Vray material reflect on backside question

[seandunderdale]

Hey guys. Ive always turned on "reflect on backside" for car CAD window glass, headlights and tail lights (at the expence of render times) as Ive always felt it gave more depth and realism. We had a chat at work and it was suggested that reflect on backside is only meant for single sided geo (arch vis windows without thickness for eg). The geo I apply it to has thickness, so I thought the reflect on backside meant that the internal glass bounces get calculated more accurately.

This is from the documentation...
Reflect On Back Side – When disabled, V-Ray will calculate reflections for the front side of objects only. Checking it will make V-Ray calculate the reflections for the back sides of objects too.

I dont feel like that really helps describe whether the feature results in a more real world result, or if it is a fix for single sided geo like windows without thickness.

Any thoughts?

[damaggio]

I never turn it on, just leave at default, as for windows the IOR has to be lowered quite a lot if it has no thickness, faking it.
Vray is so fast now and optimized that has no problems tracing thru glass with thickness .

[seandunderdale]

The IOR for refraction needs to be 1.0 if the glass has no thickness, but I find the 1.49 IOR of glass works fine for the reflection. (unticking lock reflect + refract IOR). Ive got no issue with the speed of Vray, and I know its fine handling complex raytracing with geo that has thickness, but I think what Im trying to understand from the devs is whether "reflect on backside" is a real word effect, or a render time workaround for insufficient geo. If Im going for total realism, and Ive modelled my glass to have bevels and thickness, do I need it on for the best result or not? I can look at my results and say whether I like it or not, but when there is a debate about what the tick box is for, the documentation isnt too helpful with regards to what "reflect on backside" is meant for.

At the risk of repeating myself, is it for single sided glass? Or glass with real world thickness, but it gives you a more realistic result?

[SonyBoy]

Hope to get an answer from the Chaos devs too. My guess is that it IS a workaround, since obviously a single sided surface will not have normals facing away from the camera and therefore will not have any reflections. A surface with thickness (assuming back surface normals face correctly away from the camera) should generate reflections which ought to be visible through the refracted front.

[seandunderdale]

Any thoughta from the devs or anyone else about the purpose of reflect on backside? Do people use it to make realistic glass effects? Or use it only to make single sided geo have a reflection?

[yolov]

Single-sided objects might be a bit misleading here. The effect adds realism in some cases, where it would be obvious that it's missing.
One such example would be diamonds. Since they refract, the ray hits the 'back' side of the face inside the diamond and it can add reflection there too.
Don't know how much it helps with glass, but the effect will be more prominent when there's multiple panes of glass stacked behind one another, for example.

[seandunderdale]

Thanks Yolov. Thats a big help. What we've found in our automotive renders is that turning it on can boost windscreen reflections by quite a lot. I might be answering my own question here, but would you suggest the reason to have it on in this case would be a subjective one, eg. if it looks good, rather than its technically correct, or recommended, to create a more realistic glass effect. Similarly would a tail light or headlight with many glassy layers match your example of multiple glass panes stacked behind each other do you think?

[matthew999]

hey sean, i found that i get a doube refection if turned on, one being in the refract pass as its has passed through one side of the glass so now appears in refect render element. This is not helpfull for us to add drivers in post as we now have a refect in the refract pass. So we just leave it off and boost reflecton in post or on shader to compensate

[seandunderdale]

Hey Matthew. You're right, it probably doesn't help in some circumstances, but that second reflection I've found shows up in the stand in car photography I'm given, so I usually just try to find a way to make it work.