Density Control for Photon Maps
Frank Suykens | Yves Willems |
Contact: Computer Graphics Research Group
Proceedings of the Eurographics Workshop on Rendering 2000, p. 23-34
Brno, Czech Republic, 26-28 June
Abstract
The photon map method allows efficient computation of global illumination in general scenes. Individual photon hits, generated using Monte Carlo particle tracing, are stored in the maps and form a geometry independent representation of the illumination. Two important issues with the photon map are memory requirements to store the photons and the question how many photons are needed for an accurate representation of illumination in a certain scene. In this paper we introduce a method to control the density of photon maps by storing photons selectively based on a local required density criterion. This reduces memory usage significantly since in unimportant or over-dense regions less photons are stored. Results for caustic photon maps and global photon maps representing full illumination show a decrease in number of photons of a factor of 2 to 5. The required density states how accurate the photon map should be at a certain location and determines how many photons are needed in total. We also derive such a criterion based on a novel path-importance-based first pass, taking some steps towards solving the difficult 'how many photons' question.
Keywords: global illumination, Monte Carlo, multipass rendering, photon map, error control, importance
Images
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Image computed using a caustic photon map with density control. 90.000 photons were used instead of 190.000 without density control. |
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Scene computed using a global photon map, constructed with importance driven density control. |
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Required density based on camera-view of the previous image. |
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Final density of the global map after construction. |