[mattnewport]

What you seem to be describing are micro facet models which while commonly used in PBR renderers are not really fundamental to PBR. One of the major things that distinguishes PBR from earlier approaches is trying to ensure BRDFs for non emissive surfaces aren't adding energy by reflecting more total outgoing radiance than incoming radiance. Micro facet models help achieve that but there is also research that tries to more or less directly use measured BRDFs as well as trying to fit micro facet based and other models' parameters to measured data.

Of course there are non local scattering effects that are not captured by BRDFs and that can be perceptually important for some materials so there are also extensions like BSDFs used in some PBR renderers. Overall the PBR approach is to try and understand the underlying physics and drop or approximate (in a more or less principled way) the parts that are impractical to simulate or have small perceptual effects in most situations. It's not really about any specific such approximation like micro facet models. "Physically Based" seems like a perfectly good name for this approach, perhaps "Physically Inspired" would have been an appropriate alternative.

[tagrun]

Microfacets is just one example (which is what PBR approximations today ubiquitously use BTW). The very idea of a B*DF function for light is a rough approximation, and can never capture a variety of quantum effects. You can simply never recover the first-principles quantum dynamics of a macro-scale matter + light.

Approximations and models need to be made, that's how it works. Yes, you can make it better and better as the hardware improves. And calling it "physically based" sounds fine to me. What doesn't sound fine is to claim that any PBR-related work in the graphics literature today starts from "first principles". First-principles or "ab initio" in physics is a very specific technical word reserved for calculations which really start from first principles (see https://en.wikipedia.org/wiki/Ab_initio_quantum_chemistry_me... for an example) that doesn't get thrown around cheaply.

[dahart]

> What doesn’t sound fine is to claim that any PBR-related work in the graphics literature today starts from “first principles”.

The use of that term was entirely my sloppiness alone. I used “first principles” only to mean the opposite of a phenomenological model, as @s-macke put it. What’s the right term there? Theoretical model?

You’re okay with “physically based”, but not “first principles”, @s-macke was okay with “first principles” but not “physically correct”. Personally, I wouldn’t mind being able to talk about graphics in a way that doesn’t immediately rub physicists the wrong way, but I’m not sure how, and I’m bound to cause some problems flinging around terms that have a precise technical meaning in physics. Sorry about that. ;)

It’s definitely true that in graphics we are not usually setting out to simulate quantum effects, unless the result is visible at the macro scale. Despite that, we’re trying to incorporate all the physics that matter at the macro scale.

[tagrun]

I think an accurate description would be classical (meaning not quantum mechanical) model of light (which covers both geometrical optics and wave optics) and phenomenological model (in physicist's sense: https://en.wikipedia.org/wiki/Phenomenological_model) of matter and light-matter interaction. It is a phenomenological model, because it's either derived from a cartoon description of matter at microscopic scale and how light bounces off it, or fitted to some experimental data, and is without an underlying actual first-principles physical theory either way.

BTW, I hope you don't think this is just me being picky about words; if you talk to any physicists in condensed matter matter physics and say the magic words "first principles", they'd immediately start thinking that you did ab initio calculations. First-principles calculations and DFT is a sub-field, and entire careers and communities are built on it!

[dahart]

> I hope you don’t think this is just me being picky about words

Not at all, thank you for taking the time to explain!