It’s entirely my fault.
I spent a wee bit of time with hundreds of VRScans, profiling their measured response in RGB in a way which returned the exact albedo value (as Normal fraction, or 0.0-1.0, or %, as you wish to read it) at each incidence angle (more precisely, i sampled it at 10 bit, or 1024 samples across the 90 degree angles), and then we all got lost into other stuff, and I personally never came around to preparing the post.
I’ll sync with the VRScans team and see if we have fixes for some old issues in place, and the latest advancements made available, and if not, i’ll post some of the usable ones anyway.
As for the reflectance, i speak of the maximum reflectance (so yes, for glass at 90deg.), not the minimum one.
Eyes are often not enough to perceive it (damned, or blessed, be the log-like perception), but reflecting 15% of the sky/sun albedo is a heck of a lot of energy shot back.
When we use higher total reflectance, in the real world, we tend to make the surface rough enough to diffuse it (ie. plaster/paint), or if not, it turns exceedingly shiny (mirrors, and coats, over said glasses!).
Notice that mirrors for massive telescopes approach 95% reflectance (and more if and when they can!), while a house mirror with an aluminium back and a thick-ish glass on it will likely be in the 75% range, partly due to the non-smooth surface at micron-scale (aka: gloss at 0.975 or something. Old mirrors will well diffuse farther objects.), and partly due to the glass absorption.
So, as a general guideline, one should start with very low overall albedos, typically even lower than one would think plausible (white cloth at max 0.6f ? check!), and one will soon find out that going up from a too low albedo is quicker to iterate (faster GI/Caustics!), and much more controlled and pleasant to the eye (too low albedo leads to high contrast, which slowly recedes as one nails the ranges), than doing it the other way around: because of light not extinguishing, and various skews, rendertimes explode, and contrast typically is nowhere to be found, and super hard to retrieve.
Attached, you can see a few of those measurements: X is the angle from view, 0 to 90Deg., Y is the albedo, the curves are conveniently in R, G and B (but may be overlapping in places.); the left Sphere is the “Average Symmetrized” BTF mode used to measure the response, the right one right the full BTF, albeit likely off-scale (as the script didn’t adjust the UVs to the sample size, my mistake.), and of course, it goes without saying that the measurement was done properly, no inter-reflections, or coloured light. These renders are for display of the results only.
Notice how the curves are quite characteristics of the type of material response, with a diffuse-like paint displaying darkening at grazing angles, while a coated carpaint will show the expected grazing accentuation of reflectance (to 70% or so!), a White Matte carpaint showing the same-ish behaviour as a wall paint, and a mirror construction can be even inferred from its curves, in this case likely a poor quality, very absorbing glass on top of a poor reflective substrate, to produce said ~75% maximum reflectance, and showing the glass’ high absorption, and lack of grazing-angle reflectance, as the view gets shallow.
p.s.: in theory, if you replicated those curves in a falloff map, you’d get identical results (to the left sample!).
