Mesh light sampling/noise

[Rumlab]

Rendering with a thin ring disk mesh light, I'm having a hard time getting rid of noise. Noise threshold is set to 0.001 and sampling to 2 to 48 - render takes one and half day on i7 5820K @4Ghz. Using Environment Fog.
Normally I would up the samples only for the specific light, but this is not possible anymore?

EDIT: GI is set to Brute Force + Light Cache (2000 samples). Rez is 2400x1800px.

[Peter.Chaushev]

Hello,

One and a half day for render this size (+env. fog) seems quite excessive! Try toning down the quality settings substantially and enabling V-Ray Denoiser (does a great job at speeding up Environment Fog output).

Keep in mind emissive material / light object edges cannot be filtered properly if their intensity is high. Same applies the other way around - e.g. having a high intensity background may cause object silhouettes to appear jagged.
It stems from the fact that when the Antialiasing filter is applied it averages neighboring pixels based on the unclamped pixel colors (the VFB by default clamps values of the final pixels, see the "Force color clamping" button). Therefore, high-intensity lights produce colors with excessive color values, which even when averaged are still too bright to produce a smooth result.

As a workaround, you can smooth bright areas of your image (lights, highlight reflection, emissive materials, etc) by applying VFB Lens effects and using their intensity mask options.
Alternatively, you can clamp the prior to the filtering using Asset Editor's right Settings panel > Raytrace (advanced view) > Optimizations > Max Ray Intensity and/or Subpixel Clamp. This, however, may take a few tries until you got it right.

Kind regards,
Peter

[Rumlab]

Hi Peter. Thanks for reply.
Denoising with afore mentioned setting will actually render noise free Tested with normal "High" settings, but denoiser becomes splotchy in fog light sampling close to mesh light - and still takes a about 3 hours.
So maybe the standard very high settings + denoiser will do it + patience.
As teaser I can render a noise free image with redshift (softimage + 1 GTX1070 + 1 GTX1080) in about 5 minutes

Thx

[Peter.Chaushev]

If you would like, send over the scene and we'll take a look how it can be optimized.
The Rhino menu V-Ray > Pack Project produces an archive which you can send through a file-sharing service of your choice and addressed to support@chaosgroup.com or simply link it in a forum PM to me.

Kind regards,
Peter

[Rumlab]

Thanks, PM'ed scene with link

[Rumlab]

Another issue observed with scene is, that the denoiser introduces tonal steps across gradient surfaces (like the dome).

[Peter.Chaushev]

Hello, and thank you once again for providing the scene!
After examining it the conclusion we've reached is that there is nothing in v3.60.02 which can substantially improve the render times without degrading the quality.
The fog splotchiness you described is produced when the affecting Mesh (or Rectangle) light shape is very thin (e.g the light hoop visible in your project's 03 view). Our developers are already familiar with the issue and are exploring potential solutions.
The only workaround I can advise of in the current version is to enable Environment Fog's Scatter GI toggle. It may add some time to the render process but should smooth out the end result noticeably. VFB's Lens bloom effect can also help a bit.

I'm afraid there is nothing else I can offer you right now for this edge case.
GPU support for Environment fog will be included in the next major V-Ray for Rhino release. It should provide a speed boost on a system spec like yours and hopefully the core issue will be resolved by then as well.

Let me know if I can assist you with anything else.

Kind regards,
Peter

[Rumlab]

Thanks for checking - looking very much forward to Next architecture.