I wanted to start a small discussion on behavior of glossiness parameter.
How the model currently works, is not unique to Corona, but is equally shared by Vray and MentalRay and is more a trait of the specular/glossy shader model legacy.
And since both Maxwell engine, and currently various real-time renderers integrated Disney's PBR model, where this is instead, swapped to "Roughness" parameter.
On paper: Roughness=Inverted Glossiness. Almost. The big difference between these two, is that while in Specular/Glossy shader model, glossiness is purely a subset of specular reflection,
a value (numeric or texture driven) that defines how spread out the reflection appears (and being in-line with energy conversation, more spread out obviously appears weaker),
Roughness defines the surface property both in how the specular reflection spreads but most importantly, how much of the grazing angle reflectance (white 1.0 for non-metals, white or tinted 1.0 intensity for metals)
becomes visible.
And in line with that behavior manifests the following difference in practice:
Spec/Gloss: 0.0 Glossy (matte)= Visible specular sheen, or rather overlay of grazing color.
Roughness: 100perc. (matte)=100perc. lambertian shader, or rather, diffuse color for non-metals, and base specular reflectivity for metals becomes fully visible with no overlay of grazing color at all.
Why the roughness model is more logical and easier to use with photorealistic materials in my opinion:
Consistency: Polished wood (not laquered!) and rough,matte wood is the very same material. The only difference is microscopic in its surface. Both have the same reflectivity, which is IOR 1.52, or rather, 0.04 base reflectivity, and 1.0 grazing reflectivity.
The only thing you need to change to achieve either, is change the roughness parameter. Since its linear value, its same across all engines (from Maxwell, to Unreal Engine 4 and many others soon) and is absolutely easily eye-balled from reference.
We are only changing one value.
In Spec/Gloss (Vray,Corona,MentalRay,etc..) to simulate identical behaviour, we have to "guess" the reflective value. Because full reflectivity, but zero glossiness produces visible sheen, and overlay of our 90 degree value (white by default). And they don't go against each other in linear fashion, i.e, we can't just set it to 30perc. glossy, and deduct 70 from our reflectance value,i.e 0.3. There is no such direct relationship, so we are juggling 2 values, and at any step, we can't be sure if this is actually physically correct material. The fact that it looks "right" to us, doesn't change anything about the fact that it's more complicated and much less logical.
Biggest problem for most users here is, they often end up with incorrect albedo, because they have both too high specular reflectance and diffuse (which is also 'reflectance'). Corona recently introduced small algorithm that "corrects" this for you in background (by dimming your diffuse if your specularity is too high).
Physical correctness=/=physical correctness. This term quickly became buzzword for markerting and is in such fashion used by all major renderers. But all it means is basically, they follow physical laws. You are still allowed to create material wildly differing from its real-world counterpart. That is actually good, the problem is, you don't know when you crossed that line.
Some random illustrations:
1)Vray: 0.0 glossy produces very strong, velvet like sheen.
2)Corona 0.0 glossy produces weaker sheen but exhibits similar behavior.
3)Corona with 128 grey material and Red 1.0 reflection, 0.0 glossy the overlay is quite visible
4)Roughness chart (this one is from Maxwell website, but its always the same). On left, 100perc. roughness (=0.0 glossy) = ideal lambertian surface, zero overlay of grazing angle color (which is blue)
End: Nothing :- ) Not actually saying I want this strongly right now or anything. It's not really any sort of request. I just wish to talk about your understanding of this issue, and generally, what you think.
Me personally, I am big fan of the pbr approach, as you see me joining all the tidbits here and there in forum. It's small, but revolutionary thing, something I am much bigger fan of than
features everyone else already has. In free time, I am creating stuff in Unreal4 (and no, it's not going to replace off-line rendering at all, it won't even become popular in archviz...trust me ;- ) not any soon),
and boy...I am having so much fan using the material system. It's vastly superior imho.
Cheers, all in good faith:- ) !