Questions related to Volume Grid

[KushKush]

Hello,
I'm working with lots of Volume grids in the scene, even when most of them are hidden or disabled, the viewport is playthrough is quite laggy, I use VDB cache imported from FumeFX . Often volume grids overlap one another, in those cases I cannot hide them. I end up rendering for prievew 5-10 minute a frame(vray settings render time limit) for acceptable resolution, 5-10 minutes adds up quickly into few hours of render just to see if smoke animation is acceptable. Are there more practical way to work with Volume grids other than what I'm currently doing?
Another question regarding Volumetric Geometry. It seems that GPU does much faster work compared to CPU, are the results on the same quality? Documentation states that Volumetric Geometry should be used when volumetrics overlap each other and for render element export. I don't see the difference when I'm trying to export atmosphere, can anyone explain what exactly the purpose and proper usage of Volumetric Geometry.

[Joelaff]

Some of the newer nighties starting around November or December of 2020 have some good speed improvements in this department that were added when we were working on a project with a lot of volume grids. Made a big difference.

There was also a script posted that was helpful. The thread is here:

https://forums.chaosgroup.com/forum/...ces-for-clouds

[Joelaff]

Also make sure probabilistic shading is on in the atmosphere settings, and experiment with how low you can go on those settings.

Use the bucket render and the volume light cache if possible with the light cache speed up as high as you can get away with.

[KushKush]

There was also a script posted that was helpful. The thread is here:

https://forums.chaosgroup.com/forum/...ces-for-clouds

Interesting thread, thanks for sharing it. It's gonna take a while to figure out what Svetlin says there.
Why bucket instead of Progressive?

[Joelaff]

Interesting thread, thanks for sharing it. It's gonna take a while to figure out what Svetlin says there.
Why bucket instead of Progressive?

The Volume Cache doesn't work with progressive. So usually bucket is faster (with the volume cache). However, probabilistic shading sometimes makes this less important.
https://docs.chaosgroup.com/display/...ering+In-Depth (about halfway down about the volume light cache)

Note that it says with multiple grids it may be better without the light cache (so maybe progressive is OK). (this is the PHX light cache, not the GI light cache). I don't recall what we ended up using for our project in terms of the light cache or not.

Test, test, test!

[KushKush]

I've rendered this frame 6 times. It contains 3 smoke volume grids and vray fog, floor vray displace.
1. CPU Prog- light cache off - 08:26
2. CPU Bucket - light cache on - 08:25
3. CPU Bucket - light cache off - 08:26
4. GPU Bucket - light cache - on - 04:32
5. GPU Prog -light cache off - 5.3 ( the vfb shows 4:32+30 seconds preparing gpu renderer)
6. CPU Bucket - light cache off, vray displace off - 5:45
7. CPU Bucket - light cache on, vray displace off - 6:18 ---> light cache on is slightly longer time ( likely due to multiple volume grids)
8. CPU Prog - light cache off, vray displace off - 5:57

It seems that light cache does not make much difference except for the GPU test (which I want to run again to make sure its consistent)
If I understand correctly Vray GPU does not support Vray displacement modifier and that is likely why when turning it off in CPU mode, render times are closer to GPU render times.

I've got other question that bothers me, how do I isolate each athmospheric effect such as smoke? I want to have flexibility in Post
Here is a image of atmosphere render element . Right side of image there is a smoke(1) overlapping other smoke(2), behind them there is a fog in the horizon. Lets say, in post I want to change color of smoke (1) and turn toggle the opacity of smoke (2) how would I achieve that kind of flexibility?

[Joelaff]

Interesting results. Thanks for sharing.

It is definitely scene dependent. I have had pretty massive speedups from the light cache when close to clouds, but other scenes it makes little difference. I think it also makes little difference when you are using Probabilistic Shading, which is the default with Volume Grid, but must be enabled (last I checked) with Phoenix.

[Joelaff]

I think if you want to change each one independently the best bet is to render it entirely independently.

However, if you go to Geometry mode you should be able to get Multi_Matte Element mattes out of each one. Not the perfect solution in terms of the comp, but better than nothing.

[KushKush]

I think if you want to change each one independently the best bet is to render it entirely independently.

However, if you go to Geometry mode you should be able to get Multi_Matte Element mattes out of each one. Not the perfect solution in terms of the comp, but better than nothing.

1. What do you mean by "render it entirely independently"? Turning off "visible to camera" on all other objects in scene?
2. Geometry is something I'm looking at lately, I'm not entirely sure yet of how to use it, Documentation says it has longer times and there are certain limitations.

[Joelaff]

I mean use a matte black material override for all your objects (to knock them out) and then render each grid that way. So now you have just your smoke or whatever on a "layer."

Geometry is typically slower, though sometimes faster if you have a lot of grids and a lot of GI, if I recall. I don't think it works in GPU, but not sure.

[KushKush]

I mean use a matte black material override for all your objects (to knock them out) and then render each grid that way. So now you have just your smoke or whatever on a "layer."

Geometry is typically slower, though sometimes faster if you have a lot of grids and a lot of GI, if I recall. I don't think it works in GPU, but not sure.

1. Thanks Joelaff, I'll give it a try.
2. A lot of GI? I Haven't seen that on documentation.

[Joelaff]

I think GI was the completely wrong term (sorry I was on the phone). I meant a lot of reflective surfaces (especially soft reflections) that have a lot of secondary rays.

Basically, best to take a moment to test different settings when possible.