How to deal with VrayMaterial and Modo Fresnel

[Robert]

I wonder how to use the Vray Material when you want to translate Modos settings with Fresnel.
For example:
Reflection: 20%
Reflection Falloff: 70%

How would set that in the Vray Material where you have just a color input and an IOR ?
Or do I need to do that with a gradient ramp ?

[Christopher3D]

I'm testing out Vray for Modo and I'm seeing this also. Vray isn't using Modo's Fresnel reflectance and it looks like it's because Vray's material doesn't support it. I'm kind of astonished by this, unless there's something I'm missing. It's a fundamental feature of most modern BRDFs for conductive type materials.

[vlado]

Despite your astonishment, there hasn't been a specific need to build this functionality into the V-Ray material; the falloff effect is usually achieved with a Falloff map. Besides we also translate the Modo material and this effect is taken into account in that case. So you can just keep using it.

See also my reply in the other thread.

Best regards,
Vlado

[Christopher3D]

Ok, so I did some digging and see that there's a V-Ray Fresnel texture layer that modifies a V-Ray material when set to 'vrml Reflection Color' mode. This makes the metal behave with the edge reflectance which is awesome.

But I'm not seeing that a Modo's Fresnel function is taken into account, the surface reflects with a uniform color. But at least I know how to achieve this so I'm happy.

Thanks.

[Robert]

Yes that exist. The Problem however remain. The Modo material is not translated correctly. I noticed that before that the renderings with lot reflective materials look more realistic in Modo. However its a very seldom case, the only strong colored materials people might run into is Gold and there is so much more about correct shading of Gold then just the base shader let you do.
Also for Modo user its quite uncommon to develop shaders with lot of blends, like whats really common with Vray. Yet its not the best workflow and I hope the alShader will replace most of that in the future. Really glad that Vlado is aware of the problem and has a solution.

[vladimir.nedev]

How would set that in the Vray Material where you have just a color input and an IOR ?
Or do I need to do that with a gradient ramp ?

For completely matching results, you would need a gradient texture, yes.
You can convert between specular amount (i.e. incident reflection amount) and IOR, by using the formula:
specular_amount = ((ior-1)/(ior+1))^2 . To get the IOR from specular amount, you need to solve a quadratic equation.

There are two things you cannot match without a gradient texture:
- the MODO and V-Ray materials use different Fresnel formulas (MODO uses the Schlick Fresnel approximation, V-Ray uses the actual formula for dielectrics).
- the glancing reflection color is always white in the MODO material, while the V-Ray material uses the Reflection Color for this

But I'm not seeing that a Modo's Fresnel function is taken into account, the surface reflects with a uniform color. But at least I know how to achieve this so I'm happy.

That's not quite true, the fresnel is taken into account, and we even use the Schlick Fresnel approximation for MODO's material.

There are some things that are indeed wrong in the translation:
- the glancing reflection color is not white
- the fresnel channel is not used the same way that MODO uses it

There are other problems with the translation of the MODO material at the moment:
- we don't yet have the Ashikhmin BRDF implemented, which is used by MODO's "Energy Conserving" model
- the roughness translation is wrong in the "Energy Conserving" model
- the clear coat layer is not using Fresnel
- clear coat roughness and bump are ignored

I plan to fix all of these in one update, so the material behavior doesn't change too often.
Won't happen for the next update that will have a demo ( 3.40.02 ), hopefully for the update after that.

Greetings,
Vladimir Nedev

[Robert]

interesting.. my preference goes for alShader. The main reason is that it has so many things possible that are not so hard to do with VrayMats.