I am coming from a game-ready, PBR workflow where it is easy to think about a material as having metallic (metal or non-metal) and roughness (light scattering coherence) parameters
Actually, like many things in CG, this is mostly a matter of getting used to one thing. For me, the specular/glossiness workflow is easier to comprehend.
Main question: Does this shader framework offer any corollaries to the PBR metal-rough model, and if so, how can these concepts be mapped most appropriately?
I *think* anything that can be achieved with the metalness/roughness workflow can be as well achieved with the specular/glossiness workflow, however you will need slightly different maps. There are also some advantages of one over the other, as far as I remember it is easier to set up a material without "outlines" visible in some masked areas using the metalness workflow.
- How do opacity and translucency differ?
Opacity is straightforward - how much light is reflected vs how much light passes through. It can be also seen as having microscopic holes in a material. If light hits a solid part of the material - it reflects; if it hits a hole - it passes through. Opacity makes materials see-through, so you can see some objects behind them.
Translucency is more complex - it simulates subsurface scattering in a very thin object (like a sheet of paper of a tree leaf). Even if translucency fraction is set to 1, you won't see objects behind such material, it is opaque. If light hits a material with translucency, some of it will reflect, some of it will be scattered inside, and some of it will be "transmitted" to the other side.
If you have a material using translucency, and then you lower its opacity - it will work like lowering opacity of a layer in Photoshop. You will simply start seeing the objects behind it, regardless of other effects enabled in this material (translucency, refraction, reflection,...).
- How does glossiness relate to roughness (or specularity) without reflection or refraction?
If reflection or refraction is not enabled, then glossiness will not do anything.
If reflection is enabled, lowering the reflection glossiness will blur the reflections.
Same with refraction and refraction glossiness.
A glossiness map is the inverse of a roughness map. In a glossiness map white parts mean sharp reflections and black parts will be completely rough.
- Do reflection and refraction have any corollary to metallic and roughness?
I think this should be pretty much explained by the above, and also by the video tutorial.
