ABSTRACT
The appearance of an illuminated surface depends on how light is reflected from the surface. Reflectance in computer graphics sometimes has a specific meaning, such as the ratio of incident flux to radiance exitance, but the term is usually used in a broader sense to describe any model or function that describes how light is reflected. A general reflectance function quantifies the amount of incident light that is reflected from a surface. How light is reflected from a surface depends on the shape of the surface, the microscopic roughness of the surface, and the physical properties of the material at and below the surface itself. Various reflectance models take different kinds of surface properties into account. The simple bidirectional reflectance distribution function models (BRDFs) introduced in Chapter 1 really only consider surface roughness in a very approximate sense. The more complicated bidirectional subsurface scattering (BSSRDF) models described in Chapter 4 include transport in the material below the surface in the reflectance model. In this chapter, BRDF models, including physically based models, are examined in greater detail. Other ways of representing reflectance are considered, and methods for separating direct illumination from global (indirect) illumination are introduced. The chapter concludes with a particular problem of great recent interest in movie production: the modeling and rendering of hair.
