Comment by dahart
6 years ago
To add a little to what @mattnewport said, yes some reflectance models are based on a microfacet theory. But some people are just using measured reflectance (sometimes along with curve-fit approximations). How does an empirical statistical table of reflectance fit into the world of physics models? Would you consider that more correct than a microfact model?
> starting from a second-quantized form of the electromagnetic field + the lattice of the build or surface material + itinerant electrons.
I’d love to see a model derived that way that we can use in graphics, and I don’t have the physics chops for it. Feel like writing a paper for siggraph? ;)
> a very incomplete toy model... which might barely be enough to deceive human eye for most everyday objects.
Yep :) That is the end goal of most graphics, we do stop when it looks good enough.
> How does an empirical statistical table of reflectance fit into the world of physics models? Would you consider that more correct than a microfact model?
Reflectance would be an emergent property of a material. No such thing exists in a microscopic quantum model (that is, first principles). It may make sense in certain situations, but it doesn't make sense in others.
Think about a simple process of an electron absorbing a photon, which later goes back to it's initial state by spontaneously emitting two photons of different colors (like from |0> -> |2> by absorption, and |2> -> |1> -> |0> by two consequent emission) with random delays in time (which has to obey the distribution is determined by Fermi's golden rule, which is in turn determined by the microscopic Hamiltonian). How do you assign a reflectance to this process?
Or a valance electron going into conduction band by absorbing a photon, moving around a bit, and emitting a similar photon at a different point after merging with a hole.
I can give you dozens of examples of physical processes of light-matter interactions that no kind of B*DF function can emulate.
> I’d love to see a model derived that way that we can use in graphics, and I don’t have the physics chops for it. Feel like writing a paper for siggraph? ;)
Solving the many-body problem in the presence of an EM field which is also treated quantum mechanically, without approximations that washes away all the quantum mechanical effects? That's probably worth a lot much more than a siggraph paper, like a couple dozen of Nobel prizes.
> Yep :) That is the end goal of most graphics, we do stop when it looks good enough.
I completely agree, and I'm not saying it's a bad model for its purposes. It just isn't a first principles physics, is all I'm saying.