Materials

In terms of graphics rendering, a material is equivalent to a BRDF: it’s a 4-dimensional function that captures how light bounces off of objects.

Microfacet theory: Although all materials have microsurfaces (lots of bumps that make the normal vectors point in many directions), at a distance they all look smooth (macrosurface).

There should be two BRDF’s: the macroscale is flat and rough, while the microscale is bumpy and specular. Each microfacet (microscale calculation) has its own normal vector that is drawn from some distribution.

• Concentrated microfacet normals (similar directions) = glossy
• Spread out normals (pointing in lots of different directions) = diffuse

Anisotropic BRDFs #

So far, we’ve assumed that all specular highlights are elliptical. However, this is not always the case- different materials can have characteristic highlight patterns (brushed metals, fibers, etc).

The key detail that anisotropic BRDFs introduce is the directionality of the underying surface.

Isotropic BRDFs are a reflection function of the incoming direction and reflected direction of rays. Although there are four terms, the $\phi_r$ term is redundant:

Anisotropic BRDFs do not have this property.