specular reflectance of the material which is the value of Fresnel reflectance at 0° (reflectance at normal incidence) : F(0^{\circ}) [1]. The term F(0^{\circ}) is what we call specular color for our physically based shading model.
They do define all this reflectivity based on normal incidence, but not directly from those overall albedo values, as those include both diffuse and specular reflectivity.
Up to this time, I am still confused how to directly translate their data into our model, and closest and easiest is just the reflective curve for materials where that is available. But beyond that, hmm.
And that curve always goes from [specular reflectivity at 0] to 1 [at 90 angle]. When averaged from all angles + diffuse reflectivity= we get the overall value written everywhere(ie. snow 90perc.,etc..).
But we can't drop that value into our specular or diffuse slot as I've seen people doing. It's useless for us.
What does specular value in Corona/Vray/etc.. correspond to ? I don't know. If it's white[=1], and IOR of ~1.6 is used, it's almost the same as reflective curve of average dielectric material like plastic [which would have 0=0.04 and 90=1],
so maybe it's only some arbitrary number that get multiplied by the IOR curve, maybe it is 90 angle value, which then shouldn't really be moved for non-metals as long as IOR is used (i.e, it should imho always be 1=white).
But please don' take me literally, I am only curious, and dumb at that :- ) It's all my limited interpretation and I do feel lost like everyone else. I just don't like "eye-balling" and "do it like artist" answers.
Well, regarding bug and multi-material, refraction: The model of glass outside is simple box, edit poly with chamfer, single ID=1. The glass material is single material, it's not in multi-material.
The floor had multi-texture applied. Once I swapped single thing, the floor to single texture, the bug was gone, shadows were transparent again, and all those odd shadings were gone too.
This leads me it's more problem with the multi-texture somehow ? I don't really know what's happening.