How do I determine which side of the player has collided with an object?

Question

I have some static bodies (platforms) and a dynamic body (the player) in my world. The collision between them works great, but I would like to know which side of the player (rectangle) collides with the platform. How do I get this information?

Answer 1

I know this question was already answered but I needed this in one of my games as well and I found the answer in the Farseer forums and modified it for my engine. You will find the original link in the comments (Shameless link to engine http://code.google.com/p/axiosengine/ - perhaps you will find more interesting things there) :

namespace Axios.Engine.Extensions
{
    public enum CollisionDirection
    {
        Right,
        Left,
        Top,
        Bottom
    }
    public static class AxiosExtensions_Contact
    {
        /// http://farseerphysics.codeplex.com/discussions/281783
        /// <summary>
        /// Returns the direction that the collision happened.
        /// Should be used in the event OnAfterCollision
        /// </summary>
        /// <param name="c"></param>
        /// <returns></returns>
        public static CollisionDirection Direction(this Contact c)
        {
            CollisionDirection direction;
            // Work out collision direction
            Vector2 colNorm = c.Manifold.LocalNormal;
            if (Math.Abs(colNorm.X) > Math.Abs(colNorm.Y))
            {
                // X direction is dominant
                if (colNorm.X > 0)
                    direction = CollisionDirection.Right;
                else
                    direction = CollisionDirection.Left;
            }
            else
            {
                // Y direction is dominant
                if (colNorm.Y > 0)
                    direction = CollisionDirection.Bottom;
                else
                    direction = CollisionDirection.Top;

            }

            return direction;
        }
    }
}

To use it:

 switch (contact.Direction())
                {
                    case CollisionDirection.Left:
//ect

Answer 2

There's a few ways I can think of to do this:

1. You could make the player into several fixtures representing each side. Then you could either give each one an OnCollision event, or give each one an ID in the UserData to figure out which one is the "collider" in a shared OnCollision method. This was good for me because I could set the side and top fixtures to have zero friction so the player doesn't stick to platforms.
2. If you're stuff is all axis aligned rectangles, you could calculate it based on their positions and dimensions. This is probably not a great idea for more complex situations since it can get difficult to follow and Farseer already has to calculate this anyways.
3. I think you can check the Contact's world manifold's normal. In your OnContact event, there are three parameters: two Fixtures and a Contact. Contact has a method called GetWorldManifold which has an out parameter called normal. This is a Vector2 that, I think (someone please correct me if I'm wrong), represents the angle of contact.