max planck institut informatik

Principal-Ordinates Propagation for Real-Time Rendering of Participating Media
@ Computers and Graphics (CaG) 2014

Oskar Elek^1,2      Tobias Ritschel^1,2      Carsten Dachsbacher³      Hans-Peter Seidel^{1     
1}MPI Informatik      ²MMCI / Saarland University      ³Karlsruhe Institute of Technology     

We extend our previous work on Principal Ordinate Propagation by splitting the main transport stage in two parts: anisotropic and isotropic propagation. We heuristically estimate the number of anisotropic iterations that need to be executed; after that, we assume the directional radiance distribution to be isotropic. As a consequence the residual radiance can be propagated in a single domain without even keeping track of its directionality, resulting in much higher simulation speeds. The figure above shows both the anisotropic and the isotropic portions of the propagated energy for two different illumination conditions and two opposing camera positions.

Abstract

Efficient light transport simulation in participating media is challenging in general, but especially if the medium is heterogeneous and exhibits significant multiple anisotropic scattering. We present Principal-Ordinates Propagation, a novel finite-element method that achieves real-time rendering speeds on modern GPUs without imposing any significant restrictions on the rendered participated medium. We achieve this by dynamically decomposing all illumination into directional and point light sources, and propagating the light from these virtual sources in independent discrete propagation domains. These are individually aligned with approximate principal directions of light propagation from the respective light sources. Such decomposition allows us to use a very simple and computationally efficient unimodal basis for representing the propagated radiance, instead of using a general basis such as spherical harmonics. The resulting approach is biased but physically plausible, and largely reduces the rendering artifacts inherent to existing finite-element methods. At the same time it allows for virtually arbitrary scattering anisotropy, albedo, and other properties of the simulated medium, without requiring any precomputation.

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CaG Article (20.7 MB)

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Citation

Interactive Light Scattering with Principal-Ordinate Propagation
Best Student Paper @ Graphics Interface (GI) 2014

Oskar Elek^1,2      Tobias Ritschel^1,2      Carsten Dachsbacher³      Hans-Peter Seidel^{1     
1}MPI Informatik      ²MMCI / Saarland University      ³Karlsruhe Institute of Technology     

Our method's pipeline. The surface portion of the light transport is obtained from a Reflective Shadow Map (RSM), attenuated by the medium. Importance-sampling the RSM yields a set of Virtual Point Lights (VPLs). These are then used to calculate 1st-bounce global illumination, as well as indirect multiple scattering from surfaces (using a separate propagation grid for each VPL). Scattering from environment illumination is obtained similarly from VPLs generated by importance-sampling the environment map. Combining these partial solutions with the light directly scattered from light sources produces the final solution.

Abstract

Efficient light transport simulation in participating media is challenging in general, but especially if the medium is heterogeneous and exhibits significant multiple anisotropic scattering. We present a novel finite-element method that achieves interactive rendering speeds on modern GPUs without imposing any significant restrictions on the rendered participated medium. We achieve this by dynamically decomposing all illumination into directional and point light sources, and propagating the light from these virtual sources in independent discrete propagation volumes. These are individually aligned with approximate principal directions of light propagation from the respective light sources. Such decomposition allows us to use a very simple and computationally efficient unimodal basis for representing the propagated radiance, instead of using a general basis such as Spherical Harmonics. The presented approach is biased but physically plausible, and largely reduces rendering artifacts inherent to standard finite-element methods while allowing for virtually arbitrary scattering anisotropy and other properties of the simulated medium, without requiring any precomputation.

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GI Paper (15.8 MB)

Supplement (0.2 MB)

Video (60 MB)

Slides (67 MB)

Slides (3.1 MB)

Video

Citation