A Fast Rendering Method for Optically Thick Participating Media Based on Next-Next Event Estimation

The traditional rendering technique for participating media solely considers the influence of transmittance and phase function while sampling scattering points. This implies that under certain conditions, a substantial contribution could be sampled with a very tiny probability by the participating medium's traditional rendering methods, resulting in a very huge numerical noise. A novel approach to forward rendering for participating media is put forth in this paper in order to address this issue. It involves sampling extra scattering points using a heuristic proxy distribution and then combining these single scattering contributions using multiple importance sampling techniques. With this approach, the issue of the multiple importance sampling method failing under direct illumination from a tiny light source can be avoided. These single scattering contributions can also be combined with the resampling method to obtain a better multiple scattering path without bias or extra computation. Furthermore, these single scattering contributions can be stored in memory and reused at subsequent scattering points along a path to provide additional single scattering contributions. Experiments demonstrate that the suggested approach can guarantee unbiased estimations while achieving a faster rate of rendering convergence than the traditional method.