The scene nodes usually contain bounding boxes (or spheres, etc.), and these boxes are organized into a hierarchy, i.e., the bounding box of a node contains all the bounding boxes of the child nodes. You can take advantage of this to perform hierarchical frustum culling.

The basic idea is that if a node's bounding box is not visible, then you know for sure that it's children are not visible as well, and so you don't need to check the children for visibility - you just discard the entire "branch" (node + all it's descendants) in one check! That way you don't need to check all the nodes each frame.

You can speed up collision detection and other spatial queries in a similar way.

About removing nodes from the scene graph - I'm not sure that's a good way to do it. I think the common way to do this is to add all visible nodes to a render queue. This queue can also be sorted according to some criteria, for example, opaque objects can be sorted by material for faster rendering while transparent objects need to be depth sorted back to front. For this reason, you typically have several render queues, each one holding a different "category" of objects (opaque, transparent, etc.). Of course, at the beginning of each frame the queues start out empty.

Quote:Original post by ehmdjii
so if you create a new list of renderable objects every frame, that means that you also have to do the frustum-check for each object every frame, right?

No. You traverse the scene graph, testing the bounding box of each node. If it's not visible, you discard the node and all it's child nodes. If it's visible, you recursively check each child node for visibility. I assume that the renderable objects are stored in the leaves (I think that that's usually how it works), so when you get to the leaves you check each object for visibility and add the visible ones to the render queue.

Notice that the above process does not check each object in the graph. If you have a node that has 100 children that are renderable objects and the node's bounding box is not visible, then you can reject all 100 objects in one check.