Department of Science and Technology, C-Research, Media and Information Technology, Linköping University, Norrköping, Sweden.
IEEE Trans Vis Comput Graph. 2012 Mar;18(3):447-62. doi: 10.1109/TVCG.2011.35.
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.
我们提出了一种算法，该算法可使用一般照明（包括方向光、点光源和环境贴图）在直接体绘制中实现实时动态阴影。通过使用存储在体积范围内的高效多分辨率网格中的球谐（SH）基函数对局部和全局体积可见性进行编码，实现了实时性能。我们的方法在空间域中实现了高频阴影，但在角度域中仅限于可见性和照明的低频近似。在第一遍中，网格中的细节级别（LOD）选择基于当前传递函数设置。这使得可以快速在线计算并将局部球形可见性信息的 SH 投影。然后，通过在局部区域上对 SH 系数进行分段积分，计算体积内的全局可见性。通过使用它们的 SH 投影来表示光源，可以在渲染过程中有效地计算照明、可见性和各向同性相位函数的积分。通过展示几个示例，演示了该方法的通用性和交互性能，证明了我们方法的实用性。
