Improving occlusion query efficiency with occupancy maps

Image space occlusion culling is an useful approach to reduce the rendering load of large polygonal models. Like most large model techniques, it trades overhead costs with the rendering costs of the possibly occluded geometry. Meanwhile, modern graphics hardware supports occlusion culling, whereas they associate a significant query overhead, which hurts in particular, if the occlusion culling query itself was unsuccessful. We propose the occupancy map - a compact, cache-optimized representation of coverage information - to reduce the number of costly but unsuccessful occlusion culling queries and to arrange multiple occlusion queries. The information of the occupancy map is used to skip an occlusion query, if the respective map area is not yet set $the respective area has not yet received rendered pixels -, hence an occlusion query would always return not occluded. The remaining occlusion information is efficiently determined by asynchronous multiple occlusion queries with hardware-supported query functionality. To avoid redundant results, we arrange these multiple occlusion queries according to the information of several occupancy maps. Our presented technique is conservative and benefits from a partial depth order of the geometry.