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Compositing is the process or technique of combining visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene.
Live-action Live action (or live-action) is a form of cinematography or videography that uses photography instead of animation. Some works combine live-action with animation to create a live-action animated film. Live-action is used to define film, video ...
shooting for compositing is variously called "
chroma key Chroma key compositing, or chroma keying, is a visual-effects and post-production technique for compositing (layering) two images or video streams together based on colour hues ( chroma range). The technique has been used in many fields to ...
", "blue screen", "green screen" and other names. Today, most, though not all, compositing is achieved through digital image manipulation. Pre- digital compositing techniques, however, go back as far as the trick films of Georges Méliès in the late 19th century, and some are still in use.


Basic procedure

All compositing involves the replacement of selected parts of an image with other material, usually, but not always, from another image. In the digital method of compositing, software commands designate a narrowly defined color as the part of an image to be replaced. Then the software (e.g.
Natron Natron is a naturally occurring mixture of sodium carbonate decahydrate ( Na2CO3·10H2O, a kind of soda ash) and around 17% sodium bicarbonate (also called baking soda, NaHCO3) along with small quantities of sodium chloride and sodium sulfate. ...
) replaces every pixel within the designated color range with a pixel from another image, aligned to appear as part of the original. For example, one could record a television
weather presenter A weather presenter (also known colloquially in North America as a weatherman or weather broadcaster) is a person who presents the weather forecast daily on radio, television or internet news broadcasts. Using diverse tools, such as projected weath ...
positioned in front of a plain blue or green background, while compositing software replaces only the designated blue or green color with weather maps.


Typical applications

In
television studios Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
, blue or green screens may back news-readers to allow the compositing of stories behind them, before being switched to full-screen display. In other cases, presenters may be completely within compositing backgrounds that are replaced with entire "virtual sets" executed in
computer graphics Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
programs. In sophisticated installations, subjects, cameras, or both can move about freely while the computer-generated imagery (CGI) environment changes in real time to maintain correct relationships between the
camera angle The camera angle marks the specific location at which the movie camera or video camera is placed to take a shot. A scene may be shot from several camera angles simultaneously. This will give a different experience and sometimes emotion. The diff ...
s, subjects, and virtual "backgrounds". Virtual sets are also used in motion picture
filmmaking Filmmaking (film production) is the process by which a motion picture is produced. Filmmaking involves a number of complex and discrete stages, starting with an initial story, idea, or commission. It then continues through screenwriting, cast ...
, usually photographed in blue or green screen environments (other colors are possible but less common), as for example in '' Sky Captain and the World of Tomorrow''. More commonly, composited backgrounds are combined with sets – both full-size and models – and vehicles, furniture, and other physical objects that enhance the realism of the composited visuals. "Sets" of almost unlimited size can be created digitally because compositing software can take the blue or green color at the edges of a backing screen and extend it to fill the rest of the frame outside it. That way, subjects recorded in modest areas can be placed in large virtual vistas. Most common, perhaps, are set extensions: digital additions to actual performing environments. In the film '' Gladiator'', for example, the arena and first tier seats of the
Roman Colosseum The Colosseum ( ; it, Colosseo ) is an oval amphitheatre in the centre of the city of Rome, Italy, just east of the Roman Forum. It is the largest ancient amphitheatre ever built, and is still the largest standing amphitheatre in the world to ...
were actually built, while the upper galleries (complete with moving spectators) were computer graphics, composited onto the image above the physical set. For motion pictures originally recorded on film, high-quality video conversions called "
digital intermediate Digital intermediate (typically abbreviated DI) is a motion picture finishing process which classically involves digitizing a motion picture and manipulating the color and other image characteristics. Definition and overview A digital intermediat ...
s" enable compositing and other operations of computerized
post production Post-production is part of the process of filmmaking, video production, audio production, and photography. Post-production includes all stages of production occurring after principal photography or recording individual program segments. Th ...
. Digital compositing is a type of matting, and one of four basic compositing methods. The others are physical compositing,
multiple exposure In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be id ...
, and background projection, a method which utilizes both
front projection A front projection effect is an in-camera visual effects process in film production for combining foreground performance with pre-filmed background footage. In contrast to rear projection, which projects footage onto a screen from behind the pe ...
and rear projection.


