recreate VRayAlSurface with VrayBlendMtl

[jasonhuang1115]

Hi, edit: sample scene attached. Render comparison attached.
VrayAlSurfaceVSVrayBlendMtl.zip


Is it possible to recreate the result of a VrayAlSurface from blending two or three VrayMtl?
For example, in a simple case where the VrayAlSurface has gray diffuse and one reflection lobe, no SSS, no bump.
Diffuse Color: 0.5
Diffuse Strength: 1.0

Reflection 1 Color: 1.0
Reflection 1 strength: 1.0
Reflection 1 roughness: 0.35
Reflection 1 IOR: 1.4
Reflection 1 distribution: GGX

I tried to recreate it by using two VrayMtls and blend them in VrayBlendMtl in additive mode.
Base Material: VrayMtl 1 (the diffuse component)
Diffuse Color: 0.5
Amount: 1.0
Reflection color: 0

Coat Materil 0: VrayMtl 2 (the reflection 1 component). Additive mode. Blend Amount 1.
Diffuse Amount: 0
Brdf type: GGX
reflection color: 1.0
reflection amount: 1.0
reflection roughness: 0.35
Glossy fresnel: on
Fresnel IOR: 1.4
GGX tail falloff: 2
metalness: 0

The results I got are different between the two. Mostly in reflection closer to the glancing angles.
Why do they differ? Also,

Maya 2018.2, Vray 4.03.04 build 28563)

Thanks!

[dgruwier]

You have glossy fresnel enabled in the VRayMtl, that affects the glancing reflections quite a bit. Also it would be helpful to see your test renders

[jasonhuang1115]

Attached is a wipe between VrayAlSurface vs VrayBlendMtl applied on a sphere in the full white dome.
A sample scene that generates the output is also attached.

I observe the VrayBlendMtl result seems to have an overall stronger reflection as if the fresnel is of a higher value.
The glossy fresnel is turned on as if I turn it off, the reflection on the edge would be even brighter and more different from the VrayAlSurface output.

[jasonhuang1115]

bump this thread up with sample scene and renders attached to the first post.

[yolov]

I don't think you will be able to match alSurface with anything, as it does things the other shaders don't, especially VRayMtl. The big question here is why you need to do that?

[jasonhuang1115]

I don't think you will be able to match alSurface with anything, as it does things the other shaders don't, especially VRayMtl. The big question here is why you need to do that?

Hey Alex, couple reasons:
1) I would like to be able to use the material_select_RE to output each individual spec lobe. If I can successfully match Alsruface by using a VrayBlendMtl (in additive mode) with 3 VraMtl (1 for diffuse + 2 for spec lobes), then I can dial the diffuse, spec 1, and spec 2 in post.
2) I am curious how VrayAlsurface handle the blending of the two spec lobe (or reflection lobe). When both reflection 1 strength and reflection 2 strength are set to 1.0, does that mean is a 50/50 split?

[sharktacos]

The AL surface is energy conserving. So the two spec lobes are normalized, and all the lobes together (in your case the diffuse too) balence each other out with the fresnel to be energy conserving. A blend material in additive mode is not energy conserving. That’s why it gets brighter. To have multiple spec lobes with a blend material it would need to be re-written to use glossy fresnel to blend the spec lobes in an energy conserving PBR way.

[sharktacos]

You should be able to get the same results with a AL surface & a Vray Mtl with the setting you describe (diffuse and 1 spec lobe) since a Vray Mtl is also energy conserving. And you would not be able to recreate a Vray Mtl with a blend material for the same reason.

[jasonhuang1115]

You should be able to get the same results with a AL surface & a Vray Mtl with the setting you describe (diffuse and 1 spec lobe) since a Vray Mtl is also energy conserving.

sharktacos This is exactly what I was wondering. I am only blending two VrayMtl (so diffuse + 1 spec lobe) using the additive mode and hence expecting a match

And you would not be able to recreate a Vray Mtl with a blend material for the same reason.

Can you elaborate on this part? A VrayMtl is energy-conserved with one spec lobe. I should be able to re-create that with two VrayMtl blended in the additive mode where one represents the diffuse and the other represent the spec lobe.

[sharktacos]

Jason,

A VrayMtl, like any BRDF, is made to be energy conserving. The basic way this works is that before the spec is added to the diffuse, it is first subtracted from the diffuse. That is how energy conservation is ensured. You can see this when looking at the diffuse RE on a VrayMtl with dielectric Fresnel (for example IOR 1.5). Render a sphere and notice how instead of being a flat color it is dark on the edges. That is the spec in the Fresnel being subtracted from the diffuse before it is added.

In contrast, a BlendMtl in additive mode does not subtract the spec from the diffuse before adding it, it simply adds it. It is not energy conserving at all. In order to create this with a BlendMtl in additive mode, the shader would need to be re-written to use Fresnel masking.

Personally I hope they just add a second spec lobe to the VrayMtl for VrayNextNext. Seems to the trend in PBR rendering now (we have 2 spec lobes in ALsurface, ArnoldStandard and DisneyPxr shaders). It's super useful to have that second spec lobe artistically.

[jasonhuang1115]

Thank you Derek. I suppose that explains the result I got with VrayBlendMtl where the edge of the sphere is a bit brighter. I will do the diffuse_RE test to learn.