Call of Duty 4: Modern Warfare runs on a proprietary engine and with features that include true world-dynamic lighting, HDR lighting effects, dynamic shadows, and depth of field.[12] "Bullet Penetration" is calculated by the engine, taking into account factors such as surface type and entity thickness. Certain objects, such as cars and some buildings, are destructible. This makes distinguishing cover from concealment important, as the protection provided by objects such as wooden fences and thin walls do not completely protect players from harm. Bullet speed and stopping power are decreased after penetrating an object, and the decrease is dependent on the thickness and surface type of the object. The game makes use of a dynamic physics engine, not implemented in previous Call of Duty titles. Death animations are a combination of pre-set animations and ragdoll physics.