Physical compositing

In physical compositing the separate parts of the image are placed together in the photographic frame and recorded in a single exposure. The components are aligned so that they give the appearance of a single image. The most common physical compositing elements are partial models and glass paintings. Partial models are typically used as set extensions such as ceilings or the upper stories of buildings. The model, built to match the actual set but on a much smaller scale, is hung in front of the camera, aligned so that it appears to be part of the set. Models are often quite large because they must be placed far enough from the camera so that both they and the set far beyond them are in sharp focus.Professional Cinematography, Clarke, Charles G., A.S.C., Los Angeles, 1964, p 152 ff. Glass shots are made by positioning a large pane of glass so that it fills the camera frame, and is far enough away to be held in focus along with the background visible through it. The entire scene is painted on the glass, except for the area revealing the background where action is to take place. This area is left clear. Photographed through the glass, the live action is composited with the painted area. A classic example of a glass shot is the approach to Ashley Wilkes' plantation in ''
Gone with the Wind Gone with the Wind most often refers to: * ''Gone with the Wind'' (novel), a 1936 novel by Margaret Mitchell * ''Gone with the Wind'' (film), the 1939 adaptation of the novel Gone with the Wind may also refer to: Music * ''Gone with the Wind'' ...
''. The plantation and fields are all painted, while the road and the moving figures on it are photographed through the glass area left clear. A variant uses the opposite technique: most of the area is clear, except for individual elements (photo cutouts or paintings) affixed to the glass. For example, a
ranch house Ranch (also known as American ranch, California ranch, rambler, or rancher) is a domestic architectural style that originated in the United States. The ranch-style house is noted for its long, close-to-the-ground profile, and wide open layout. ...
could be added to an empty valley by placing an appropriately scaled and positioned picture of it between the valley and the camera.


Multiple exposure

An in-camera
multiple exposure In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be id ...
is made by recording on only one part of each
film frame In filmmaking, video production, animation, and related fields, a frame is one of the many '' still images'' which compose the complete '' moving picture''. The term is derived from the historical development of film stock, in which the sequenti ...
, rewinding the film to exactly the same start point, exposing a second part, and repeating the process as needed. The resulting negative is a composite of all the individual exposures. (By contrast, a "double exposure" records multiple images on the entire frame area, so that all are partially visible through one another.) Exposing one section at a time is made possible by enclosing the camera lens (or the whole camera) in a light-tight box fitted with maskable openings, each one corresponding to one of the action areas. Only one opening is revealed per exposure, to record just the action positioned in front of it. Multiple exposure is difficult because the action in each recording must match that of the others; thus, multiple-exposure composites typically contain only two or three elements. However, as early as 1900 Georges Méliès used seven-fold exposure in ''L'homme-orchestre/The One-man Band''; and in the 1921 film '' The Playhouse'', Buster Keaton used multiple exposures to appear simultaneously as nine different actors on a stage, perfectly synchronizing all nine performances.


Background projection

Background projection throws the background image on a
screen Screen or Screens may refer to: Arts * Screen printing (also called ''silkscreening''), a method of printing * Big screen, a nickname associated with the motion picture industry * Split screen (filmmaking), a film composition paradigm in which mul ...
behind the subjects in the foreground while the camera makes a composite by photographing both at once. The foreground elements conceal the parts of the background image behind them. Sometimes, the background is projected from the front, reflecting off the screen but not the foreground subjects because the screen is made of highly directional, exceptionally reflective material. (The prehistoric opening of '' 2001: A Space Odyssey'' uses front projection.) However, rear projection has been a far more common technique. In rear projection, (often called process shooting) background images (called "plates", whether they are still pictures or moving) are photographed first. For example, a camera car may drive along streets or roads while photographing the changing scene behind it. In the studio, the resulting "background plate" is loaded into a projector with the film "flipped" (reversed), because it will be projected onto (and through) the back of a translucent screen. A car containing the performers is aligned in front of the screen so that the scenery appears through its rear and/or side windows. A camera in front of the car records both the foreground action and the projected scenery, as the performers pretend to drive. Like multiple exposure, rear projection is technically difficult. The projector and camera motors must be synchronized to avoid flicker and perfectly aligned behind and before the screen. The foreground must be lit to prevent light spill onto the screen behind it. (For night driving scenes, the foreground lights are usually varied as the car "moves" along.) The projector must use a very strong
light source Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terah ...
so that the projected background is as bright as the foreground. Color filming presents additional difficulties, but can be quite convincing, as in several shots in the famous
crop duster A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydroponi ...
sequence in Alfred Hitchcock's ''
North by Northwest ''North by Northwest'' is a 1959 American spy thriller film, produced and directed by Alfred Hitchcock and starring Cary Grant, Eva Marie Saint and James Mason. The screenplay was by Ernest Lehman, who wanted to write "the Hitchcock picture ...
''. (Much of the sequence, however, was shot on location.) Because of its complexity, rear projection has been largely replaced by digital compositing with, for example, the car positioned in front of a blue or green screen.


