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Quad trees, octrees, etc in scene graph.

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I'm working on the rendering portion of my engine, and I started wondering about quad trees, octrees, k[d] trees, bsp trees, basically all the different tree structures that are used to improve rendering performance. Then I started wondering: isn't this what my scene graph architecture already does? It seems like once you have a scene graph engine, then all those tree structures would involve is a little bit of processing at load time to split the meshes up and then render them like normal nodes in the scene graph. It seems like I must be missing something, what is it? How do these tree structures fit into a scene graph (which is already a tree of nodes)? As a special node, or just a preprocessing step?

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Depending upon how you build your scene graph there may be overlap or there may not. My understanding is that a scene graph is about hierarchies, e.g. which objects conceptually are grouped together. An octree (or any of the other types) are about spatial layout, e.g. which items are physically close to each other. If you have two car models next to each other, in the scene graph each car might be a node which has child nodes of door, wheel, etc. Note that in a hierarchical sense the right wheel of the left car and the left wheel of the right car are far apart, even though spatially they are very close together. Similarly the front left tire and rear right tire are close in the hierarchy, despite being far apart spatially.

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