rectangles

I'm using OpenCV and want to group together rectangles that have significant overlap. I've tried using groupRectangles for this, which takes a group threshold argument. With a threshold of 0 it doesn't do any grouping at all, and with a threshold of 1 is only returns rectangles that were the result of at least 2 rectangles. For example, given the rectangles on the left in the image below you end up with the 2 rectangles on the right: What I'd like to end up with is 3 rectangles. The 2 on the right in the image above, plus the rectangle in the top right of the image to the left that doesn't

I've been working on adding a Quadtree to a program that I'm writing, and I can't help but notice that there are few well explained/performing tutorials for the implementation that I'm looking for. Specifically, a list of the methods and pseudocode for how to implement them (or just a description of their processes) that are commonly used in a Quadtree (retrieve, insert, remove, etc.) is what I'm looking for, along with maybe some tips to improve performance. This is for collision detection, so it'd be best to be explained with 2d rectangles in mind, as they are the objects that will be stored

Sorry for the awful title. The purpose of the Java applet is as such: A ball is bouncing around the screen. The size and speed of this ball can be changed via scrollbars. The user can press and drag the mouse on the screen to draw rectangles. The ball will bounce off of these rectangles as well. The bounds of these rectangles are stored in a vector. When a rectangle is clicked, it (and all other rectangles at that point) are removed from the vector (and the screen). The problem I'm having is two-fold: One, when I click a rectangle to remove it, it doesn't get removed, but that can be solved