Instant Multiple Scattering for Interactive Rendering of Heterogeneous Participating Media

Rendering participating media with multiple scattering is costly and often even challenging for off-line methods. In this paper we present a novel method for efficiently rendering such effects that achieves interactive speed for dynamic scenes with both homogeneous and heterogeneous media. It is based on instant radiosity, which is typically used to approximate indirect illumination between surfaces by computing direct lighting from a set of virtual point lights (VPLs). The same principle can be applied to participating media: we describe a particle tracing algorithm to create a set of VPLs within the medium, such that the combined single scattering contribution thereof yields full multiple scattering. Compared to indirect illumination, it is even more important to avoid clamping and singularities from VPLs when rendering participating media effects. For this we derive a GPU-friendly bias compensation for high-quality rendering of participating media with VPLs.