Ray Space Factorization for From-Region Visibility

We present a fast conservative from-region visibility method capable of handling 3D models using arbitrary 3D triangles as occluders. The from-region visibility problem is considered significantly harder than the from-point visibility problem, since it is inherently 4-dimensional. Our occlusion culling method is based on factorizing the 4D visibility problem into horizontal and vertical components. The visibility of the horizontal component is based on a novel bounded ray space, and the visibility of the vertical component is solved by incrementally merging umbrae. The technique is designed so that the horizontal and vertical operations can be efficiently realized together by advanced graphics hardware. The framework of the method is structurally similar to frompoint hierarchical image-based methods; however, the image-space occlusion map is in the ray space rather than in the primal space. This leads to a from-region culling method that can be fast enough to be applied on-line. Our results show that conservative occlusion culling of large urban scenes from large regions takes only a fraction of a second. Since the visibility computation is valid for many frames, its cost is further amortized and imposes a negligible overhead on the rendering. CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Visible line/surface algorithms