Animation is a dynamic medium in which images or objects are manipulated to appear as moving images. In traditional animation, images are drawn or painted by hand on transparent celluloid sheets to be photographed and exhibited on film. Today most animations are made with computer-generated imagery (CGI). Computer animation can be very detailed 3D animation, while 2D computer animation can be used for stylistic reasons, low bandwidth or faster real-time renderings. Other common animation methods apply a stop motion technique to two and three-dimensional objects like paper cutouts, puppets or clay figures. The stop motion technique where live actors are used as a frame-by-frame subject is known as pixilation. Commonly the effect of animation is achieved by a rapid succession of sequential images that minimally differ from each other. The illusion as in motion pictures in general is thought to rely on the phi phenomenon and beta movement, but the exact causes are still uncertain. Analog mechanical animation media that rely on the rapid display of sequential images include the phénakisticope, zoetrope, flip book, praxinoscope and film. Television and video are popular electronic animation media that originally were analog and now operate digitally. For display on the computer, techniques like animated GIF and Flash animation were developed. Apart from short films, feature films, animated gifs and other media dedicated to the display moving images, animation is also heavily used for video games, motion graphics and special effects. The physical movement of image parts through simple mechanics in for instance the moving images in magic lantern shows can also be considered animation. Mechanical animation of actual robotic devices is known as animatronics. Animators are artists who specialize in creating animation.