11 Volume Scattering
So far, we have assumed that scenes are made up of collections of surfaces in a vacuum, which means that radiance is constant along rays between surfaces. However, there are many real-world situations where this assumption is inaccurate: fog and smoke attenuate and scatter light, and scattering from particles in the atmosphere makes the sky blue and sunsets red. This chapter introduces the mathematics to describe how light is affected as it passes through participating media—large numbers of very small particles distributed throughout a region of 3D space. Volume scattering models are based on the assumption that there are so many particles that scattering is best modeled as a probabilistic process, rather than directly accounting for individual interactions with particles. Simulating the effect of participating media makes it possible to render images with atmospheric haze, beams of light through clouds, light passing through cloudy water, and subsurface scattering, where light exits a solid object at a different place than where it entered.
This chapter first describes the basic physical processes that affect the radiance along rays passing through participating media. It then introduces the Medium base class, which provides interfaces for describing participating media in a region of space. Medium implementations return information about the scattering properties at points in their extent, including a phase function, which characterizes how light is scattered at a point in space. (It’s the volumetric analog to the BSDF, which describes scattering at a point on a surface.) In order to determine the effect of participating media on the distribution of radiance in the scene, Integrators that handle volumetric effects are necessary; this is the topic of Chapter 15.
In highly scattering participating media, light can undergo many scattering events without any appreciable reduction in its energy. The cost of finding a light path in an Integrator is generally proportional to its length, and tracking paths with hundreds or thousands of scattering interactions quickly becomes impractical. In such cases, it is preferable to aggregate the overall effect of the underlying scattering process in a function that relates scattering between points where light enters and leaves the medium. The chapter therefore concludes with the BSSRDF base class, which is an abstraction that makes it possible to implement this type of approach. BSSRDF implementations describe the internal scattering in a medium bounded by refractive surfaces.