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LSL Wiki : occlusion

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Occlusion (occlusion culling or occlusion rendering), basically means not performing rendering calculations on blocked/covered things which shouldn't be shown anyway. SL's occlusion rendering is based on an octree ("oct" meaning 8, as opposed to bintree) that divides up a sim into a hierarchy of cubes, with each cube containing the 8 smaller cubes that take up the same volume. Tight-fit bounding boxes are then generated for the objects within each cube and its children, which can be shown via the Debug > Rendering > Info Displays > Octree and Occlusion menu options. Hardware occlusion queries are performed against the hierarchy using an algorithm designed to find the topmost occluded node for a given branch in the tree over the span of several frames. Terrain, prims, avatars, and trees--any polygon rendered to depth--can act as occluders and will occlude other prims and terrain, as well as avatars and their attachments. As of 1.14.0, the simulation of particles, flexiprims, and LOD recalculations are also affected by occlusion rendering (with "octree" on, anything with a box--the bounding box--around it, is occludable).

Enable Debug > Rendering > Info Displays > Octree to see various occlusion indicators (see Debug for more info).
As of 1.14.0, enable Debug > Rendering > Info Displays > Occlusion to highlight geometry that has failed the occlusion test (i.e. what the renderer thinks is visible).

...occlusion culling has nothing to do with privacy. It only has to do with not drawing things that are just going to be drawn over by things which obscure them, from the camera's perspective. You can't "see" it, because anything that is visible should always be drawn. The only way you can tell is by turning it on and off and watching the change in framerate.

...the algorithm is a smart one, designed to not impact framerate when nothing is being culled and everything is visible.
(Yadweb Linden, 4/14/06)


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