What are 'physically-based' lighting/rendering/materials?

Question

What's the difference between physically based lighting, physically based rendering and physically based materials? How they are different from deferred and forward rendering? And physically based rendering is used in films and deferred rendering is used in games?

Answer 1

It would appear that "Physically-based rendering" (PBR), "Physically-based materials", and "Physically-based shading" are different names for the same general concept of creating materials with shading that lend themselves to looking more "real" or physically-based when rendered.

PBR can be used in both movies (offline rendering systems, Pixar has a paper) and games (realtime: Unity and Unreal have docs). It can also be done in WebGL, check out Marmoset's PBR-enabled WebGL Viewer, find the picture of the camera lens, and try rotating that around. Another WebGL example can be found in OpenSceneGraphJS's PBR Demo.

Different PBR implementations have different parameters, but most center around a concept of specifying how "metallic" and how "rough" a particular material is, and allowing the rendering engine to make sure that the material doesn't reflect light in an unrealistic way, or reflect more light than it receives, etc. For example:

This example is a composite of screenshots from the OSGJS demo above, and shows a single base color with varying degrees of metallic and roughness. The idea is that the artist can vary these parameters per-material and even per-texel, and not wind up with something that appears to violate the laws of physics when it comes to lighting. This is considered better than more traditional rendering techniques, where you can create a specular highlight that overexposes objects even in low light conditions.

Disclaimer, I'm still learning about PBR myself. I've been doing a lot of reading up on it lately in hopes of being able to contribute to the current effort underway to add PBR to the glTF 3D model spec. Khronos (the standards organization behind OpenGL and WebGL) is creating an open standard 3D model delivery format that eventually will support PBR, and can convey PBR materials to a wide variety of rendering engines, on the web, mobile, and desktop. But, currently the PBR portion of this is still a draft extension (as of this writing), still under development for some time to come.

I'm not a deferred/forward rendering expert, perhaps someone else can chime in with differences there. In the meantime, I'd suggest to read more from Marmoset about the theory of PBR, and PBR in practice. Good luck.