Direct and Indirect elements broken rendering volume

[pablo_rohtau]

Hello I'm rendering some volumes and the Direct and Indirect elements (AOVs) are broken, the rest of the elements are fine as well as the beauty render.
I'm render the volume as Volume Grid and using Bucket sampler for the final render.
I guess the artifact is due to the bucket sampler, in the IPR I use Progressive and I don't get this problem.
Also the pattern indicates the culprit is the Bucket sampler.

Thanks

[Gosho.Genchev]

Hey pablo_rohtau ,

Any chance you could share the following:
1. V-Ray Core version ( top menu -> V-Ray -> About V-Ray -> Copy to Clipboard) that is used to render the scene
2. If possible, the hip file + a single frame of the volume

You could try disabling the Volume Light Cache on the Volume Grid Shader's "Smoke" tab to check if the issue will persist.
There is a similar issue that the Core team is looking into resolving in the future but it's surprising this has popped up for you.

Best regards!

[pablo_rohtau]

Thanks Gosho.
I always forget to put the vray version:
5.00.50 using H.18.5.408

---
V-Ray for Houdini hash:49f1173 from 15 Dec 2020 20:24
V-Ray Core 5.00.50 [c3af6122] from 15 Dec 2020
---

I'm going to try the "Volume Light Cache" thing
Thanks

[pablo_rohtau]

So I disabled the Volume Light Cache and the elements are working now.
The problem is the render time increased dramatically so for the time being I have to sacrifice these AOVs in favor of getting the renders out.
But definitely is something related to the Volume Light Cache.
Thanks

[david.anastacio]

So I disabled the Volume Light Cache and the elements are working now.
The problem is the render time increased dramatically so for the time being I have to sacrifice these AOVs in favor of getting the renders out.
But definitely is something related to the Volume Light Cache.
Thanks

Hey pablo_rohtau,

This should not be the case actually, nowadays that internal volume grid light cache should not be used, it's an old concept and things are faster without it as Svetlin.Nikolov explained some months ago.

3 things you need to always consider when rendering volumes:

- Always use packed disk primitives in the "file" node and vdbs.



-Never use the internal Light cache from the volume grid shader.



-Use "probabilistic volume rendering" in the ROP options.




Best,
David.

[pablo_rohtau]

Thanks a lot for the info david.
So basically the docs are wrong and all the defaults in the volumeGrid shader and in the Vray ROP are wrong as well.
I assume Probabilistic Volumetrics is what is called Stochastic Transparency in Mantra.

Thanks!

[david.anastacio]

Hi again Pablo,

So basically the docs are wrong and all the defaults in the volumeGrid shader and in the Vray ROP are wrong as well.

Pretty much that, yes. We bumped into this issues here also, and that was also a complain from us, the defaults for volumes are not usable nowadays and should be changed. To address this issue here, I wrote a "working with volumes in V-Ray" page in our wiki here for the team to follow.

I assume Probabilistic Volumetrics is what is called Stochastic Transparency in Mantra.

Yes. From the V-Ray manual: "the volumetrics will select a few samples along each ray based on the smoke density, and evaluate the volume lighting at those points. If disabled, the volumetrics will evaluate the lighting at each step of the raymarching algorithm."

Best,
David.

[Svetlin.Nikolov]

Ah, let me drop in with some clarification - the Volume Light Cache is still faster in many cases and this is why it's on by default, but with time more and more setups are better off without it. If the Volume Light Cache OFF + Probabilistic Volumetrics ON combination works better for your setup, this is important for us to know!

[LeoYfver]

I can just bump in and say that we independently came to using the same settings and david.anastacio. Most common question is why the volume render has blotches and we always ask people to turn the light cache off.

Regards Alexis

[ronald_a]

yes, same here. lightcache renderings always have blotches, always rendering without. Probabilistic is noisier so we have to up samples, but still worth it.

[pablo_rohtau]

Ah, let me drop in with some clarification - the Volume Light Cache is still faster in many cases and this is why it's on by default, but with time more and more setups are better off without it. If the Volume Light Cache OFF + Probabilistic Volumetrics ON combination works better for your setup, this is important for us to know!

In my case, at the moment I'm getting visually similar results, the difference is that using the Volume Light Cache the Indirect and Direct elements are broken, but the render is definitely faster.
What about using Volume Light Cache and Probabilistic Volumetrics?

[david.anastacio]

Hey,

In our experience here the internal light cache of the volume grid shader is unusable, yes there are cases where it's a bit faster, but in almost every scenario the amount of memory it needs makes it unusable, scenes can go from 10-20gb to 100+gb just by turning the LC on. Just throw some decently big volumes into a scene and throw a set of AOVs that are normaly used in production (30+ AOVs) and see your ram amount explode.
We have no complain about the render times of volumes when the LC of the volume grid shader is off, it's fast enough and ram stays under control.

Best,
David.

[ronald_a]

out of interest: are those optimizations that vlado shows in this video https://www.youtube.com/watch?v=KRStiWWtopI in the current version of the volume grid?

[pablo_rohtau]

out of interest: are those optimizations that vlado shows in this video https://www.youtube.com/watch?v=KRStiWWtopI in the current version of the volume grid?

Would be good to know more details about that optimizations.
Looks great BTW

[Svetlin.Nikolov]

Sure, these are in V-Ray 5 now. There are a few smaller optimizations in there as well.