A wall describes an infinite two-dimensional plane that physics bodies can collide with. To define a wall, you must give it a distance (from the center of the physics engine's origin, and a normal defining the plane of the wall. (wall) | | (normal) (origin) | ---> * | | (distance) ................... (100px) e.g., Wall({normal : [1,0,0], distance : 100}) would be a wall 100 pixels to the left, whose normal points right