Special effects embrace a wide array of photographic, mechanical, pyrotechnic, and model-making skills.

The most important resource of the special effects department is the optical printer, essentially a camera and projector operating in tandem, which makes it possible to photograph a photograph. In simplest form this apparatus is little more than a contact printer with motorized controls to execute simple transitions such as fades, dissolves, and wipes. A 24-frame dissolve can be accomplished by copying the end of one film scene and the beginning of another onto a third film so that diminished exposure of the first overlaps increased exposure of the second. Slow motion can be created by reprinting each frame two or three times. Conversely, printing every other frame (skip printing) speeds up action to create a comic effect or to double the speed when filming action such as collisions. A freeze frame is made by copying one frame repeatedly.

The optical printer can also be used to replace part of an image. For example, a high-angle long shot in a western may reveal what looks like an entire frontier town surrounded by wilderness. Rather than take the time and trouble to actually build and film on location for a shot that may last less than a minute, filmmakers can make the shot using standing sets on the studio backlot, with skyscrapers and freeway traffic visible in the distance. One frame of the original scene is then enlarged so that a matte artist can trace the outline of the offending area on paper. When the copy negative is made, the offending area is masked and remains unexposed. The negative can then be rewound to film a matte painting of suitable location scenery. In addition to combining artwork with live action, optical printing can combine two or more live-action shots.

In the aerial image optical printer, the camera is aimed straight down at a ground glass easel on which an image is projected from below. The large image allows the artist to make a very precise alignment of the artwork and live action so that they can be filmed in one pass.

Optical printing can be combined with blue-screen photography to produce such effects as characters flying through the air. Ordinary superimposition cannot be used for this effect because the background will bleed through as the character moves. To create a traveling matte shot, it is necessary to obtain an opaque image of the foreground actors or objects against a transparent background. This is done by exploiting film’s special sensitivity to blue light. In a traditional blue-screen process the actor is posed before a primary blue background, which, to avoid shadows, is illuminated from behind (see Figure 4A). Eastman No. 5247 color negative is used to film the shot because its blue-sensitive layer yields a dense black-silver image in the area of the blue screen. On the positive print, the foreground action appears against a transparent field (see 4B). This image, printed with red light onto high-contrast panchromatic film, produces the action, or female, matte (see 4C). An additional generation yields a countermatte known as the background, or male, on which the action appears as an opaque silhouette (see 4D). This silhouette is placed with a separately photographed background (see 4E) in an optical printer. In the first pass through the optical printer, the background is “printed in” (see 4F). In the second pass, the actor and action matte are combined and the foreground is printed in (see 4G). All the elements are thus composited on one film (see 4H). There are many variations using more or fewer generations. In some systems the foreground is printed first. With a negative, or reverse, matte, the action matte is made from the camera negative and is opaque against a transparent background. The blue-screen process, in a form more complex than that described here, was used to create many spectacular effects in such films as Star Wars (1977) and E.T.—The Extraterrestrial (1982). The term blue-screen need not be taken literally. Blue-garbed Superman required a differentiated backing, and sodium vapor (yellow) light was used on the screen to yield a transparent background for the flight scenes in Mary Poppins (1964).

In the past two actors talking in a car were likely to be filmed in the studio using rear projection (process) shots; that is, the actors were photographed in front of a translucent screen through which previously filmed footage of passing scenery was projected. Location shooting and lightweight sound equipment have all but eliminated this formerly common practice in feature films, although it survives in television. When routine background replacement is still used in expensive productions, it is more likely to be done with blue-screen than with rear projection.

The light loss and lack of sharpness (especially noticeable in color) that made rear projection shots obvious has also inspired some interest in front projection. The camera is placed facing the screen, and the background projector is positioned in front of and to the side of the camera so that the beam it projects is perpendicular to the camera’s line of sight. A semitransparent mirror is angled at 45 degrees between camera and projector; the camera photographs the scene through the glass while the mirror particles reflect the projection beam onto the screen. The screen is made of Scotchlite, the trade name for a material that was originally devised to make road signs that would reflect light from a car’s headlight to the driver’s eyes. Because camera and projector are in the same optical axis in the front projection process, the background illumination is reflected directly to the camera lens so brilliantly that it is not washed out by the lighting on the actors. The actors also mask their own shadows. Front projection was used to great effect in “The Dawn of Man” sequence in 2001 (1968) wherein a leopard’s eyes lit up in facing the camera. Scotchlite screens have been used to reflect powerful lights that have been shone through tanks of dyed water to produce large-scale blue-screen effects.

To reduce the graininess that each generation of film adds to the original, concerns such as George Lucas’ Industrial Light and Magic produce their effects on 65-mm film. Others, notably Albert Whitlock, have revived the old practice of making matte effects on the camera negative. In the silent film days, this was achieved using a glass shot in which the actors were photographed through a pane of glass on which the background had been painted. The Whitlock method employs a black matte in front of the camera. A hole is cut in the matte to expose the live action, which may account for only a small portion of the image. The partially exposed negative is rewound, and the background is photographed from a matte painting on glass on which the corresponding area of live action is absent.

Miniatures (scale models) are often used in special effects work because they are relatively inexpensive and easy to handle. Great care is needed to maintain smooth, proportionate movement to keep the miniatures from looking as small and insubstantial as they really are. Models may be filmed at speeds greater than 24 frames per second (i.e., in slow motion) to achieve more realistic-looking changes in perspective and time scale. John Dykstra’s Apogee, Inc., is a leader in the field of motion control, the use of computer-controlled motors to regulate the movement of models and camera in relation to one another, thereby improving the illusion of motion. The model aircraft or spacecraft can even be made to swoop and turn as they approach the camera.

Until recently it was difficult to introduce camera movement into special effects shots. Limited camera movement was achieved by moving the camera in the optical printer, thereby creating an optical zoom, but this method did not create a convincing illusion of three-dimensionality because the foreground and background elements, as well as the grain pattern in the film, were enlarged or reduced at the same rate. When a crane or dolly was used to shoot the live portion of the scene, the background had to be animated frame-by-frame, involving considerable expense in draftsmanship. Computer-enhanced animation has made it possible to store and recall the algorithms needed to model shapes and surfaces at varied perspectives.

The increased interface of film and video techniques has great implications in the effects area. The ease with which color components can be separated and reformed makes the electronic medium especially well suited to blue-screen and similar image replacement techniques. The creation of mattes through computer graphics rather than the laborious process of laboratory development is an obvious area of cost savings. Digital image storage on laser videodiscs, as in the Abekas system, enables images to be manipulated with ease.