objective c - In Cocos2D, how do I check for collisions against an arbitrary user-drawn shape?

I want to detect a collision between a sprite and a user-generated shape of some sort.

For example. There are 3 objects on the screen. The user takes their finger and draws an abnormal shape around 2 of the objects. I need to detect those 2 objects.

How do I approach this efficiently using Cocos2D?

Answer

There's a very easy solution for that particular example.

I'm assuming your arbitrary shape is just a series of points.

Draw a ray from each of your objects in any direction. If the number of times it intersects a line segment on your shape is even (including zero), you are outside of the shape.

If the number of times you intersect is odd, you are inside the object.

Ray/line segment collision is a pretty easy algorithm to find/implement.