Unexpected Fresnel Factor with Principled BSDF
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I've been using the Principled BSDF node for the last few months. During this time I have noticed a fresnel transition between transparency and reflection which is too sudden. Below I will document examples which work and examples which exhibit the broken behavior.



Example on water surface using Principled BSDF as a volumetric shader. IOR is set to 1.33 and Specular to 0.5

Example showing the Fresnel node with expected smooth Fresnel response

Original V-Ray scene

Converting to Cycles, note reflection on tea using IOR 1.32 and Specular 0.5:

IOR set to 1.0:

Brecht Van Lommel (brecht) triaged this task as Incomplete priority.Nov 10 2017, 1:29 PM

Please follow the bug reporting template and provide operating system, Blender version, .blend file, etc.:

OK I will try to reproduce this problem in Blender. Currently I am experiencing it when using the cycles API directly.

I have been unable to reproduce this in the current version of blender yet. Was there any updates to the principled shader between 1.79 and 1.79.1?

There were some fixes: rBc040ded, rBd85a0a7, rB9e929c9.

Or maybe the normals are pointing the wrong way?

Hi Brecht, thanks so much for the suggestion, the problem seems to be that my normals in particular cases were flipped. I can reproduce it in blender by manually flipping the normals. Please close this for now and I will re-open if I have further problems.

Brecht Van Lommel (brecht) claimed this task.