db4a2de620
Schlick's approaximation used by EEVEE is not accurate near `IOR == 1`, especially when IOR is exactly one, there is no specular reflection and the material should appear diffuse. Cycles bypass the issue by lerping between the f0 and f90 color using the factor derived from real Fresnel curve. In EEVEE we can use the same trick as in Glass BSDF to smooth the transition at `IOR == 1`. Note that at `IOR < 1` there is still mismatch, because f0 is prebaked in the BTDF look up table. In the future if we color f0 using `specular_tint`, we can split the table and use the BTDF LUT for the specular component too.
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