Calculating Bidirectional Reflectance

Besides ordinary reflectance and scattering, PBR also integrates a bidirectional reflectance distribution function (BRDF), which considers how a specular reflection will fall off or how fuzzy it appears around the edges. It is a function that considers the four factors of the incident ray, the vector to the viewer, the surface normal, and radiance (how well the surface reflects light). In fact, the Lambert (diffuse) and Phong (specular) models we considered in Chapter 18, Customizing the Render Pipeline, are examples of BRDFs. The BRDF for Phong, which calculates specular lighting that can be added to the diffuse of Lambert for a final effect, can be stated as the following:

In this formula, R is the vector of reflection of the incoming light, E is the vector from the point of contact to the viewer’s eye, and p is the specular power. All vectors involved in calculating reflections are shown in Figure 19.4:

Figure...