Need help on Quadtrees java

Question

I've been searching for some way to implement quadtrees in my 2D simulation in order to make the collision detection a whole lot faster, thing is I find the concept quite hard to grasp. The sim works great as it is now its just that once I get past the 160-180 particles it gets very slow since the collision detection is going through absolutely all particles needlessly and is just plain stupid. Right now it's just a bunch or circles colliding with each other all around the screen and I'm able to spawn new ones with new speeds and location by clicking and dragging the mouse.

edit1:

Well I think I was able to create the tree now as you can see in the image

My Quadtree image: http://i.stack.imgur.com/c4WNz.jpg

So the question now is how do I make it useful for my collision detection...

Do I have to create the whole tree from zero each time I check?

What I'm about to do while I await, hope it's right in some way :P

1_Check if there's a ball in each node and leave that node out if there's none. 2_Keep checking intersections until I hit the leaf level and add those intersecting balls to the leaf node. 3_Collide balls in the leaf node before I keep going??

Here's my QuadTreeNode class:

public class QuadTreeNode { 
    private QuadTreeNode parent;
    private QuadTreeNode[] children;    
    private int id;
    private double x;
    private double y;
    private double width;
    private double height;

public QuadTreeNode(double x, double y, double width, double height, int id, QuadTreeNode parent){
    this.x = x;
    this.y = y;
    this.width =  width;
    this.height = height;
    this.children = new QuadTreeNode[4];
    this.id = id;
    this.parent = parent;
    //System.out.println("<x:>"+x+"<y:>"+y+"<w:>"+width+"<h:>"+height);     
    if (this.width>=1000/12 && this!=null){
        nodes+=1;
        this.children[0] = new QuadTreeNode(x, y, width/2, height/2, id+1, this);
        this.children[1] = new QuadTreeNode(x + width/2, y, width/2, height/2, id+2, this);
        this.children[2] = new QuadTreeNode(x, y + height/2, width/2, height/2, id+3, this);
        this.children[3] = new QuadTreeNode(x + width/2, y + height/2, width/2, height/2, id+4, this);
    }
}

Answer 1

I find the concept quite hard to grasp

Idea:

• recursive partition of a cell since:

  • the cell contains a fix count of primitives.
  • a maximum partition count happened.

• Start with a bounding-box for the whole scene.

• Recursive: Divide the cells if there are too much primitives in it.
• no dividing if the room is empty (adaptive partition)
• fine partition where the geometry is.

Hint: traversal is relative expensive because of many up and down movements in the hirarchy.

• Save the primitives in the inner nodes or in the leaves.

Traversal: - test with axis aligned bounding box of the root (first: whole scene)

Hint: the first hit could not be the nearest.

That's the principle of a quadtree or octree (3D) in short. I have a cg slide here which described it with images but I didnt want to upload them all. I think thats not the full answer on your question but maybe it helps a bit.