This paper presents a new method for computing global illumination in scenes with participating media. The method is based on bidirectional Monte Carlo ray tracing and uses photon maps to increase efficiency and reduce noise. We remove previous restrictions limiting the photon map method to surfaces by introducing a volume photon map containing photons in participating media. We also derive a new radiance estimate for photons in the volume photon map. The method is fast and simple, but also general enough to handle nonhomogeneous media and anisotropic scattering. It can efficiently simulate effects such as multiple volume scattering, color bleeding between volumes and surfaces, and volume caustics (light reflected from or transmitted through specular surfaces and then scattered by a medium). The photon map is decoupled from the geometric representation of the scene, making the method capable of simulating global illumination in scenes containing complex objects. These objects do not need to be tessellated; they can be instanced, or even represented by an implicit function. Since the method is based on a bidirectional simulation, it automatically adapts to illumination and view. Furthermore, because the use of photon maps reduces noise and aliasing, the method is suitable for rendering of animations.
Key words: participating media, light transport, global illumination, multiple scattering, volume caustics, nonhomogeneous media, anisotropic scattering, rendering, photo-realism, photon tracing, photon map, ray marching, Monte Carlo.
Henrik Wann Jensen and Per H. Christensen: "Efficient Simulation of Light Transport in Scenes with Participating Media using Photon Maps". In Proceedings of SIGGRAPH'98, pages 311-320, Orlando, July 1998