Smoke 'noise' when adding more lights

[Tore S]

Hello,

I'm currently working on a fire simulation in a tunnel. The software I am using is 3ds max 2018, phoenix 3.04 and vray 3.60.01.

The problem I'm facing is noise in my smoke as i add more vray lights to the scene.

Smoke with 4 lights



Smoke with 8 lights







This problem only comes when I am using Irradiance map and Light cache and rendering for animation using prepass. If I render a single frame I do not get this problem, I have found that the problem is the amount of lights i have. I don't even need to recalculate the prepass as long as i delete some lights and only have 4-5 it looks like it's supposed to. I also tried to have the additional lights outside the tunnel, it did in no way supply light to the camera or the smoke cloud. So it does not matter where the lights are as long as they are in the scene. I tried using other lights, and i got the same problem.

I have now tested with Vray 3.60.03, didn't work. I tired to recreate the scene with 3ds max 2015, vray 3.60.01 and phoenix 3.04 and i get the same problem.

I have tried to use custom settings for the irradiance map and animation med/high presets. Interp.frames set to 2, interp samples tested at 5, 20 and 50, no difference.

I must have done something wrong, but I can't figure out why. Does anyone have any ideas. It just seems strange that it only happens when i render animation and i have more then 4-5 lights in the scene.

Thanks for any help, I'm about to go the bed so I won't be able to read any answers before i get to work.

[gregvfx]

I have had this problem for very long with no valid answer, basically can't use the light cache with grid renders and it is like up to 10 times slower without the light cache...
My investigations also showed that if the light source is inside the grid, you have bad sampling like your example... Maybe that has changed since.

[Tore S]

Thank you very much for the tip, I will try this out right away.

[George Barzinski]

Hi,
Do you use Phoenix nightly build? And can you send us a scene to check what's wrong? I've tried to reproduce the problem with 8 plane lights but everything looks normal.

[Svetlin.Nikolov]

I don't think a scene is needed. When you see cubic voxel-like artifacts, this means that it's time to turn off the Phoenix Light Cache.

Cheers!

[Tore S]

I have tested and it seems turning off light cache is the only way forward, the render time increases about 3 fold, but there seems to be no way around that. Thank you all so much for the help.

[Svetlin.Nikolov]

Alternatively, you could try with Light Cache on, but decrease the Light Cache speedup. However, please check if setting the speedup to 0 does not produce these artifacts.