I'll discuss using a bounding sphere for occlusion culling today. The basic idea is that large objects, occluders, are likely to occlude smaller objects in a scene. In Point Break, an obvious occluder is the Earth, and satellites, ground stations, vehicles, et.al. are objects likely to be occluded. Occlusion culling applied in this manner is sometimes called horizon culling, since objects are no longer visible when they go below the horizon.
I recently read an entry in the Journal of Ysaneya regarding horizon culling that got me thinking about the topic again. (Check out Ysaneya's blog for incredible planet and space screen shots from his game under development. I know you'll be asking when we'll be able to render such amazing scenes.) We have horizon culling code that is very fast, but is sometimes wrong. For Point Break, we needed to address this.
|A satellite about to disappear below the horizon|