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要旨:
This work presents a novel 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 initially used, and is then upsampled to the density
field resolution on a physical basis in each frame. We also present a technique
to store and reconstruct the angular illumination information by exploiting
properties of the standard Henyey-Greenstein 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 above 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.
