Path Traced Subsurface Scattering using Anisotropic Phase Functions and Non-Exponential Free Flights

An important distinction of diffusion models is that they only model multiple scattering. We generally distinguish between scatter events at different path depths: Zero-scatter events are photons that enter a surface and immediately reach an exit point without having scattered in the medium itself. Single scatter events do the same with a single interaction inside the volume. Multiple scatter events have two or more scatter events before exiting through the surface of the object. Diffusion models only capture the multiple scattering contribution, leaving zero and single scatter events to be treated explicitly as separate BxDF lobes.