Conservative visibility preprocessing for walkthroughs of complex urban scenes

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Abstract
Visibility preprocessing is a useful method to reduce the complexity of scenes to be processed in real-time, and so enhances the overall rendering performance for interactive visualization of virtual environments. In this paper, we propose an efficient visibility preprocessing method. The proposed method is able to handle more general environments, like urban environments, and remove invisible polygons jointly blocked by multiple occluders. The proposed method requires O(nm) time and O(n+m) space. By selecting a suitable value for m. user can select a suitable level of trade-off between the preprocessing time and the quality of the computational result. In the proposed method, we assume that navigatable areas in virtual environments are partitioned into rectangular parallelepiped cells or sub-worlds. To preprocess the visibility of each polygon for a given partitioned cell, we should determine at least the area-to-area visibility. That is inherently a four-dimensional problem. In the proposed method, we efficiently express four-dimensional visibility information on two-dimensional spaces and keep it within a ternary tree. which is conceptually similar to a BSP(Binary Space Partitioning) tree, by exploiting the characteristics of conservative visibility.

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