Binary space partitioning: Difference between revisions

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The specific choice of partitioning plane and criterion for terminating the partitioning process varies depending on the purpose of the BSP tree. For example, in computer graphics rendering, the scene is divided until each node of the BSP tree contains only polygons that can be rendered in arbitrary order. When [[back-face culling]] is used, each node, therefore, contains a convex set of polygons, whereas when rendering double-sided polygons, each node of the BSP tree contains only polygons in a single plane. In collision detection or ray tracing, a scene may be divided up into [[Geometric primitive|primitives]] on which collision or ray intersection tests are straightforward.

Binary space partitioning arose from the computer graphics needneeding to rapidly draw three-dimensional scenes composed of polygons. A simple way to draw such scenes is the [[painter's algorithm]], which produces polygons in order of distance from the viewer, back to front, painting over the background and previous polygons with each closer object. This approach has two disadvantages: the time required to sort polygons in back-to-front order, and the possibility of errors in overlapping polygons. Fuchs and co-authors<ref name="fuchs80"/> showed that constructing a BSP tree solved both of these problems by providing a rapid method of sorting polygons with respect to a given viewpoint (linear in the number of polygons in the scene) and by subdividing overlapping polygons to avoid errors that can occur with the painter's algorithm. A disadvantage of binary space partitioning is that generating a BSP tree can be time-consuming. Typically, it is therefore performed once on static geometry, as a pre-calculation step, prior to rendering or other real-time operations on a scene. The expense of constructing a BSP tree makes it difficult and inefficient to directly implement moving objects into a tree.

BSP trees are often used by 3D [[video game]]s, particularly [[first-person shooter]]s and those with indoor environments. [[Game engine]]s using BSP trees include the [[Doom engine|Doom (id Tech 1)]], [[Quake engine|Quake (id Tech 2 variant)]], [[GoldSrc]] and [[Source (game engine)|Source]] engines. In them, BSP trees containing the static geometry of a scene are often used together with a [[Z-buffer]], to correctly merge movable objects such as doors and characters onto the background scene. While binary space partitioning provides a convenient way to store and retrieve spatial information about polygons in a scene, it does not solve the problem of [[Hidden surface determination|visible surface determination]].