Computer Graphics Forum (Proceedings of Eurographics Symposium on Rendering 2017)
Heterogeneous emissive volumes are a challenging form of light source. The images show an equal-time comparison of an explosion, in two versions that differ in overall density. The thin version (left, 10min) greatly benefits from a line integration estimator that accumulates emission for path segments and not only at path vertices. In addition, we propose a next event technique called forward next event estimation (FNEE) that samples the length of next event segments proportional to transmittance. This prevents long segments that do not contribute to the final image due to high extinction (right, 30min) and increases the efficiency of path tracing compared to regular next event estimation (NEE).
Emissive media are often challenging to render: in thin regions where only few scattering events occur the emission is poorly sampled, while sampling events for emission can be disadvantageous due to absorption in dense regions. We extend the standard path space measurement contribution to also collect emission along path segments, not only at vertices. We apply this extension to two estimators: extending paths via scattering and distance sampling, and next event estimation. In order to do so, we unify the two approaches and derive the corresponding Monte Carlo estimators to interpret next event estimation as a solid angle sampling technique. We avoid connecting paths to vertices hidden behind dense absorbing layers of smoke by also including transmittance sampling into next event estimation. We demonstrate the advantages of our line integration approach which generates estimators with lower variance since entire segments are accounted for. Also, our novel forward next event estimation technique yields faster run times compared to previous next event estimation as it penetrates less deeply into dense volumes.