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Issue No. 03 - March (2012 vol. 18)
ISSN: 1077-2626
pp: 447-462
Joel Kronander , Linköping University, Norrköping
Daniel Jönsson , Linköping University, Norrköping
Joakim Löw , Linköping University, Norrköping
Patric Ljung , Siemens Corporate Research, Princeton
Anders Ynnerman , Linköping University, Norrköping
Jonas Unger , Linköping University, Norrköping
We present an algorithm that enables real-time dynamic shading in direct volume rendering using general lighting, including directional lights, point lights, and environment maps. Real-time performance is achieved by encoding local and global volumetric visibility using spherical harmonic (SH) basis functions stored in an efficient multiresolution grid over the extent of the volume. Our method enables high-frequency shadows in the spatial domain, but is limited to a low-frequency approximation of visibility and illumination in the angular domain. In a first pass, level of detail (LOD) selection in the grid is based on the current transfer function setting. This enables rapid online computation and SH projection of the local spherical distribution of visibility information. Using a piecewise integration of the SH coefficients over the local regions, the global visibility within the volume is then computed. By representing the light sources using their SH projections, the integral over lighting, visibility, and isotropic phase functions can be efficiently computed during rendering. The utility of our method is demonstrated in several examples showing the generality and interactive performance of the approach.
Volumetric illumination, precomputed radiance transfer, volume rendering.

D. Jönsson, P. Ljung, J. Kronander, J. Löw, A. Ynnerman and J. Unger, "Efficient Visibility Encoding for Dynamic Illumination in Direct Volume Rendering," in IEEE Transactions on Visualization & Computer Graphics, vol. 18, no. , pp. 447-462, 2011.
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