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Paolo Giandoso avatar image
Paolo Giandoso asked ·

PBR and Diffuse Roughness, Specular Weight and Specular Color attributes

Hi all.

I have 3 questions on the PBR Workflow using maps from Substance Painter or Substance Deaigner in Arnold 5:

If you are using a set of PBR maps from Substance, so Base Color, Metallic, Roughness and Normals,what value or map should be used in

Diffuse Roughness,

Specular Weight and

Specular Color?

  • Diffuse Roughness is a slider that varies between Lambert and Oren-Nayar diffuse system right? So concrete and plaster and sand should get higher values, but is this slider something to map with the same Roughness map we use for the Specular Roughness? If not, is there a list of values for materials like skin, wood, sand etc? Is this attribute something allegorithmic does not consider?
  • Specular color should be white for dielectrics and for conductors it represent the color of the Specular reflection at glancing angle, this is clear. So if I know my material is, let say, bronze, I can find out the exact rgb values from the internet.

But what if my material is rusty, so the Metallic Map from Substance has a mix of areas of black and white. Should the Specular Color map be white in the same areas where the metallic map is black (dielectric) and colored when the metallic map is white (conductor)?

  • Specular weight tells how strong the Specular reflection are, regardless of their roughness in a range of 0 to 1. Should it be left on 1 when using the Substance PBR workflow? Why? Or they should be connected to a map?

Many thanks for the help!

P

arnold5shadersubstance painterpbr
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Max Tarpini avatar image
Max Tarpini answered ·

You better answer some of these questions yourself by doing a little project in painter for example. I mean what's better than get visual feedback while you paint in your channels, you'll see in an instant what's needed and what not.

About diffuse roughness.. it's complicated.. read on at your own risk.. you'll just get more confused if you do probably :) Basically it's a broken thing and really not integrated in the whole physically based model if there's something like that at all (roughly speaking .. in a PBR mat.. everything stays together just by obeying energy conservation and fresnel reflectance factor). It should model backscattering that depends heavily on reflections .. ie. multiple-scattering between substrate and interface (the real intuition here is that the diffuse is the refractive portion of the BSDF.. ie light has to pass the reflection layer to get to the diffuse right !?.. so at the interface we really should account for transmission and that would model automagically the diffuse roughness aka backscattering). But.. first reflections layers don't come in a unified model (ie. clear coat is almost indipendent from base reflections) .. second they do not interact with the base diffuse as they should.

Generally you just eyeball stuff :) that also means that PBR is just a workflow .. you won't get straight answers to resolve let's say scientifically your problems but just 'modi operandi' .. you'll find ppl saying they do that and that in that way for that reason and so on, it's all good until it looks good :)

So no exact values for the diffuse roughness, sure you can account for your intuition where for very rough dielectrics you may increase accordling your diffuse roughness value if that make the overall material looking better.

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Paolo Giandoso avatar image
Paolo Giandoso answered ·

This is also interesting. In this Disney paper available online :

Physically-Based Shading at Disney by Brent Burley, Walt Disney Animation Studios

, if I got it correctly (big chance I didn't), they seem to say that in the Disney PBR model the one roughness parameter controls both the Diffuse (backscattering) and Specular Roughness.. This makes so much sense :)

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Paolo Giandoso avatar image
Paolo Giandoso answered ·

Thanks for the answer Max. It's very clear.

I didn't know that backscattering was a synonymous for diffuse roughness. Very interesting!

I'm doing tests with Substance of course, but the reason why I asked here on the forum is that, since Arnold 5 added the PBR metallicity workflow and there is such an exhaustive online manual on substances integration, I was hoping Solidangle also had specifically integrated the other parameters in someway and I was simply missing something.

For some specific things, like the Specular color and weight, I think the help section is maybe a bit incomplete and it should be addressed. It can not hurt right? After all we all look at the help section to benefit from the creators and each other experience and minimize the testing times :)

Also, if it is a broken thing wouldn't be the right thing to address this in the manual?

Nevertheless I'm happy to eyeball things the usual way as you suggested ;)

I'll test if there is any benefit to plug the Roughness map also in the diffuse roughness :)!

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