Chroma key

[4] It is commonly used for live weather forecast broadcasts in which a news presenter is seen standing in front of a CGI map instead of a large blue or green background.

Chroma keying is also common in the entertainment industry for visual effects in movies and video games.

[6]: 4 In the 1920s, Walt Disney used a white backdrop to include human actors with cartoon characters and backgrounds in his Alice Comedies.

At RKO, Linwood Dunn used an early version of the travelling matte to create "wipes" – where there were transitions like a windshield wiper in films such as Flying Down to Rio (1933).

Credited to Larry Butler, a scene featuring a genie escaping from a bottle was the first use of a proper bluescreen process to create a travelling matte for The Thief of Bagdad (1940), which won the Academy Award for Best Special Effects that year.

In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began working on an ultraviolet travelling matte process.

He also began developing bluescreen techniques: one of the first films to use them was the 1958 adaptation of the Ernest Hemingway novella, The Old Man and the Sea, starring Spencer Tracy.

An optical printer with two projectors, a film camera and a "beam splitter", was used to combine the actor in front of a blue screen together with the background footage, one frame at a time.

In the early 1970s, American and British television networks began using green backdrops instead of blue for their newscasts.

For the film The Empire Strikes Back, Richard Edlund created a "quad optical printer" that accelerated the process considerably and saved money.

For decades, travelling matte shots had to be done "locked-down", so that neither the matted subject nor the background could shift their camera perspective at all.

Meteorologists on television often use a field monitor, to the side of the screen, to see where they are putting their hands against the background images.

Some films make heavy use of chroma key to add backgrounds that are constructed entirely using computer-generated imagery (CGI).

Performances from different takes can be composited together, which allows actors to be filmed separately and then placed together in the same scene.

Advances in computer technology have simplified the incorporation of motion into composited shots, even when using handheld cameras.

Reference points such as a painted grid, X's marked with tape, or equally spaced tennis balls attached to the wall, can be placed onto the coloured background to serve as markers.

Modern advances in software and computational power have eliminated the need to accurately place the markers ⁠— ⁠the software figures out their position in space; a potential disadvantage of this is that it requires camera movement, possibly contributing to modern cinematographic techniques whereby the camera is always in motion.

Green is used as a backdrop for TV and electronic cinematography more than any other colour because television weather presenters tended to wear blue suits.

Broadcast-quality colour television cameras use separate red, green, and blue image sensors, and early analog TV chroma keyers required RGB component video to work reliably.

In the digital television and cinema age, much of the tweaking that was required to make a good quality key has been automated.

Removing these spots could be done by a suitable double-exposure with the colour positive (thus turning any area containing red or green opaque), and many other techniques.

In analog television, colour is represented by the phase of the chroma subcarrier relative to a reference oscillator.

[6]: p20  This can look unnatural or cause portions of the characters to disappear, so must be compensated for, or avoided by using a larger screen placed far from the actors.

An example of intentional use of this is when an actor wears a blue covering over a part of his body to make it invisible in the final shot.

was far easier to manufacture and make reliable than film that somehow excluded both frequencies higher and lower than the screen colour.

Red is avoided as it is in human skin, and any other colour is a mix of primaries and thus produces a less clean extraction.

[6]: 16 A newer technique is to use a retroreflective curtain in the background, along with a ring of bright LEDs around the camera lens.

[14][15] For Star Trek: The Next Generation, an ultraviolet light matting process was proposed by Don Lee of CIS Hollywood and developed by Gary Hutzel and the staff of Image G. This involved a fluorescent orange backdrop which made it easier to generate a holdout matte, thus allowing the effects team to produce effects in a quarter of the time needed for other methods.

[18] A background with a repeating pattern alleviates many of these issues, and can be less sensitive to wardrobe colour than solid-colour backdrops.

A very simple f() function for green screen is A(r+b) − Bg where A and B are user adjustable constants with a default value of 1.0.