# 14 Light Transport I: Surface Reflection

This chapter brings together the ray-tracing algorithms, radiometric concepts, and Monte Carlo sampling algorithms of the previous chapters to implement two different integrators that compute scattered radiance from surfaces in the scene. Integrators are so named because they are responsible for evaluating the integral equation that describes the equilibrium distribution of radiance in an environment (the light transport equation).

Recall the scattering equation from Section 5.6.1; its value can be estimated with Monte Carlo:

with directions sampled from a distribution with respect to solid angle that has PDF . In practice, we’ll want to take some samples from a distribution that approximates the BSDF, some from a distribution that approximates the incident radiance from light sources, and then weight the samples with multiple importance sampling.

The next two sections derive methods for sampling from BSDFs and
light sources.
After these sampling methods have been defined, the
`DirectLightingIntegrator` and the `PathIntegrator` will be
introduced. Both find light-carrying paths starting from the camera,
accounting for scattering from shapes’ surfaces. Chapter 15 will
extend this approach to scattering from participating media as well, and
Chapter 16 will introduce bidirectional methods for
constructing light-carrying paths starting from both the camera and from
light sources.