Another important concept in real time is that of collision. Once players are allowed to travel wherever they wish in a 3D world, physical constraints must be set to prevent them from traveling through walls, yet allowing them to move over uneven terrain, stairs, and other obstacles. In a 3D engine, typically, the constraints will be invisible primitive objects, or they may be derived from the mesh object’s geometry directly. The former are the most economical, as far as frame rate goes. With complicated meshes, one often creates a lower poly stand-in for collision. Many game engines will do this for you.