This chapter discusses a class of algorithms for computing global illumination pictures known as path-tracing algorithms ^[1]. The common aspect of these algorithms is that they generate light transport paths between light sources and the points in the scene for which we want to compute radiance values. Another, although less vital, characteristic is that they usually compute radiance values for each individual pixel directly. As such, these algorithms are pixel-driven, but many of the principles outlined here can be equally applied to other classes of light transport algorithms, such as finite element techniques (to be discussed in the next chapter).

First, we present a brief history of path-tracing algorithms in the context of global illumination algorithms (Section 5.1). Then, we discuss the camera set-up that is common to most pixel-driven rendering algorithms (Section 5.2) and introduce a simple path-tracing algorithm in Section 5.3. In Section 5.4, we introduce various methods for computing the direct illumination in a scene, followed by similar sections for the special case of environment map illumination (Section 5.5) and indirect illumination (Section 5.6). Finally, in Section 5.7, the light-tracing algorithm is discussed, which is the dual algorithm of ray tracing.