Dome Light / GI - Light Select Bug?

[tylerART]

Hey guys!

Ran into a possible bug. I have a scene being lit by a directional light and dome light. I have a light select set to raw connected to the dome light. I render my scene with GI off, and my dome light select renders correctly. I then turn GI on, and the raw dome light select gets brighter, even though a diffuse dome light select stays the same. Not sure what's going on, but this simple test shows the issue.



In this scene, I have a VRayMtl shader with diffuse set to pure white on both the cube and plane. In addition to the raw light select, I added an additional light select on the dome set to diffuse. In theory, both light selects should look the same, as my diffuse is 1 on everything. When GI is off, both elements are the same, however once I turn GI on, the raw light select is brighter than the diffuse light select.

Dome Raw


Dome Diffuse


This problem only appears to affect dome lights.

GI should not be affecting the dome light, and if it was, it should be consistent across all the light select render elements. Is there something I'm missing here? As far as I understand it, this is incorrect, and is currently causing my studio a headache.

This scene is easy to setup and test yourself, but I can email it to you if you need. Currently, I'm testing on the 3.05.03 build in Maya 2014 on Windows 8, but we're getting the problem at work on VRay 2.4 Maya 2013, Windows 7 as well.

Thanks for the help!

tylerART

[Dariusz Makowski (Dadal)]

Heya

This was already reported once if I'm correct.

Its LC speeding up calculations of direct light. If you untick your "Store direct light" in LC it should come back to old brightness however it was not always the case. And I might be wrong here

[tylerART]

Thanks for the reply Dariusz,

I've tried that and it doesn't fix the problem, this happens even when the lc is off and I'm just using just brute force GI. If I was getting the same result on both light selects, I could see this being ok, but the two light select render elements are different in the same frame, and adding the render elements up doesn't give me the same result as the beauty.

[Svetlozar Draganov]

This is not a bug - it's the way that DomeLight is designed to work in terms of optimization.
When GI is not enabled the dome-light is calculated as regular light but once GI is enabled Vray uses GI rays in order to speed up the process, it works similarly as Store with Irradiance Map option.

In Vray 3.x there is a option to disable that optimization, it called Use MIS(multiple importance sampling) and it is located under the Advanced Options of DomeLight properties.

[tylerART]

Thank you for your help Svetlozar in pointing this out to me. Sure enough I turned off MIS in 3.xx and it renders at the correct value.

Could someone explain to me how then to properly use the brighter (MIS on) raw dome light select render element when compositing a shot together? I've been multiplying my diffuse and raw pass together to get my lighting pass. This works on all my lights, except my dome, and only when I have GI on (or when MIS is on in 3.xx).

Dome (MIS on) + GI on:
(Raw Light Select * diffuse) + spec ≠ lighting

Dome (MIS off) + GI on OR Dome + GI off:
(Raw Light Select * diffuse) + spec = lighting

My studio wants the ability to use color correct and add to the diffuse pass independently of the raw lighting info. Short of subtracting the diffuse and re-adding it, is there a way to composite with a raw light select on a dome (MIS on), with GI on?

Thanks for the help community,

Brent Tyler

[tashko.zashev]

Hi Brent,

This workflow is only possible with MIS option Off for now, otherwise part of the lighting goes to the GI element and messes up the lighting comp .
I have to check if we can do something on that regard.

[vlado]

Hmm, the raw light select element by the diffuse should always equal the lighting element, whether MIS is on or off (because in both cases, the light select and the lighting elements are computed in the same way). If something else comes out, it may mean that there is a bug somewhere.

If you can attach a scene here, however simple, it will be very helpful.

Best regards,
Vlado

[tylerART]

Hey Vlado,

Thanks for checking into this. The forum wouldn't let me upload a .mb file, so here's my file on dropbox.

https://db.tt/4tQHqEzq

I also emailed it to vlado@chaosgroup.com for good measure.

Thanks everyone!

Brent Tyler

[tashko.zashev]

Hi Brent,

Thank you for the file. We are checking it right now and will update the topic when we have any information.