How does 2sidedMTL + VrayMTL translucency work?

[LudvikKoutny]

Hi,

I've just found out something that almost broke my brain. When I have VrayMTL fed into 2sidedMTL for translucency, setting Translucency in VrayMTL to any other modes than none actually does something, despite the fact that VrayMTL has no refraction what so ever. But ONLY parameter that works is Back-side color, even though other controls being unfrozen as well.

While this is great, as now I don't need two separate materials if i want leaf backside to be brighter, it's also extremely confusing. Why is it not documented anywhere? And how does it exactly work?

And is there any way to prevent loss of reflection energy when using 2sidedMTL? I mean if I want for example leaves after rain, with VRay2Sided MTL, that is something impossible to achieve, while it is easy to achieve with any other mainstream renderer. Of course I would like to avoid having to wrap every such material in VrayBlend just to have correct reflection intensity, so please don't suggest that

[vlado]

While this is great, as now I don't need two separate materials if I want leaf backside to be brighter, it's also extremely confusing. Why is it not documented anywhere? And how does it exactly work?

While it sort of works, it doesn't mean that it makes any sense It's definitely not a setup that I would recommend. Translucency in VRayMtl is more like volume sub-surface scattering rather than thin-walled diffuse translucency.

And is there any way to prevent loss of reflection energy when using 2sidedMTL? I mean if I want for example leaves after rain, with VRay2Sided MTL, that is something impossible to achieve,while it is easy to achieve with any other mainstream renderer.

Can you show me a) what you mean by energy loss and b) how you would set it up in the other renderer that you liked since before it was mainstream?

Best regards,
Vlado

[vlado]

Ok, nevermind, I seem to get it - you want the diffuse translucency to not affect reflections, just the base diffuse color. For the moment as you noted there's no other way than using a blend material for the reflection layer. It will work fine with the blend, although you are right that the reflections might take more tinkering to get the right strength.

For future versions, we can probably think of a an easier way to set it up.

Best regards,
Vlado

[LudvikKoutny]

Yes, for the easier implementation, I have an idea:

AFAIK, reflection in VrayMTL has a priority over refraction, therefore increasing refraction keeps reflection strength intact. How about, aside from current modes (hard, soft, hybrid) introducing another mode called "thin", which would do exactly the same type of backside diffuse scattering as 2sidedMTL does, just without need to have two materials to build anything translucent, and with correct reflective energy preservation. And refraction color would then act the same way Translucency amount color works in 2sidedMTL.

As for the first thing I mentioned, I am afraid there was a misunderstanding, so I've made a video :

https://www.youtube.com/watch?v=dwge4WJxCpY

[vlado]

How about, aside from current modes (hard, soft, hybrid) introducing another mode called "thin", which would do exactly the same type of backside diffuse scattering as 2sidedMTL does, just without need to have two materials to build anything translucent, and with correct reflective energy preservation.

Yes, I thought the same exact thing will make a note to look into it.

Thanks a lot for the video - I've no idea what happens in that case either, but I'll debug and will let you know.

Best regards,
Vlado

[LudvikKoutny]

Just two more things:

1, I think I once saw this workflow being used by some of the Evermotion archmodels tree libraries. They had something plugged in translucency slot of VrayMTL, and SSS mode set to hybrid, but refraction was off, and that VRayMTL was plugged into 2sidedMTL. At the time I thought they had just no idea what they were doing, but now it starts to make sense. So in case it would turn out to be a bug, I would definitely not fix is at is could break backwards compatibility with lots of stuff. But it looks more like some old forgotten mechanism than a bug.

2, About the new thin mode suggestion, a drawback here would be users would then not be able to have a material that's translucent and refractive at the same time. Mental Ray's arch&design material uses this solution, but it has one additional parameter that defines how much % of refraction is actually used for translucency. So to create for example leaves, you first have to set that parameter to 1, so that 100% of refraction transparency is used as translucency. But then again, I don't know how many people would even want to have partially refractive, partially translucent material, so I am not sure if it would be worth cluttering VrayMTL UI with another spinner.

[LudvikKoutny]

Has the mystery been resolved? :O