Microfacet BRDF model

Even if our virtual geometry data is “smooth,” we often use a mathematical model to pretend that at a microscopic level (the “microfacet”) the surface has some roughness that causes rays to bounce at random angles when a ray intersection occurs.

• This can help model the surface material properties in the LTE more realistically.
• The rougher we pretend the surface is, the more random surface normals we’ll get, and the less normals will be aligned with our regular macrofacet surface normal.

The Torrance-Sparrow microfacet model assumes the microfacet surface is perfectly specular, while the Oren-Nayar BRDF model assumes it to be perfectly diffuse.

Lighting effects

Microfacet-level lighting can cause three events to occur.

• Masking occurs when ${\omega }_o$ is blocked by bumps on the surface and never reaches the light source. In other words, ${\omega }_i$ can never be calculated.
• Shadowing occurs ${\omega }_o$ is able to bounce off the surface and we get ${\omega }_i$, but ${\omega }_i$ is never able to reach a light source due to bumps that block its path.
• Interreflection occurs when a ray bounces across multiple bumps before reaching a light source.