View Full Version : Mental Ray - GI only vs GI with FG - brightness changes

04 April 2006, 09:10 PM

I need some help with a strange result I have when I render any scene using GI and FG together in MR. I'm not an expert user, but I'm not a beginner neither... So... It may look simple, but I just can't figure it out. Some time ago I spent a lot of time trying to solve it. I think I read almost any article or tutorial or thread that could possibly have anything to do with this, but no success. Then I let it go because I could work my way out using FG multibounces in most cases. But I know in some situations it is better to use GI combined with FG, and I really have got to RElearn how to do that. Well... Maybe there isn't anything wrong and I just couldn't get used to MR 3.4. Let's see...

Here are two simple scenes so that you can understand the problem (if there's any).

In each scene there's a render using GI only and a render using GI and FG together. Shouldn't the overall brightness of both images in each scene be the same?! I mean... Isn't it strange that the image gets much darker when FG is on?! I always heard that GI and FG should be better tweaked separately, right? And so I did until some time ago. But if the image is much brighter when FG is off, it's not very usefull to set GI separetely! The image has to be blown out so that the final result is acceptable when I turn FG on.

And what about the bright areas at the bottom corners of the first scene, for example? The shader is a simple lambert. No Occlusion shaders were used. But anyway, shouldn't there be some contact shadows right there instead of blown out areas?!

What am I missing here?! Is it just how it's supposed to be after the changes done to FG in MR 3.4? I mean... Does it have to do with things like final gather only considering a single diffuse bounce when GI is on or maybe the multipliers added to GI and FG? Or can it be on purpose so that the first bounce of light only comes from direct lighting?

Could anyone show me how to setup that simple first scene, for example, only for a GI pass using photons and FG?! The image posted is definitely not ok!

My settings were the same in both cases:
- a room with a window;
- a spot light emitting photons;
- GI settings: radius 0.5, accuracy 100
- FG setting: 100 rays, min 1, max 20, pixel size.

I think these settings are alright. Just a simple setup to ilustrate the problem.

Well... Any help appreciated...
Thank you,


04 April 2006, 10:21 PM
I'm sorry... I thought the problem was gone with the reinstallation, but the fact is that it only worked in a couple of tests I made.

The way I really solved the problem was by using only mental ray shaders. Including photonic materials, obviously. This way, the FG really works as it used to, not making the whole scene much darker.

So maybe the real answer is: when using GI in mental ray, use mental ray shaders!!! It's a pitty because I love Maya shaders... I think they're much easier to setup, specially in more complex shading networks. And sometimes a simple maya material would do. But now I don't feel secure to use them very often anymore.


Old post:
"Ok... You'll think I'm kidding you guys, but things just got back to normal now.
Today I got other Maya installation files to see if that could solve the problem. That's the reason why I waited until today to post this thread. Then I installed it. Made like 3 or 4 tests and as the problem continued, I started the thread.

Some minutes later I opened Maya again to try the same last test and guess what... It's working great! The only explanation is that it was a problem with my other installation files.

I'm really sorry to bother you guys with that. I swear that I made the test with the new installation before I started the thread. And I had already reinstalled with old files a couple of times. I can't believe I had this thing going on for months...

Anyway... If someone gets the same problem, try reinstalling with different files.

Thank you,

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04 April 2006, 10:21 PM
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