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I’ve always seen “physically based” to mean the renderer does a physical simulation (photons bouncing, refracting, having wavelengths stretched, subsurface scat

I’ve always seen “physically based” to mean the renderer does a physical simulation (photons bouncing, refracting, having wavelengths stretched, subsurface scattering, BSDF calculations, etc.). So much in computer graphics are just (clever) hacks upon hacks to get something that looks “good enough” but isn’t really simulating physics in any meaningful way (like SSAO, texture baking, bump mapping, etc.). These hacks are much, much faster than simulating the physical process of photons interacting with the world. I don’t know where “physically based” originated but my first introduction to it was pbrt, which I suspect popularized “physically based” naming. Differentiable rendering is the name of going from the final image and “reverse rendering” it to reconstruct a 3D model of it.

fluxem 4y ago
Some clarification regarding the differentiable rendering vs inverse rendering. Differentiable rendering is what it says: differentiating the rendering process. Imagine that x is the scene, then f(x) is the function that renders the image. Then, Differentiable rendering is simply taking the derivatives: f'(x). Inverse rendering is a process of finding scene parameters x, such that f(x) produces the given image y. This is often achieved by using differentiable rendering together with an optimization algorithm (like SGD or Adam). However, due to the nature of rendering, it's easy to get stuck in a local minimum. Therefore, even a perfect differentiable rendering engine is not sufficient for the inverse rendering.
ohgodplsno 4y ago
Mind you, physically based renderers also use a bunch of clever hacks upon hacks to be able to run in realtime. Nobody is simulating photons bouncing in the nanostructure of a material. We just have formulas thay say "for this roughness and metallicness, your material will look like that and it'll be good enough". We still abuse the hell out of UV maps to fake things, still include ambient occlusion (as a stylistic choice most of these days). It's a different workflow that gives you consistency once you know what you're doing, as opposed to the days of Phong shading.

fluxem 4y ago

Some clarification regarding the differentiable rendering vs inverse rendering. Differentiable rendering is what it says: differentiating the rendering process. Imagine that x is the scene, then f(x) is the function that renders the image. Then, Differentiable rendering is simply taking the derivatives: f'(x). Inverse rendering is a process of finding scene parameters x, such that f(x) produces the given image y. This is often achieved by using differentiable rendering together with an optimization algorithm (like SGD or Adam). However, due to the nature of rendering, it's easy to get stuck in a local minimum. Therefore, even a perfect differentiable rendering engine is not sufficient for the inverse rendering.

ohgodplsno 4y ago

Mind you, physically based renderers also use a bunch of clever hacks upon hacks to be able to run in realtime. Nobody is simulating photons bouncing in the nanostructure of a material. We just have formulas thay say "for this roughness and metallicness, your material will look like that and it'll be good enough". We still abuse the hell out of UV maps to fake things, still include ambient occlusion (as a stylistic choice most of these days). It's a different workflow that gives you consistency once you know what you're doing, as opposed to the days of Phong shading.