8.5 Fresnel Incidence Effects
Many BRDF models in graphics do not account for the fact that Fresnel reflection reduces the amount of light that reaches the bottom level of layered objects. Consider a polished wood table or a wall with glossy paint: if you look at their surfaces head-on, you primarily see the wood or the paint pigment color. As you move your viewpoint toward a glancing angle, you see less of the underlying color as it is overwhelmed by increasing glossy reflection due to Fresnel effects.
In this section, we will implement a BRDF model developed by Ashikhmin and Shirley (2000, 2002) that models a diffuse underlying surface with a glossy specular surface above it. The effect of reflection from the diffuse surface is modulated by the amount of energy left after Fresnel effects have been considered. Figure 8.23 shows this idea: when the incident direction is close to the normal, most light is transmitted to the diffuse layer and the diffuse term dominates. When the incident direction is close to glancing, glossy reflection is the primary mode of reflection. The car model in Figure 12.19 uses this BRDF for its paint.
The model takes two spectra, representing diffuse and specular reflectance, and a microfacet distribution for the glossy layer.
This model is based on the weighted sum of a glossy specular term and a diffuse term. Accounting for reciprocity and energy conservation, the glossy specular term is derived as
where is a microfacet distribution term and represents Fresnel reflectance. Note that this is quite similar to the Torrance–Sparrow model.
The key to Ashikhmin and Shirley’s model is the derivation of a diffuse term such that the model still obeys reciprocity and conserves energy. The derivation is dependent on an approximation to the Fresnel reflection equations due to Schlick (1993), who approximated Fresnel reflection as
where is the reflectance of the surface at normal incidence.
Given this Fresnel term, the diffuse term in the following equation successfully models Fresnel-based reduced diffuse reflection in a physically plausible manner:
We will not include the derivation of this result here.