[Bf-cycles] Cycles Fresnel behaviour

Matthew Heimlich matt.heimlich at gmail.com
Mon Jan 5 19:10:48 CET 2015

Cycles' Fresnel term is definitely different than most other renderers I've
used, which is why I almost solely use "facing" from an artistic point of
view. Not a huge deal I don't think, unless working with real-world
absolutes (in which case a spectral renderer should be used anyway), but it
would be interesting to hear what the issue is that causes this disparity
in Cycles.

On Mon, Jan 5, 2015 at 11:35 AM, Sergey Sharybin <sergey.vfx at gmail.com>

> Hi,
> Can't really give exact link to what Cycles use, from quick glance it it
> seems to be somewhat close to what embree does.
> Improvements are always possible, but it's also useful to see the .blend
> file with the setup which you consider is faulty. To investigate if it's
> setup properly and use for comparison of results and so.
> On Mon, Jan 5, 2015 at 8:38 PM, Marco G <marco.gzt at gmail.com> wrote:
>> 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
>> _______________________________________________
>> Bf-cycles mailing list
>> Bf-cycles at blender.org
>> http://lists.blender.org/mailman/listinfo/bf-cycles
> --
> With best regards, Sergey Sharybin
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