/// Initialize the bodies, anchors, axis, and reference angle using the world
/// anchor and world axis.
public void Initialize(Body b1, Body b2, Vector2 anchor, Vector2 axis)
{
body1 = b1;
body2 = b2;
localAnchor1 = body1.GetLocalPoint(anchor);
localAnchor2 = body2.GetLocalPoint(anchor);
localAxis1 = body1.GetLocalVector(axis);
}