2.4 Light Scattering

When light encounters an obstacle it is either scattered or absorbed. The obstacle can be the surface of a different material or medium. It can also be a very small particle or a molecule, but this case requires special scattering techniques which we will describe in Chapter 10. In this section we will introduce tools for modeling the local light scattering at surfaces: i.e., what happens when a beam of light strikes a given surface. In graphics this is also known as local illumination.

First we present the theoretical framework used to describe light scattering, and then we present a few reflection models commonly used in computer graphics. Unless stated we assume that the wavelength of light does not change as a result of scattering (i.e., fluorescence), and we therefore omit the wavelength parameter in our description.

2.4.1 The BSSRDF

When a beam of light is scattered by a material it normally enters the material and then scatters around before leaving the surface at a different location. This is particularly noticeable for translucent materials such as marble and skin, but it happens to some degree for all non-metallic materials. We can describe this scattering using the Bidirectional Scattering Surface Reflectance Distribution Functional BSSRDF [69]. The BSSRDF, S, relates the differential reflected radiance, dL _r, at x in direction , to the differential incident flux, d ? _i at x' from direction : ^[1]

Figure 2.3: The BRDF models the...