GI Light bounce

[Stirling]

Trying to get my head around why this is happening. I imagine it's quite simple but it's just not clicking.

My camera is pointing up at a building, on top of which is a green roof. I have a grass texture on there (not that I need it as it's not going to be seen but I'm keeping it to illustrate the point). There's a wall next to this grass on top of the building which is a grey fibre cement with no reflection. When I render the wall picks up the green of the grass a huge amount which I'm certain wouldn't happen. Why is it doing this? If I change the colour of the surface to, say, white, the wall turns very bright despite the fact that it's in full shadow.

Think I'm missing something here so any help greatly appreciated.

[ramon_perez]

In real life, if you painted a roof top green and had sunlight hitting it, I'm pretty sure the wall next to it would get a green tint, even if this wall was in shadow. IMO.

Use the Vray Override material in your roof top, and put a dark gray material on the GI override tab..

[Stirling]

I agree that there would be a slight green tint, but this is what I'm getting. Surely it wouldn't make as much difference as this.

[Ihno]

looks like too high diffuse amounts.
Whites shouldn't go over 190x190x190 (unless ists snow) most whites in fact are around 153x153x153
Grass should have an average of something between 45 and 58 roughly.

Or did you change gi multipliers or color mapping settings? Those things can also result in pretty odd bleeds.
Images of your settings/the materials wouldn't harm.

[andy51]

Put your grass tezture inside a VrayOverrideMtl, then in the GI slot, add a VrayMtl, default grey, this will prevent the the colour bleed.. You can also adjust the GI colour from white to black, to increase or decrease the GI effect or even add a very feint green colour to get a more realistic but less saturated green tint as you would get in real life

[Stirling]

Thanks for the advice, the VrayOverrideMtl did the trick. I've never seen the bounce as pronounced as this, trees that are close to the building are casting a bright green tint too.

[^Lele^]

As Ihno said, your shaders look overloaded in the diffuse channel (too bright/too saturated/both), and the Override trick won't solve the general issue with your scene (albeit it will fix the specific problem.).
If you reworked it to maintain physically plausible Albedo values for shaders, not only the green cast would be "just right", but your image contrast would go up, and your rendertime would go down.
Something to keep in mind for the next one, perhaps.

[Stirling]

Will do, thanks. Good to know.

Incidentally I was using the GI Environment override to help soften the shadows. I have to use the sun/sky system to match shadows exactly in the photograph into which the render will be composited but find that sometimes this makes shadows far too dark. Finding a balance is something I always struggle with as the more environment override I add the better the shadows look but the less contrast I get.

[^Lele^]

Incidentally I was using the GI Environment override to help soften the shadows. I have to use the sun/sky system to match shadows exactly in the photograph into which the render will be composited but find that sometimes this makes shadows far too dark.

Exactly!
As the Albedo is too high, the directly lit part will nearly glow under sun and sky (a light type which is by nature a lot more intense, and a lot less forgiving, than "man-made" fixtures.).
To cover for that, one exposes down, meaning the shadows too get darker.
In fact, one could "expose down" the shaders, lowering their Albedo, and that would lower the direct component, without touching shadow brightness.