[Bf-cycles] Cycles Fresnel behaviour

[Marco G]

Hi all,
i always thought Cycles Fresnel was somewhat incorrect/weak compared to
other engines i've used, but never got what's wrong, so I never mentioned.
I'm still unsure if it's the case, i'd like to hear from you devs what
about it, but seems it's really a known problem and Cycles still does not
implement the correct approach.

So, Fresnel in Cycles ("Shader A" Mixed via Fresnel Node with "Glossy
BSDF") always gave pretty dull facing highlights, almost absent, and
extreme reflections (and dark edges) at grazing angles. Turns out that
making correct looking human faces, lacquered wood, floors, plastics...etc
it's been always difficult.
An RGB Curves node added between Fresnel and the Mix Factor has been a sort
of solution (tuning down whites and raising blacks), but a proper
implementation should be priority IMHO.

Example human head with highlighted errors, current Cycles behaviour:
Head_Fresnel <https://db.tt/gaO1ZUBu>

I stumbled upon these slides below, which exactly explain (and solve) this
problem, page 29-34, the same head is used for demonstration. CC Lee Perry
Link slides: Slides Anders L.
<http://blog.selfshadow.com/publications/s2014-shading-course/langlands/s2014_pbs_alshaders_slides.pdf>

As you can see by using a correct approach, the dark artifacts and dull
front highlight problems are gone, the slides says "for rough BSDFs you
really need to do it based on the normal of each microfacet (i.e, the
half-angle vector) and the light direction" page 31.

Is this the case or am i missing something? Could this be improved by
implementing such method?

Thanks,
Regards. MG
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