12 Light Sources
In order for objects in a scene to be visible, there must be a source of illumination so that some light is reflected from them to the camera sensor. To that end, this chapter first presents the Light interface, which allows specification of a variety of types of light sources. (Before reading this chapter, you may wish to review Section 4.4, which describes the physical processes underlying light emission.)
The implementations of a number of useful light sources follow. Because the implementations of different types of lights are all hidden behind a carefully designed interface, the light transport routines in Chapters 13 through 15 can generally operate without knowing which particular types of lights are in the scene, similar to how acceleration structures can hold collections of different types of primitives without needing to know the details of their actual representations.
A wide variety of light source models are introduced in this chapter, although the variety is slightly limited by pbrt’s physically based design. Many non-physical light source models have been developed for computer graphics, incorporating control over properties like the rate at which the light falls off with distance, which objects are illuminated by the light, which objects cast shadows from the light, and so on. These sorts of controls are incompatible with physically based light transport algorithms and thus cannot be provided in the models here.
As an example of the problems such lighting controls pose, consider a light that does not cast shadows: the total energy arriving at surfaces in the scene increases without bound as more surfaces are added. Consider a series of concentric shells of spheres around such a light; if occlusion is ignored, each added shell increases the total received energy. This directly violates the principle that the total energy arriving at surfaces illuminated by the light cannot be greater than the total energy emitted by the light.
In scenes with many lights, it is impractical to account for the illumination from all of them at each point being shaded. Fortunately, this issue is yet another that can be handled stochastically. Given a suitable weighting factor, an unbiased estimate of the effect of illumination from all the lights can be computed by considering just a few of them, or even just one. The last section of this chapter therefore introduces the LightSampler, which defines an interface for choosing such light sources as well as a number of implementations of it.