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Video Compositing is the Process of combining of 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.

    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 replaces every pixel within the designated color range with a pixel from another image, aligned to appear as part of the original.
blue or green screens may back news-readers to allow the compositing of stories behind them, before being switched to a 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. 
Virtual sets are also used in motion picture filmmaking, a few of which are photographed entirely in blue or green screen environments, as for example in Sky Captain and the World of Tomorrow.

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.
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.
Main article: Multiple Exposures
An in-camera multiple exposures 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.

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. 

In rear projection, or (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. 


Main article: Matte (filmmaking)
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 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. 

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 the 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 spaceship, and a second spaceship could be shot separately against the blue screen, each "moving" differently. The individual shots could then be composited with one another, and finally with a star background. 

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