 "Left 4 Dead" uses the 2008 version of Valve's Source engine, with improvements such as multi-core processor support and physics-based animation to more realistically portray hair and clothing, and to improve physics interaction with enemies when shot or shoved in different body parts.Animation was also improved to allow characters to lean realistically when moving in curved paths.Rendering and artificial intelligence were scaled up to allow for a greater number of enemies who can navigate the world in better ways, such as climbing, jumping, or breaking obstacles.