LightSelect broken

[TheScope]

We have an issue where it is not possible to recreate the lighting of the scene with light selects. This seems to happen in scenes with sun, sky and aerial perspective.

Steps to reproduce (or just grab the Maya test scene).

• Create a new scene with some objects.
• Add a sun and sky system
• Add a dome light and use the sky as the texture.
• Add Light Select render elements for the sun and dome light.
• Add an aerial perspective and enable Affect Environment.
• Render

The resulting sky Light Select is too dark. If a Light Mix render element is used instead, the missing light from the sky is in the self illumination layer. But when rendering with Light Select and Self Illumination render elements, the self illumination is much darker.

Here are some renders to illustrate the problem.




System Info:

• Windows 10
• Maya 2023.3
• V-Ray 6.10.00 (Also tested this with some nightly builds).

[TheScope]

And here is the Maya file.
23040_renderError_render_v010.zip

[vladimir_krastev]

Hi, thanks for posting. I tested your scene. I found no difference between the Self Illumination render element and LightMix. Here is a gif about it:

[TheScope]

The SelfIllum only looks correct if your render with LightMix. If your render without the LightMix RE, the SelfIllum is very dark and it is not possible to get the beauty render by compositing LightSelects + SelfIllum.

[vladimir_krastev]

Hi, we will need to investigate this a bit further. The VRaySelfIllumination render element looks like this



regardless if we have LightMix or not. The SelfIllumination layer in the LightMix looks different. I will find our if this is expected or a bug and get back to you.

[petya.georgieva]

Hi there. This might be a long explanation so bear with me

What you're missing from your beauty rebuild in the scene with the light selects is the atmosphere and its Indirect contribution. Aerial perspective is an emitting object and as such it has effect on the overall light. Light selects in their regular modes accept only lights (hence the name). In order to get your missing contribution you will need to add another light select, set it to Light path expression mode and do one of the following:
1. set it to C.*O for and equivalent of Full light contribution of the emitting objects (Aerial Perspective)
2. add two light select LPEs and set one to CO for Self Illumination of the emitting object (Aerial Perspective) and another to C.+O for the Indirect light caused by it, in case you want separate control.
3. Add a regular Self Illumination render element and set your LPE to C.+O

What you're seeing in your render is not wrong or broken but it has been designed for different workflows. The self Illumination render element doesn't include Aerial Perspective (even though it is emissive) as it would be included in Atmospheric Effect. It doesn't include the GI caused by the Aerial perspective as that would usually be included in the GI render element. Those three render elements (Self Illumination, Atmospheric effect and GI) are done this way as they would be composited together in a beauty composite and would provide control over the different components without them overlapping.
A Light select beauty composite is slightly different as it would need to rebuild some of those other Beauty render elements. Since you can't add Emissive objects to the Light select you would need to also use Light Path expressions to get your full beauty rebuild as I described above. And because of the reason I listed above, if you add Self Illumination and Atmospheric effect render elements the will be correct but won't include the contribution you're looking for (because if it were included, it would make the regular beauty rebuild wrong )
As for the Self Illumination in Light Mix - albeit having a similar name, it is not an equivalent to the regular Self Illumination render element. It contains either all parts of the scene which have a self illumination material, or only the parts of the scene which haven't been included in any groups specified by Light Selects. Because it's different it contains both self illumination of Aerial Perspective and its actual indirect light contribution. This is why you see it differently in your render in comparison to a regular Self Illumination.

I hope the above was comprehensive and helped somewhat. I can put forward a request to be able to include Aerial Perspective and Environment Fog as sources for light select if you think that would be helpful and less confusing but otherwise, working with an LPE to add the missing contribution would be the advisable workflow

(As a note, LPEs work best with Brute Force or with Light cache with a higher retrace threshold. If You don't want to do either of those things and don't have any Background added to your scene you can use the expression C.*[OB]; C[OB]; C.+[OB])

[TheScope]

Thanks,
that seems to be working.

It is somewhat confusing that the LightMix RE generates a layer called Self Illumination which actually is different than the SelfIllumination RE.

I hope the above was comprehensive and helped somewhat. I can put forward a request to be able to include Aerial Perspective and Environment Fog as sources for light select if you think that would be helpful

In general we are OK with this solution or using LightMix instead. One thing that is bothering us with LightMix are the very long layer names if lights with one (or several) namespaces are in the scene. Some way to get sensible layer names would be great.