A sequence of still images (called frames) varying slightly from image to image that, when projected at high speed, produce the illusion of movement. Animation can display at any speed, but is usually somewhere between 22:30 frames per second. Most familiar types of animation are two-dimensional (2-D) animation, or flat images overlayed and moving against a flat background. The most common type of 2-D animation is cel animation, in which the animated images are drawn or painted on transparent overlays. Computer graphics programs are increasing the quality of three-dimensional (3-D) animation, or images that appear to have depth.