Combining of visual elements from separate sources into single images
This article is about visual effects. For the process of combining several democratic motions, see compositing (democracy). For the technique of compositing typeset by hand, see typesetting. For compositing in graphic design and still photography, see photomontage. For other uses, see Compositing (disambiguation).
Exchanging the background of a video clip with a Compositing toolA composite image of a basketball shot, with six basketballs added to the initial image to depict the arc of the shot.
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 shooting for compositing is variously called "chroma key", "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.
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) 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 positioned in front of a plain blue or green background, while compositing software replaces only the designated blue or green color with weather maps.
Composite of photos of one place, made more than a century apart
Typical applications
In television studios, 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 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 angles, subjects, and virtual "backgrounds".
Virtual sets are also used in motion picture filmmaking, 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 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 intermediates" enable compositing and other operations of computerized post production. Digital compositing is a type of matting, and one of four basic compositing methods. The others are physical compositing, multiple exposure, and background projection, a method which utilizes both front projection 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.[1]
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. 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 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
The Playhouse composited using multiple exposures to show nine copies of Buster Keaton on screen at once.
An in-camera multiple exposure is made by recording on only one part of each film frame, 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 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 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 sequence in Alfred Hitchcock's North by Northwest. (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.
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, 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
Four images of the same subject, removed from their original backgrounds and composited onto a new background
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.
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 passes or layers and then combine the resulting 2D images into a single, final image called the composite. Compositing is used extensively in film when combining computer-rendered image elements with live footage. Alpha blending is also used in 2D computer graphics to put rasterized foreground elements over a background.
Widescreen images are displayed within a set of aspect ratios used in film, television and computer screens. In film, a widescreen film is any film image with a width-to-height aspect ratio greater than 4:3 (12:9).
Chroma key compositing, or chroma keying, is a visual-effects and post-production technique for compositing (layering) two or more images or video streams together based on colour hues. The technique has been used in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture, and video game industries. A colour range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as colour keying, colour-separation overlay, or by various terms for specific colour-related variants such as green screen or blue screen; chroma keying can be done with backgrounds of any colour that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from any human skin colour. No part of the subject being filmed or photographed may duplicate the colour used as the backing, or the part may be erroneously identified as part of the backing.
Special effects are illusions or visual tricks used in the theatre, film, television, video game, amusement park and simulator industries to simulate the imagined events in a story or virtual world.
A view camera is a large-format camera in which the lens forms an inverted image on a ground-glass screen directly at the film plane. The image is viewed, composed, and focused, then the glass screen is replaced with the film to expose exactly the same image seen on the screen.
Cinematography is the art of motion picture photography.
Visual effects is the process by which imagery is created or manipulated outside the context of a live-action shot in filmmaking and video production. The integration of live-action footage and other live-action footage or CGI elements to create realistic imagery is called VFX.
An optical printer is a device consisting of one or more film projectors mechanically linked to a movie camera. It allows filmmakers to re-photograph one or more strips of film. The optical printer is used for making special effects for motion pictures, or for copying and restoring film material.
Mattes are used in photography and special effects filmmaking to combine two or more image elements into a single, final image. Usually, mattes are used to combine a foreground image with a background image. In this case, the matte is the background painting. In film and stage, mattes can be physically huge sections of painted canvas, portraying large scenic expanses of landscapes.
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 performers, front projection projects the pre-filmed material over the performers and onto a highly reflective background surface.
Rear projection is one of many in-camera effects cinematic techniques in film production for combining foreground performances with pre-filmed backgrounds. It was widely used for many years in driving scenes, or to show other forms of "distant" background motion.
The sodium vapor process is a photochemical film technique for combining actors and background footage. It originated in the British film industry in the late 1950s and was used extensively by Walt Disney Productions in the 1960s and 1970s as an alternative to the more common bluescreen process. Wadsworth E. Pohl is credited with the invention or development of both of these processes, and received an Academy Award in 1965 for the sodium vapor process used in the film Mary Poppins.
Reverse bluescreen is a visual effects technique pioneered by Jonathan Erland of Apogee Inc.(John Dykstra's company) for shooting the flying sequences in the film Firefox. Its objective is to enable the matting of subjects that confound the conventional process, such as those exhibiting reflective surfaces. It derives its name from the fact that it reverses, or inverts, the basic bluescreen process. Erland received a Scientific and Engineering Award from the Academy of Motion Picture Arts and Sciences for this technique.
In cinematography, bipacking, or a bipack, is the process of loading two reels of film into a camera, so that they both pass through the camera gate together. It was used both for in-camera effects and as an early subtractive colour process.
Primatte is a brand of chroma key software used in motion picture, television and photographic host applications to remove solid colored backgrounds and replace them with transparency to facilitate ‘background replacement’. It uses a unique algorithm based on three multi-faceted polyhedrons floating in RGB colorspace that are used to isolate color regions in the foreground image. Primatte is often referred to as a compositing technology and is usually used as a plug-in for host products such as Adobe After Effects, Adobe Photoshop, Autodesk Media and Entertainment Inferno or Flame, Eyeon Fusion and several other compositing and editing software packages.
Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film. Film is typically segmented in frames, that give rise to separate photographs.
Introvision was a variation on a front-projection process that allowed film makers to view a finished composite of live action and plate photography through the camera's viewfinder on set and in real time. During its heyday, starting with the feature film, Outland in 1981, Introvision enjoyed the novelty of visual effect compositing in-camera, thus eliminating the need to wait for photo-chemical compositing to determine if the effect shot was successful.
Technicolor is a series of color motion picture processes, the first version dating back to 1916, and followed by improved versions over several decades.
Multi-image is the now largely obsolete practice and business of using 35mm slides (diapositives) projected by single or multiple slide projectors onto one or more screens in synchronization with an audio voice-over or music track. Multi-image productions are also known as multi-image slide presentations, slide shows and diaporamas and are a specific form of multimedia or audio-visual production.
The Williams process or Williams doublematting process is a matte creation technique patented by the American cinematographer Frank D. Williams in 1918. Unlike prior matte techniques, it allowed for the integration of the actors' movements with previously shot backgrounds.
References
↑ Professional Cinematography, Clarke, Charles G., A.S.C., Los Angeles, 1964, p 152 ff.
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 ( ISBN0-12-133960-2)
Steve Wright, Digital Compositing for Film and Video, Second Edition ( ISBN0-240-80760-X)
American Cinematographer Manual, 2nd ed., Mascelli, Joseph V., A.S.C. and Miller, Arthur, A.S.C, eds. Los Angeles, 1966, p.500 ff.
External links
Look up compositing in Wiktionary, the free dictionary.
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