Refraction colour massivly impacting the rendertimes

[Neilg]

A guy in our office discovered this last night while having major problems with a render.

He originally sent it and had a refraction colour of 254 on all his glass materials - when testing to figure out the crazy rendertimes, changed the colour to 255 and it rendered in nearly 1/4 of the time.
We did another test this morning on a more glass heavy scene - they're identical apart from the refraction colour, and got the same results.

Is this normal?
It's nowhere near as noticable with IR/LC, this is with IR/BF.

Here's an image he knocked up for comparison:

[jacksc02]

it could come from a mesh problem.
Is the glass modelled in max?

[Neilg]

Yeah, splines, outline, extrude. Bit of edit poly for chopping bits out.

[jacksc02]

very odd indeed.
Is the glass excluded from GI?

[Neilg]

Nope, it's just object & material.

I just did this in a fresh scene. It's not as big a difference as the render, but then again adding 30 seconds onto a 45 second render is still a lot.

edit: new image, that scaling on this forum is awful.

[jacksc02]

Try something for me, change the diffuse to black and change the IOR to 1.35 (ior of glass) and see if the problem is still there

[Neilg]

Refract at 255 - 38 seconds.
Refract at 254 - 38 seconds.

By jove, I think he's got it.

[jacksc02]

Nice, your glass will probably look a little better too

[Neilg]

haha, thanks a lot man

Add another beer to the queue!

[jacksc02]

sheesh i should probably claim those one day

[vlado]

If you have non-black diffuse color for your glass, V-Ray will have to calculate diffuse surface lighting as soon as your refraction color gets below pure white - this means calculating direct lighting shadows and global illumination, in addition to the refractions. All of this is skipped when the diffuse color is black or when the refraction color is pure white. Unless you specifically need it, always set the diffuse color of glass materials to black.

Best regards,
Vlado

[simmsimaging]

I've been playing with a scene off and on that has some frosty ice cubes in it. The diffuse color needs to be non-black to help get the right look, but the material is extremely slow to render. Is there any benefit to having the diffuse be pure white? It won't look as good as mixed tones, but would be better than black for sure.