Principled BSDF uses fresnel reflections for dielectric materials. Dielectrics have two fresnel knobs: specular for facing reflections and IOR for transmissions.
- Principled BSDF
- Does the Principled Shader IOR value effect specular reflection on a transmission material
My googling suggests that principled BSDF uses complex fresnel reflections for metallic materials even though the IOR parameters cannot be tuned:
- Complex Fresnel texture node for Cycles
- What complex index of refraction values are used in metallic input in the Principled BSDF
- Does the Principled BSDF node have built-in Fresnel? How could I adjust it if I wanted to?
Metallic materials have a very different reflectance curve vs. angle of incidence. This arises from the complex component of the IOR known as the coefficient of extinction.
The complex component results not only in a shift in reflectance based on angle of incidence, but also a shift in reflectance tint. The base color knob sets the tint for facing reflectance in metallic materials.
- Why don't dielectric materials have coloured reflections like conductors?
- How can metals absorb light?
A fully metallic principled BSDF node represents an unknown metal, in the sense that the complex component is unknown. Such a metal should have:
- Shift in reflectance based on angle of incidence.
- Shift in tint based on angle of incidence.
I don't see that. I see a perfectly specular metal with what appears to be no fresnel. I believe this is why online tutorials often plug a layer weight fresnel node into the principled BSDF node:
So is principled BSDF suitable for metals in terms of Fresnel?
I don't know how subtle metallic fresnel is. Is the shift in tint usually a shift in saturation, or an entirely new hue? It would be useful to side-by-side renders demonstrating:
- Consistent 50% specular / 50% diffuse without fresnel.
- Non-metallic fresnel demonstrating shift in reflectance.
- Metallic fresnel demonstrating shift in reflectance and tint.
There are a few complex fresnel shaders available online: