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Abstract

This paper presents an interactive algorithm for simulation of light transport in clouds. Exploiting the high temporal coherence of the typical illumination and morphology of clouds we build on volumetric photon mapping, which we modify to allow for interactive rendering speeds --- instead of building a fresh irregular photon map for every scene state change we accumulate photon contributions in a regular grid structure. This is then continuously being refreshed by re-shooting only a fraction of the total amount of photons in each frame. To maintain its temporal coherence and low variance, a low-resolution grid is used, and is then upsampled to the density field resolution in each frame. We also present a technique to store and reconstruct the angular illumination information by exploiting properties of the standard Henyey-Greenstein phase function, namely its ability to express anisotropic angular distributions with a single dominating direction. The presented method is physically-plausible, conceptually simple and comparatively easy to implement. Moreover, it operates only on the cloud density field, thus not requiring any precomputation, and handles all light sources typical for the given environment, i.e. where one of the light sources dominates.