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Efficient Visibility Encoding for Dynamic Illumination in Direct Volume Rendering
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 multi-resolution 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 on-line 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.