Unifying Points, Beams, and Paths in Volumetric Light Transport Simulation


We present a new rendering algorithm, unified points, beams, and paths (UPBP), for light transport simulation in participating media. UPBP combines a number of volumetric estimators based on points and beams with Monte Carlo path sampling techniques, all of which have complementary strengths and weaknesses. The bottom row shows the contributions of the various volumetric transport techniques to the final image at the top. UPBP excels at rendering scenes with different kinds of media, where previous specialized techniques each fail in complementary ways.


Efficiently computing light transport in participating media in a manner that is robust to variations in media density, scattering albedo, and anisotropy is a difficult and important problem in realistic image synthesis. While many specialized rendering techniques can efficiently resolve subsets of transport in specific media, no single approach can robustly handle all types of effects. To address this problem we unify volumetric density estimation, using point and beam estimators, and Monte Carlo solutions to the path integral formulation of the rendering and radiative transport equations. We extend multiple importance sampling to correctly handle combinations of these fundamentally different classes of estimators. This, in turn, allows us to develop a single rendering algorithm that correctly combines the benefits and mediates the limitations of these powerful volume rendering techniques.

Downloads and links

  • paper (PDF, 24 MB)
  • supplemental - includes additional derivations and plots (PDF)
  • poster - presented at EGSR 2014 (PDF, 53 MB)
  • citation (BIB)
  • slides - from the conference presentation (PPTX, 20 MB)
  • slides (pdf) - from the conference presentation (PDF, 4.2 MB)
  • image comparison - cool interactive JavaScript comparisons of different techniques and against previous work (pages may load slowly)
  • code (SmallUPBP) - a (not too) small open source physically based renderer that implements our algorithm
  • fxguide article - on rendering participating media via beam estimation

BibTeX reference

  author = {Jaroslav K\v{r}iv{\'a}nek and Iliyan Georgiev and Toshiya Hachisuka and Petr V{\'e}voda and Martin \v{S}ik and Derek Nowrouzezahrai and Wojciech Jarosz},
  title = {Unifying Points, Beams, and Paths in Volumetric Light Transport Simulation},
  journal = {ACM Transactions on Graphics (Proceedings of SIGGRAPH 2014)},
  volume = {33},
  number = {4},
  year = {2014},
  month = {aug},
  keywords = {global illumination, light transport, participating media, bidirectional path tracing, photon mapping, photon beams}