As far as I know, Principled shader is PBR, and the purpose of PBR is to produce materials constrained by physics (mainly, law of energy conservation). But Principled shader has controls for both Specular intensity and Roughness (blurriness of specular highlight and reflection), just like non-physical shaders of old. In other programs, such as game engines that support PBR, both of these parameters controlled by Roughness (or Smoothness) alone. The rougher the surface, the lower is the intensity of highlights. Does that mean that Principled shader can potentially produce non-physical behavior? If so, what is the correct relation between Roughness and Specular values?
The specular value gets converted to a IOR value inside the shader.IE 0.5 spec are 1.5 IOR.
here the specular formular.
The roughness is based on the GGX microfacet model.
In combination both the specular and the roughness,are PBR correct.
As sayed the specular gives the IOR Fresnel curve based on the posted formular.And the roughness is like tiny micro mirrors on the surface,the higher the roughness,the more diffuse the reflection gets,because the microfacets orientation angles getting greater.
I see, so specular doesnt mean direct intensity of the highlight, thank you.
It does, sort of. Facing highlight will get much darker with lowered specular, but it will always to towards fully specular at the extreme edge as long as specular > 0 (and roughness = 0).
Sphere on the left has specular 0.001, the right one has default specular of 0.5.
There are reasons you want to deviate from the “proper IOR” of about 1.5 for dielectrics (0.5 specular), i.e. a typical rubber microscopic structure will cause more (secondary) rays to be absorbed - shadowing is not something we obtain in shading using normal maps. 0.5 is a fine starting point for a perfectly flat structure, then use specular to adjust to match observed properties.