Standard Surface Material BSDF model? Tweet

[Andreas_Resch]

Can one of the devs maybe tell us, what BSDF model is used for specular reflections in Standard Surface Materials? As there's no option to choose one, it's hard to tell.

[elsksa]

Standard Surface Materials is from Autodesk Arnold. Their documentation should include it and I bet it's GGX (edit: not Energy Preserving).

[elsksa]

Transparency transparency simple pass-through (can be thought of as a delta BTDF)
Coating specular_brdf dielectric microfacet BRDF (GGX)
Emission emission diffuse emission
Metal metal_brdf conductor microfacet BRDF (GGX)
Specular reflection specular_brdf dielectric microfacet BRDF (GGX)
Specular transmission* specular_btdf dielectric microfacet BTDF (GGX)
Sheen* sheen_brdf retro-reflective dielectric microfacet BRDF [Estevez2017]
Subsurface scattering* subsurface subsurface scattering (e.g. diffusion or random-walk)
Diffuse transmission* diffuse_btdf diffuse microfacet BTDF (Oren-Nayar)
Diffuse reflection diffuse_brdf diffuse microfacet BRDF (Oren-Nayar)

Specular reflection
This layer models a GGX microfacet dielectric BRDF specular_brdf, conceptually right under the coating. Similarly to the coating, this BRDF is not itself energy conserving due to Fresnel laws, and the energy that is not reflected is completely transmitted to the underlying layers. The closure combination we use which expresses the energy balance between the reflected and transmitted specular lobes is then...

Coating
The topmost scattering layer is a dielectric coating with a GGX microfacet BRDF closure coat_brdf. As a dielectric, this BRDF is not energy preserving (i.e. its directional reflectance is generally less than one) as it obeys Fresnel reflection laws. The layer is assumed to be infinitely thin, and the remaining non-reflected light is passed directly to the underlying layer without refraction. The reflection color is fixed to white, though the coat medium color can be user controlled.

[Andreas_Resch]

Thanks. Looks like it's "almost" energy preservation.

[EDIT] Looks like it's "energy conserving by default". It's mentioned in the video here ... https://youtu.be/ZNLUqqACa4U?t=330

[elsksa]

Anyone can do a Furnace test to find out. No need for YouTube!

Here is one here.

[Andreas_Resch]

Nothing wrong with a confirmation from somebody who did the tests for you - or read the papers. Anyway - good to have some clarification on that.

[elsksa]

From a quick test, it appears to be Energy C, indeed.

Seems like the Autodesk Doc isn't up to date. A bit lazy to search for the latest updates, maybe their change logs or a GitHub somewhere.

[Andreas_Resch]

Thanks for the confirmation. I'll test it as well, just to learn more about the furnace test.

[elsksa]

Nothing wrong with a confirmation from somebody who did the tests for you - or read the papers. Anyway - good to have some clarification on that.

Absolutely. I just said there was no need as this is possible for everyone to proof test it. And YouTube videos aren't the most practical (longer method for seeking info, especially when it lacks time stamps and often unreliable) IMO.

[Andreas_Resch]

You have to grab info from wherever you can get it. And especially with Octane (even more so the Blender version), resources are sparse anyway. I probably have watched more C4D videos about Octane than Blender ones. But if you stick to the basis it's still a good resource.