Mirroring Dome Light with Finite Dome causes excessive noise

[kosso_olli]

Hey guys,
it seems something weird is going on when dome lights with finite dome feature and HDRi in the texture slot are mirrored along the Y-axis. The result is excessive noise which is very hard to clean and therefore leads to very high render times.
If the dome light is not mirrored, but instead the values in the HDRi texture are flipped to get the same effect, the result is as expected: Clean and fast.
It almost seems as if the dome light loses the MIS after mirroring.
The problem is not related to the material, it will also happen with a simple grey VrayMtl.

The first image is rendered with the dome light itself mirrored along the Y-Axis, and the second image is the non-mirrored dome light with the mirroring applied to the HDRi texture itself using its mapping coordinates. Render time is 12:01 minutes versus 2:04 minutes.





Max 2023, V-Ray 6.00.20

Regards

Oliver

[Philip Shounev]

Hello,

Thanks for the report. I can confirm that noise levels seem visibly increased when mirroring the dome light. For the time being I would suggest using the VRayBitmap's own UV settings to mirror the texture instead of mirroring the whole dome light. Other than that, I've logged the issue in our internal bug-tracker under VMAX-13257 for our developers to look into.

Best regards,
Philip

[kosso_olli]

It seems the issue is still present in the latest relase of V-Ray 7. Tested with 7.00.04

[kosso_olli]

Issue is still there.

[^Lele^]

But why do you need to mirror the dome light, instead of the UVs?

[kosso_olli]

We do a lot of RHD conversion of cars for UK, AU and NZ. Most of the time the camera is simply flipped over, but to get the correct lighting again, you need to flip lights as well.
Because the cars are sometimes lit with more than one HDR, mirroring the UV's means more work.

[^Lele^]

Right, thanks.
I assumed it was a practical impossibility, i can defo write you a simple script to flip the direction of any vraybitmap set to dome in a scene.

[kosso_olli]

If mirroring of dome lights could be handled on the rendering side, that would still be an improvement over manually flipping coordinates.
What I forgot to mention is that we often rotate the domelight itself using the "lock texture" feature. So this would have to be rotated manually as well, whereas it is already correct after mirroring the domelight itself.