Graphics Reference
In-Depth Information
8
Reconstruction of
Reflectance
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 phys-
ical properties of the material at and below the surface itself. Various reflectance
models take different kinds of surface properties into account. The simple bidirec-
tional 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 ex-
amined 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.
8.1 BRDF Models
In Chapter 1, the
bidirectional reflectance distribution function
(BRDF) was in-
troduced as one way of modeling surface reflection. A BRDF is a function that
213
