clearing it up and changing the color. It works EXACTLY like I was hoping, with beautiful volumetric lights inside a watery material.
....but only in the light cache preview. As soon as it starts actually rendering, the nice volumetrics go away.

I'm using VRay 50005, 3Ds Max 2021.

Here's the Light cache preview. The camera is pointing down onto the water, with the torus half-submerged and some random lights added for testing. Notice the nice volumetrics and shadows in the water:




Here's what it looks like as soon as the light cache is done and it starts rendering for real. All the volumetrics are gone.




Just to isolate it better, I made a test scene. I made this just using the orange jucice preset, making it white and increasing the scatter radius. This is the light cache preview (Used the "don't render final image" option to stop it at the light cache preview)
Here's the test scene download for this render




...Here's the fully rendered result. I can't tell which one is supposed to be correct, but I suppose it's the light cache one.

[fixeighted]

Can't see those first images unfortunately.

[dgruwier]

Strange, I can see them on my end. I removed and added them again, hopefully that worked.

[fixeighted]

I tried this - had to guess your setup but it works here. I upped the scale but changed nothing else from the default.
Of course it's possible this is not what you meant...

[fixeighted]

Something strange going on anyway with the LC, as you noticed...here is a similar thing in that same scene I set up...looking from above as yours is.
Where'd those shadows go?!
They are there in the previous image but from above they go away, even though it appears I still have the volumetrics.

[dgruwier]

My setup is a box with FastSSS2 applied to, using the orange juice preset with refreaction, then tweaked the colors and scatter radius.
I'm not super concerned about getting the specific look I'm going for in the first images though, I have some ideas for other ways to achieve it. But this weird discrepancy between LC and render seems like clearly a bug to me, so I wanted to report it.

[fixeighted]

Yeah I got that it was a potential bug thing - just was curious to see if I could replicate it...and could, in a similar way, as the above shows. Yes it does appear weird. And yeah, murky water has many options I think

[hermit.crab]

The light cache preview does not show the SSS effect; it only collects information for it. The correct preview is during rendering.

[^Lele^]

The LC will not consider SSS at all, i think.
You'll see a diffuse/spec/refraction components only for that shader type as the LC builds.
The correct render is the final one.

[dgruwier]

VrayFastSSS2 does work in the LC though, at least to a great extend. It's using whatever magic drives the scatter volume material too.

See the screenshot below of some nice skin SSS happening in the LC preview. Though this test had much better parity between LC and final render, I'll need to figure out what parameters in particular trigger the large disparity in the previous tests.

[^Lele^]

The LC DOES work with the SSS, to the limit of gathering info: it won't be accurate at all times.
You have scatter GI off, so for the LC that shader was diffuse (as it's the GI engine).
Turn it on, the previews will match better.
Not 100%, however, and not for very complex SSS paths (i.e. a radius 6 times the size of the object.).

VolumeScatter is where you want to go if you need such long paths.