How do you handle objects moving between quads when using quadtrees?

Question

I'm trying to use quadtrees for collision detection in a game I'm making, but I'm not sure how to handle objects that might be moving between different quads?

The only way I can think of it is by clearing out the whole tree each frame, and then adding everything back in there, but that seems like that can get cpu intensive and not very efficient. Do you check each object every frame to see if it has moved outside the boundry of it's current quad, and if so then remove it and readd it? That again seems like it can be pretty inefficient because you'd be performing collision checks on every moving object every frame.

Also, regarding quadtrees but unrelated to objects moving around in them, how do you handle multiple objects in the same quad? Most sites that I've read about them on say that you should only have one, maybe two, objects in a quad, and if you get more than that then push them down in the tree. What if you had a situation like this? You have three circles and they are all on the edges of the level below them so they can't go any further down, but there is three all in the same level, which people say you shouldn't have.

Answer 1

I don't think it's particularly inefficient to implement your suggestion: check to see if an object has moved outside its quadtree, and if so then remove and re-add it. Any object which moves from one frame to the next will need to have some collision detection performed on it, surely? And the quadtree operations are only performed if it moves quadtrees, and the CPU time spent there are probably overshadowed by the CPU time doing the more precise "Does object A touch object B?" computations. So I don't know that you can do better.

On your 2nd question: I don't know how other people implement quadtrees, but I allow objects to occupy more than one quadtree, precisely for the reason you've given in your diagram (when an object straddles a boundary). So an object has a "current list of quads" instead of a "current quad".