Graphics Reference
In-Depth Information
Chapter 27
This chapter is about the way we model the interaction of light with objects. The
first several sections are about the physics and mathematical representation of
these interactions. At the end, we briefly discuss a software interface that's well
suited to the rendering we'll do later.
The first step in our discussion is to limit consideration, for much of the chap-
ter, to light-object interactions that occur at surfaces; late in the chapter we briefly
discuss the volumetric interaction that occurs in things like fog and colored water,
etc. All these interactions, when considered
locally
(e.g., “Where does the light
arriving at this bit of surface end up going?”), are called
scattering.
Mirror reflec-
tion, for instance, is one very special kind of scattering; Lambertian reflection is
another.
Scattering is a messy process. For many materials of interest, the physical fea-
tures of the material are at a scale of just a few wavelengths of light, so diffraction
effects matter. The interaction of light with materials is dependent on the chem-
istry of the materials—the degree to which the material is a conductor or insulator,
and the distribution of electron-energy levels in the material, as we saw in Chap-
ter 26—which is highly variable. And even for the simplest of rough materials,
light interacts with the rough material in varied and complex ways. The result is
that scattering code is messy. If you peek inside almost any renderer, the represen-
tation of scattering is either oversimplified or very messy.
Operational definitions, like Le Corbusier's “A house is a machine for living,”
have an intrinsic appeal: They get to the heart of a subject from the speaker's
point of view. From the point of view of a renderer, an
object
is a machine for
converting an incoming light field into an outgoing one, through some kind of
interaction that's of no particular relevance to the renderer. The machine has a
few useful properties, determined by the laws of physics: If we sum two incoming
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