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Portals, BSP Trees, Scene Graphs and Octrees?

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I keep getting confused when reading up on Portals, BSP Trees, Scene Graphs and Octrees. Which are for collision, rendering etc.? Are several used at the same time? I'd apreicate if someone could post a small simple description of each one.

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All of the systems you mentioned can be used in the same time. The subject though is a bit too big to be answered briefly.

BSP and Octrees are mostly used for spatial partitioning which is useful for collision detection and visibility tests. The original Doom used BSPs for example for the whole scene, but nowadays 3d worlds start to be so complex that BSP (alone) isn't powerful enough. Consider a terrain of several square kilometers containing buildings and trees. Using just BSP here would be very inefficient. Combining it with the octree is a better solution (ie. put all your objects (terrain, buildings, trees etc) inside the octree and then use bsp inside these objects to accelerate collision detection etc.

Portals are used for visibility tests. Idea of portal is that it works as a link between to sectors. Simply put, if the portal "object" isn't visible the sector behind the portal is invisible too.

There was a text in www.gamearchitect.net about scenegraph. I don't use scenegraph for example. There are other ways to organize your objects.

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