The illusions or tricks of the eye used in the film, television, theatre, videogame, or simulator industries to simulate the imagined events in a story or virtual world are traditionally called special effects (often abbreviated as SFX, SPFX, or simply FX).
Special effects are traditionally divided into the categories of optical effects and mechanical effects. With the emergence of digital film-making tools a greater distinction between special effects and visual effects has been recognized, with "visual effects" referring to digital post-production and "special effects" referring to on-set mechanical effects and in-camera optical effects.
Optical effects (also called photographic effects), are techniques in which images or film frames are created photographically, either "in-camera" using multiple exposure, mattes, or the Schüfftan process, or in post-production processes using an optical printer. An optical effect might be used to place actors or sets against a different background.
Mechanical effects (also called practical or physical effects), are usually accomplished during the live-action shooting. This includes the use of mechanized props, scenery, scale models, animatronics, pyrotechnics and Atmospheric Effects: creating physical wind, rain, fog, snow, clouds etc. Making a car appear to drive by itself, or blowing up a building are examples of mechanical effects. Mechanical effects are often incorporated into set design and makeup. For example, a set may be built with break-away doors or walls to enhance a fight scene, or prosthetic makeup can be used to make an actor look like a monster.
Since the 1990s, computer generated imagery (CGI) has come to the forefront of special effects technologies. CGI gives film-makers greater control, and allows many effects to be accomplished more safely and convincingly – and even, as technology marches on, at lower costs. As a result, many optical and mechanical effects techniques have been superseded by CGI.