Like a movie, every 2D and 3D computer game is composed of scenes. For computer games, the definition of a scene is conceptually similar to that of movies, but with some distinctions. A computer game scene can be thought of as a level, a screen, an environment, or any situation that involves scene objects existing in the same coordinate space. For example, in a simple Space Invaders type game the player controls a spaceship (one scene object) that can move either left or right along the bottom of the screen (but not up or down), and UFOs (many scene objects) enter from the top of the screen and gradually move downward. The aim for the player is to shoot all the UFOs before they reach the bottom of the screen. All these objects-spaceships, background, UFOs, and so on-are part of the same scene, and all these objects exist within the same coordinate space. In other words, each can have its position defined absolutely from the origin or relatively from one another.