Microfacet models represent rough surfaces as collections of tiny facets described by a normal distribution

Microfacet models treat rough surfaces as a collection of small microfacets, each behaving like a tiny mirror. The surface’s roughness is captured by the distribution function D(w_h), which gives the differential area of microfacets whose normal is w_h. Only microfacets whose normal aligns with the half-angle vector between incoming and outgoing directions contribute to specular reflection. Smoother surfaces have a narrow normal distribution (tight highlights); rougher surfaces have a broad distribution (wide, diffuse-looking reflection). This framework underlies most modern PBR materials in games and VFX.

Examples

A near-smooth surface has D concentrated near the normal — a tight specular highlight. A rough surface has D spread broadly — a wide highlight. GGX/Trowbridge-Reitz is the most common D in real-time rendering.

Assessment

Given two materials with roughness 0.1 and 0.8, describe how their specular highlights differ, and explain which term of the microfacet BRDF controls that.