Arnold 5 for Maya, what is the correct value for specular color/weight for non metals with the Substance workflow?

[giandosopaolo]

Hi all

I have been making some tests with Substances and Arnold 5.
So far my biggest doubt is the value to give to the Specular Color (or specular Weight parameter as it is a multiplier) when you are dealing with a non metal material or a partially mixed metallic material like a rusty metal.

I assumed at the beginning that the correct way was to leave both Specular Weight and Specular Color set to 1 and "white", as the Arnold 5 guide says: "When Metalness is enabled, the Specular Weight and Specular Color only control the edge tint, while the Base is still affected by roughness."

Since in the PBR workflow from substance there is always a map connected to Metalness, even if the map is black Metalness SHOULD BE enabled (although maybe for enabled they mean a non fully black map, this is really unclear to me).

By reading the PBR book they provide I seem to understand that, at least in Substance, when a pixel in the Metalness map is black Substance assume it is a dielectric material and sets the specular weight/color to a value of 4% (0.04), which is the avarage specular weight/color for most of the dielectric materials.

Since I am dealing with non metallic substances (a roof, a patch of mud and rusty metal), I assumed that the Arnold material would have behaved similarly:that specular color/

specular weight in the aiStandard material would have had no influence on the final result for the dielectric parts and that the shader would have set it "internally" to 0.04.
However the results from leaving specular set to the max were way to bright for a correct realistic PBR.

So I made a wedge of 3 different substances(a roof, a patch of mud and rusty metal) with a specular value of 1 (max), 0.25, and 0.
The result shows that the value specified in the specular color or in the specular weight does influence the look of non metal materials a great deal, as shown by the images I linked, which are the beauty, the diffuse pass and the specular pass. Not only in the edge tint of the metal bits (as shown in the rusty substance) but also everywhere.

Am I doing something wrong? I know it is not a perfect system but I am trying ti understand what should be the default setting at least in theory.
Given these results, my question is:What is the correct value to give to the Specular Weight/Color parameters for a correct PBR? 0.04 or 1?

Many thanks

Paolo