如何快速确定用户是否触摸了许多 CGRect 之一

Question

I have an iPhone game, where there might be 30 (or more) CGRects and I need a quick way of determining if the user touched one. I have been previously been considering using the follow setup to detect if the user touched a CGRect in Cocos2d inside of touches began. So I have a vector (I am using Obj-C++) of CGRects:

for (int i = 0; i < (int) vec_of_cgrects; i++) {
    if (CGRectContainsPoint(vec_of_cgrects[i], location) {
        //Do what I need to do if user touches one of the rectangles
    }
}

But this isn't that efficient and I am wondering if this the best way to detect if the user touched a specific rect of a vector of rectangles or if there is a better way.

Answer 1

Do you want something to happen for each rectangle that the user touches, or one event should the user touch any old rectangle?

One improvement you can make in both situations is to sort your CGRects by their x-coordinate (or y-coordinate if that suits your circumstances more), so you can break as soon as the CGRect's x-coordinate is greater than the touch's x-coordinate.

If the latter, you can also check beforehand to see if any CGRect is fully contained by other CGRects and if so, remove them from the array.

Answer 2

You could consider using a Spatial Hash:

http://www.gamedev.net/index.php?app=core&module=search&do=search&fromMainBar=1

Here's a post on the Cocos2d forums about it:

http://www.cocos2d-iphone.org/forum/topic/11323#post-64132

Chipmunk and Box2d use it (physics libraries) I'm pretty sure for their collision detection...

Answer 3

Alternative inefficient approach: Store an array of width*height lists. Populate each list with the list of rectangles covering that pixel (or the "top" rectangle if you do some sort of overlap-handling). O(1) lookup!

"Hacker's" approach: Divide the screen into squares of size h*h where h ~= (width+height)/sqrt(n). Populate each square as above. O(sqrt(n))-ish lookup.

Neither is quite as good as a tree-based approach, but the tree might be painful to generate.