Slow rendering on V-Ray light material on smoke

[zjie]

Hi,

I need to use cylinder-shaped light, so I got to use V-Ray light material, however, it is taking too long to render.

On the left, I used a sphere light, turned off the V-Ray light material, it only took 15.4s

On the right, I have off the sphere light, turned on V-Ray light material, it took 2min, an 800% increment in render time.

While this is a test, when I put this on production use, with optimized subdivs, they are taking hours to render per frame.

So is there a much more efficient way to work on this?

Thanks,
Zheng Jie

[Svetlin.Nikolov]

Hey,

Could you share your Phoenix render settings - the Rendering rollout and all the volumetric render settings.

Thanks!

[zjie]

Hey yea, this was rendered without GI

[Svetlin.Nikolov]

Hmm, do you really need Volumetric Geometry mode? It will slow the rendering quite a lot, so if this is not a pass you'll need render elements from, you can switch to Volumetric to speed things up. However, 2 mins for this sim are still quite a lot - what is your grid resolution? Are you using Phoenix 3.05 or a later nightly? If possible, can we have a look at the scene so we can profile it and figure out if something can be improved?

Thanks!

[zjie]

Hi Svetlin,

We do need the depth pass from the smoke, as we are rendering the smoke separately, and all our other objects are using zdepth pass in nuke.

The grid resolution is 10gb.

We are using nightly, phoenixFD_adv_30502_maya2016.5_vray_30_x64_27942


The link of the scene file is here, with one frame of the simulation, hope it is okay.

https://drive.google.com/open?id=1Lb...zoqwodxJ4j9uXG

[zjie]

The v-ray light material was applied on geometry, laser_ILL, with primary visibility turned off so it will only emit light.

[Svetlin.Nikolov]

Hey, thanks for the scene! So the light material is indeed slowing the render quite a lot. What you can do on the Phoenix side:
- Increase the Phoenix Light Cache speedup - right now it's 0.9 and you can keep increasing it until you see artifacts, but this is the strongest speed boost you can get.
- Increase the Shadow Step - since you don't have much shadow detail, you can overcook this.
- Enable the V-Ray -> Overrides -> Volumetrics -> Probabilistic Volumetrics - here you can also experiment with the samples - note that high samples would be slow because too much unneeded rays will be cast and low samples will be slow because there will be too much noise to converge, so the ideal value would be in the middle.

Hope this helps!

[Svetlin.Nikolov]

Ah yes - another way you could approach the problem would be to turn off the Phoenix Light Cache completely and render with or without Probabilistic Volumetrics - this way the smoke would be noisier and it would be up to the V-Ray settings to clear the image.

[zjie]

Hi Svetlin,

Thanks for the help, increasing shadow step really helps alot, it could reduce the rendering to 40s.

However, there was bright red spots around the illumination of the laser, I increased the light material subdivs to 32, it reduced the flickering but there are still flickers, and the render time was increased to 6min 38s.

Is there a way to remove the flicker while not killing the render time?

[Svetlin.Nikolov]

Hey, is this with the Phoenix light cache enabled?

[zjie]

Yes it is enabled, and I put both light cache speedup to 0.100, that will give me a better quality right?

[Svetlin.Nikolov]

If you have to go so much down on the speedup, try the alternative approach - turn off the Phoenix light cache completely, try rendering like that and then try enabling the Probabilistic Volumetrics option.

[zjie]

I tried to turn on the probabilistic volmetrics option in v-ray render setting, but it doesn't seem to affect the render at all, whether if I turn it on or off, sample 20 or sample 1000

[Svetlin.Nikolov]

In this case, I guess it's up to the V-Ray guys to try to optimize the light material since it's significantly slower than a regular light - I already pinged them about it.