Determining if two rays intersect

Question

I have two rays on a 2D plane that extend to infinity, but both have a starting point. They are both described by a starting point and a vector in the direction of the ray extending to infinity. I want to find out if the two rays intersect, but I don't need to know where they intersect (it's part of a collision detection algorithm).

Everything I have looked at so far describes finding the intersection point of two lines or line segments. Is there a fast algorithm to solve this?

Answer 1

GeomAlgorithms.com has some pretty sweet algorithms dealing with lines in 3D... Generally speaking though, the probability of two lines intersecting in 3D space is really quite low.

In 2D, you have to check the slope. If the slope is not equal then they intersect. If the slope is equal, they intersect if a point on them has the same x-coordinate or the same y-coordinate.