vray domelight and GI - strange behavior?

[flipside]

Hi,

I have some problems with the vray domelight. I noticed that when rendering a simple object, turning off and on GI results in strange lighting differences, when using a domelight as the only lightsource.

Here's 2 examples with a vrayplanelight, and how it should behave.

Planelight, GI off:




Planelight, GI on. The model is lit a bit more from the bottom, because of the plane bouncing off light. This all looks normal to me. Note that the groundplane lighting doesn't change much.





Now I do the same with a domelight instead of planelight:
Image without GI:




And with GI. Notice that the groundplane gets DARKER (save images and flip back and forward). Normally the groundplane shouldn't change much in lighting, since the bounced light from the object onto the plane shouldn't make a big difference. Especially not far away from the object. And if it would make a difference, then I would expect the groundplane to be brighter because of the added light, not darker.




Any ideas on why this is happening? Is this normal behavior?

Kind regards,

wouter

[vlado]

I can't really reproduce that - works fine for me. Do you use any color mapping?

Best regards,
Vlado

[flipside]

Hey Vlado. No I'm not using any color mapping for now. Here's the scene, max2009 vraySP3: www.aversis.be/vrayrhino/std_scene_01.zip

[flipside]

when I use a hdri texture on the domelight, the problem goes away.

[vlado]

It happens because you have changed the primary/secondary GI multipliers. The calculation of the dome light is split between direct lighting and GI, so when you use GI with lower multipliers, you get a darker result.

Best regards,
Vlado

[flipside]

Can you explain that a bit more? So when using GI, 50% of the light is direct, and 50% of it is GI light (calculated by irmap+brute force in this case)?

Why isn't the same thing happening when using a hdri texture on the domelight?

Just curious (because I almost always decrease the GI multipliers, it adds some contrast and decreases the amount of bounced light and maybe therefore also calculation time?)

[vlado]

V-Ray will automatically split the calculations between direct and indirect lighting based on the required light sampling; if you use a HDR map, the direct lighting calculations will have a greater weight in the final result, so the effect of the smaller GI multipliers will not be that obvious.

In case you are wondering why V-Ray has to split the lighting this way, it is because for speed reasons - it helps to get a cleaner result faster. Here is an illustration of how this works:

http://www.spot3d.com/vray/help/150S...lights.htm#ex7

Best regards,
Vlado

[flipside]

Thanks Vlado, makes sense now