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Volumetric path tracing
Computer graphics rendering method

Volumetric path tracing is an advanced computer graphics technique that builds upon path tracing by incorporating light scattering effects within participating media such as smoke, fog, fire, and soft shadows. Introduced by Lafortune and Willems, it simulates realistic lighting by tracing rays backwards from the eye to the light source, allowing scattering events to occur along the ray path. This method enhances photorealism by accounting for how light interacts with particles in the air or other translucent materials, enabling more accurate rendering of complex atmospheric phenomena and volumetric effects in 3D scenes.

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Description

The algorithm is based on the volumetric rendering equation,3 which extends the rendering equation with a scattering term. It is composed of an absorption, out-scattering, emission and an in-scattering part. The absorption and out-scattering together form the extinction term. The in-scattering is the most expensive part to calculate because it needs an integration over all paths in the scene that consist of radiance. Therefore, thousands of paths need to be traced to achieve a result with good quality and without much noise. For a better handling, the in-scattering term can be split into two components, the single scattering and the multiple scattering.4

Algorithm

In volumetric path tracing, a distance between the ray and the surface gets sampled and compared with the distance of the nearest intersection of the ray with the surface. If the sampled distance is smaller, a scatter event occurs. In that case the path is evaluated and traced from the scatter point in the media, not from the surface point on which it falls. The rest of the procedure continues in the same manner, until reaching the light source.56

Sampling

A possible way of sampling distances is the ray marching method, which works similarly to ray tracing but operates on a distance field of the scene and acts in discrete steps. The scattering inside the media can be determined by a phase function using importance sampling. Therefore, the Henyey–Greenstein phase function7 — a non-isotropic phase function for simulating the scattering of materials like oceans, clouds or skin8 — can be applied.

Further reading

References

  1. Lafortune, Eric P; Willems, Yves (June 1996). "Rendering Participating Media with Bidirectional Path Tracing" (PDF). Rendering Techniques '96. Eurographics. pp. 91–100. doi:10.1007/978-3-7091-7484-5_10. ISBN 978-3-211-82883-0. 978-3-211-82883-0 ↩

  2. Skånberg, Robin (January 2015). "Evaluation of Visual Parameters in Volumetric Path Tracing" (PDF). Department of Science and Technology, Linköping University: 20–22. {{cite journal}}: Cite journal requires |journal= (help) http://www.diva-portal.org/smash/get/diva2:796154/FULLTEXT01.pdf ↩

  3. Chandrasekhar, Subrahmanyan (1950). "Radiative transfer". Quarterly Journal of the Royal Meteorological Society. 76 (330). John Wiley & Sons, Ltd: 498. doi:10.1002/qj.49707633016. ISSN 1477-870X. /wiki/Doi_(identifier) ↩

  4. Jarosz, Wojciech (2008). "4-5". Efficient Monte Carlo Methods for Light Transport in Scattering Media. University of California. pp. 55–100. http://www.cs.dartmouth.edu/~wjarosz/publications/dissertation/ ↩

  5. Skånberg, Robin (January 2015). "Evaluation of Visual Parameters in Volumetric Path Tracing" (PDF). Department of Science and Technology, Linköping University: 20–22. {{cite journal}}: Cite journal requires |journal= (help) http://www.diva-portal.org/smash/get/diva2:796154/FULLTEXT01.pdf ↩

  6. Kulla, Christopher; Fajardo, Marcos (June 2012). "Importance Sampling Techniques for Path Tracing in Participating Media". Computer Graphics Forum. 31 (4). John Wiley \& Sons, Inc.: 1519–1528. doi:10.1111/j.1467-8659.2012.03148.x. ISSN 0167-7055. S2CID 2534374. /wiki/Doi_(identifier) ↩

  7. "The Henyey–Greenstein phase function" (PDF). http://www.astro.umd.edu/~jph/HG_note.pdf ↩

  8. Jarosz, Wojciech (2008). "4-5". Efficient Monte Carlo Methods for Light Transport in Scattering Media. University of California. pp. 55–100. http://www.cs.dartmouth.edu/~wjarosz/publications/dissertation/ ↩