Atmosphere Volume Samples

[Andrew_Wilkins1]

Hey all,

I have a quick question here regarding atmosphere volume sampling and to some extent scene scale.

A light setup in which I have a spotlight adding volumetric lighting to the scene through a window.
The spotlight requires a radius of 800 to create a softer edge to the shadow.

In order to clean up the render noise in the volumetrics I've tried:

1. aiAtmosphereVolume: Samples are sitting the maximum of 100 (even though the Arnold Docs demonstrate that a value of 10 is clean)
2. Light Samples/Volume Samples: Don't do anything
3. Render Sampling: The only render sampling which has an effect is AA, which seems strange to me but I'm assuming it's actually increasing the atmosphere samples?

Would you say this is an issue due to scene scale? Or just the high radius spotlight making the volumetric lighting noisier which causes the max value of the atmosphere sampling to be hit?

The issue is that I can't raise the atmosphere samples any higher and raising AA samples seems inefficient?

Maya: Prefs > "Working Units": centimeter (default)
Arnold: System > Render Unit: Use Maya Unit (default)
Plugin: MtoA 3.2.1.1
Core: 5.3.1.1

Thanks,
Andrew

[lee_griggs]

I imagine it's because of the high radius, but it's difficult to tell without renders or a simplified scene.

[Andrew_Wilkins1]

Hey Lee, hope you're well!

Below setup a simple scene to demonstrate:
1x spotlight, 1x cube scaled up by about 4000 on all axis with a hole.

I've noticed dropping the light radius to around 100 does assist with the noise, but you of course start getting hard shadow edges. Even then with the atmosphere volume on 100, it's not clean.
Arealight seems to have a similar effect when scaled up.

I'd have to raise the CameraAA samples to about 7 in order to be clean.

Atmosphere samples of 10

Atmosphere samples of 100

[lee_griggs]

>I'd have to raise the CameraAA samples to about 7 in order to be clean.

>Atmosphere samples of 10

This sounds about right for final frame rendering. 3 CameraAA is meant for testing.

Have you tried a light portal?

Can you upload the scene, please?

[Andrew_Wilkins1]

Sorry, I should have specified: Atmosphere samples of 100 and CameraAA 7-9+ in order to get a clean volumetric render.

Thanks for the suggestion, I'm aware higher CameraAA is generally needed for final frame rendering. Even then, without complex assets or motionblur a value of 7+ can be overkill. We tend to average at about 4-5.
I'd want to only sample for the atmosphere volume?

Can try get you a scene—as soon as I can, its very a simple scene however and should be quickly recreate-able.

[Andrew_Wilkins1]

ie. Adaptive sampling 7-9 and 100 atmosphere volume samples.
Cube matted and all other samples to 0.
Still not exactly clean.

PS. Exposed up renderview to better visualize noise quality

Edit:
Looks like around 10 CameraAA (no adaptive) and 100 volume samples seems to be fairly okay, but if you really dive in... a value of up to 15 is even cleaner.
Thankfully, volumetric noise can be acceptable (air particulate and all)

[Andrew_Wilkins1]

No, I haven't tried a LightPortal—this sunbeam is for Key light streaming into an interior environment (not ambient sky light) but I don't mind testing that as well. 😉

Also, a skydome and hence LightPortals can't cast volumetric light. An area light substitute as a portal would work, but that would have the wrong effect unless placed and angle to shine through the window. In which case, it has a similar noise result as a spotlight.