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From what I can understand, the Principled BSDF incorporates a ton of 'PBR-friendly' parameters into one compact box of sorts. For instance, I can attenuate a material's roughness, transmittivity and what not

One thing I am not able to zero in on is as to how Fresnel is incorporated into Principled BSDF. I'd figured at first that the IOR parameter, might have a hand in doing so, since we find it on the Fresnel node as well

However, it yields no difference:

enter image description hereenter image description here

Clearly, the IOR parameter does not seem to affect how 'Fresnel-y' the material is in Principled BSDF for a lack of better words. So how exactly does Principled BSDF handle Fresnel? How would I go about changing whatever values control the Fresnel, just as I would with a Fresnel node?

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The use of Fresnel is described in Physically Based Shading at Disney by Brent Burley. The Blender Principled BSDF is an implementation of the the Disney Principled Shader.

There are several different implementations of the principled shader, including one in OSL, one as a kernel for GPU processing, and this glsl version.

Fresnel is used in several places in the principled shader, including diffuse, specular, and metallic calculations, as well as IOR calculations. You'll have to read the source code to see exactly how.

You can't change the control values directly.

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  • $\begingroup$ So in a sense it has been 'abstracted' away from user control AND is involved in various different portions of the Principled BSDF if I am getting this right? Sorry, I haven't quite started learning graphics programming yet, so GLSL and in conjunction, the GLSL code is hard for me to understand $\endgroup$
    – Hash
    Apr 21 at 17:18
  • $\begingroup$ @Hash, yes you're getting that right. I don't read glsls, either, or osl, which is the other version that's easy to find in the github repository. I think you'd be better off getting the precise details from the Disney paper. $\endgroup$ Apr 21 at 17:28

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