Matting

Traditional matting is the process of compositing two different film elements by printing them, one at a time, onto a duplicate strip of film. After one component is printed on the duplicate, the film is re-wound and the other component is added. Since the film cannot be exposed twice without creating a double exposure, the blank second area must be masked while the first is printed; then the freshly exposed first area must be masked while the second area is printed. Each masking is performed by a "traveling matte": a specially altered duplicate shot which lies on top of the copy film stock. Like its digital successor, traditional matte photography uses a uniformly colored backing – usually (but not always) a special blue or green. Because a matching filter on the camera lens screens out only the backing color, the background area records as black, which, on the camera's
negative film In photography, a negative is an image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. This reversed order occurs because t ...
, will develop clear. First, a print from the original negative is made on high-contrast film, which records the backing as opaque and the foreground subject as clear. A second high-contrast copy is then made from the first, rendering the backing clear and the foreground opaque. Next, a three-layer sandwich of film is run through an optical printer. On the bottom is the unexposed copy film. Above it is the first matte, whose opaque backing color masks the background. On top is the negative of the foreground action. On this pass, the foreground is copied while the background is shielded from exposure by the matte. Then the process is repeated; but this time, the copy film is masked by the reverse matte, which excludes light from the foreground area already exposed. The top layer contains the background scene, which is now exposed only in the areas protected during the previous pass. The result is a positive print of the combined background and foreground. A copy of this composite print yields a "dupe negative" that will replace the original foreground shot in the film's edited negative.


Advantages of digital mattes

Digital matting has replaced the traditional approach for two reasons. In the old system, the five separate strips of film (foreground and background originals, positive and negative mattes, and copy stock) could drift slightly out of registration, resulting in halos and other edge artifacts in the result. Done correctly, digital matting is perfect, down to the single-pixel level. Also, the final dupe negative was a "third generation" copy, and film loses quality each time it is copied. Digital images can be copied without quality loss. This means that multi-layer digital composites can easily be made. For example, models of a space station, a space ship, and a second space ship could be shot separately against blue screen, each "moving" differently. The individual shots could then be composited with one another, and finally with a star background. With pre-digital matting, the several extra passes through the optical printer would degrade the film quality and increase the probability of edge artifacts. Elements crossing behind or before one another would pose additional problems.


See also

*
Alpha compositing In computer graphics, alpha compositing or alpha blending is the process of combining one image with a background to create the appearance of partial or full transparency. It is often useful to render picture elements (pixels) in separate pas ...
* Broadcast designer * Character generator * Clean feed (TV) * Compositing window manager * Deep image compositing * Derivative work * Digital asset * Digital compositing * Digital on-screen graphic (Bug or DOG) * Graphics coordinator * Image stitching *
Matte painting A matte painting is a painted representation of a landscape, set, or distant location that allows filmmakers to create the illusion of an environment that is not present at the filming location. Historically, matte painters and film technicians ...
* Video matting *
Motion graphic design Motion graphic design, also known as motion design, is a subset of graphic design in that it uses graphic design principles in a filmmaking or video production context (or other temporally evolving visual media) through the use of animation or ...
*
Multiple exposure In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be id ...
* Photographic mosaic * Primatte chromakey technology


References


Further reading

* T. Porter and T. Duff, "Compositing Digital Images", Proceedings of SIGGRAPH '84, 18 (1984). * Ron Brinkmann, The Art and Science of Digital Compositing () * Steve Wright, Digital Compositing for Film and Video, Second Edition () *
American Cinematographer ''American Cinematographer'' is a magazine published monthly by the American Society of Cinematographers. It focuses on the art and craft of cinematography, covering domestic and foreign feature productions, television productions, short films, mu ...
Manual, 2nd ed., Mascelli, Joseph V., A.S.C. and Miller, Arthur, A.S.C, eds. Los Angeles, 1966, p. 500 ff.


External links

{{Wiktionary, compositing http://www.videomaker.com/article/10914-green-screen-lighting Visual effects