Mark Pauly, Thomas Kollig, Alexander Keller

• In this paper we show how Metropolis Light Transport can be extended both in the underlying theoretical framework and the algorithmic implementation to incorporate volumetric scattering. We present a generalization of the path integral formulation thathandles anisotropic scattering in non-homogeneous media. Based on this framework we introduce a new mutation strategy that is specifically designed for participating media. It exploits the locality of light propagation by perturbing certain interaction points within the medium. To efficiently sample inhomogeneous media a new ray marching method has been developed that avoids aliasing artefacts and is significantly faster than stratified sampling. The resulting global illumination algorithm provides a physically correct simulation of light transport in the presence of participating media that includes effects such as volume caustics and multiple volume scattering. It is not restricted to certain classes of geometry and scattering models and has minimal memory requirements. Furthermore, it is unbiased and robust, in the sense that it produces satisfactory results for a wide range of input scenes and lighting situations within acceptable time bounds. In particular, we found that it is weil suited for complex scenes with many light sources.