In Computer Graphics , PBR, or more commonly known as physically based rendering, is a collection of render techniques that are more or less based on the same underlying theory that more closely matches that of the physical world. As physically based rendering aims to mimic light in a physically plausible way, it generally looks more realistic compared to our original lighting algorithms like Phong Shading ( Basic shading) and Blinn-Phong.
Physically based rendering is still nonetheless an approximation of reality (based on the principles of physics) which is why it’s not called physical shading, but physically based shading. For a PBR lighting model to be considered physically based, it has to satisfy the following 3 conditions
- Be based on the microfacet surface model.
- Be energy conserving.
- Use a physically based BRDF.
The microfacet model
All the PBR techniques are based on the theory of microfacets. The theory describes that any surface at a microscopic scale can be described by tiny little perfectly reflective mirrors called microfacets. Depending on the roughness of a surface, the alignment of these tiny little mirrors can differ quite a lot:
Based on the roughness of a surface, we can calculate the ratio of microfacets roughly aligned to some vector h. This vector h is the halfway vector that sits halfway between the light l and view v vector.