Chroma keying
Chroma Keying (also color keying) is a visual effect or a post-production technique in which a chrominance is used for cropping image material. In the context of compositing , this technique is used to exchange backgrounds in photos or videos. To do this, an alpha channel is calculated from a specific color range . The most commonly used colors are green and blue, as they stand out from the skin tone in color. Green screens and blue screens are used in television and film productions.
In television, chroma keying is often used for news broadcasts or weather reports to incorporate background images and animations. The aim here is often a clipping in real time ( live keying ). If almost all visible elements except the presenter are digitally inserted in a television broadcast, it is called a virtual studio. Such a studio is e.g. B. at ZDF or RTL in use.
Chroma keying is used in film production so that actors and elements can be released without great effort. Here the keying process usually only takes place afterwards, in post-production when creating the visual effects.
The advancement of technology and computer software, such as Autodesk Smoke, Final Cut Pro , Pinnacle Studio, Adobe After Effects and dozens of other programs, mean that chroma key functions can also be used on home computers with affordable green or blue screen packages possible.
history
Even before the introduction of digital compositing, a complex and time-consuming process known as a "traveling matte" was used in film production. The blue screen process was developed by RKO Radio Pictures in the 1930s . At RKO, Linwood Dunn used an early version of the hiking mask to create transitions (wipes). Such transitions, which worked like a windshield wiper, were used in films such as Flying Down to Rio (1933). Thanks to Larry Butler, a scene in the 1940 film The Thief of Baghdad where a genie tries to escape from a bottle made the first use of a proper blue screen process, and the film won the Oscar for visual effects in that process Year. In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began work on an ultraviolet traveling matte process. He also started developing blue screen techniques. One of the first films to use the new techniques was the 1958 adaptation of the Ernest Hemingway novella " The Old Man and the Sea " with Spencer Tracy .
A disadvantage of the traditional hiking mask is that the cameras cannot be easily synchronized when rotating. For decades, mask recordings had to be made without camera movements or zoom, so that the camera perspective could not be shifted for either the main person or the background . Later motion control cameras solved this problem so that both the foreground and the background could be filmed with the same camera movements. Petro Vlahos was awarded an Oscar in 1964 for refining these techniques.
His technique takes advantage of the fact that most objects in the real world are of a color whose blue color component has a similar intensity to its green color component. An optical printer with two projectors, a movie camera, and a "beam splitter" were used to combine the actor in front of a blue screen along with the background material. Parts of the technique were contributed by Zbigniew Rybczyński .
In the early 1970s, American and British television networks began using green instead of blue backdrops for their news programs. In the 1980s, minicomputers were used to control the optical printers. For the film The Empire strikes back (Engl. "The Empire Strikes Back") created Richard Edlund a quad optical printer, which greatly accelerated the process. He received a special Academy Award for his innovation . For Spaceship Enterprise: The Next Century , a UV Light Matte Process was proposed by Don Lee and Gary Hutzel . They developed a fluorescent orange background that made it easier to generate a holdout mat, so the effects team only needed a quarter of the time.
TV meteorologists often used a field monitor on the side of the camera to see where their hands were in the background. A newer technique is to project a faint image onto the background.
Some movies make heavy use of chroma keying to add backgrounds complete with Computer Generated Imagery (CGI). Different takes / shots can be mixed together, in which actors are filmed separately and later put together in the same scene. Chroma keying enables artists to appear anywhere without leaving the studio. Computer development also made it easier to incorporate motion into composite shots, even when using handheld cameras. Points of reference can be placed on the colored background (usually as a painted grid, taped or attached to the wall). In post-production, a computer can use these references to calculate the position of the camera and thus create an image that perfectly matches the perspective and movement of the foreground. Modern advances in software and computing power have even eliminated the need to accurately place the markers - the software finds its position in space (a disadvantage of this is that it requires large camera movements).
dress
A chroma keying subject must avoid clothing that contains the same color as the chroma key color (s) (unless it is intended that body parts are not of natural origin) or the background will replace the clothing. This technique can be used to achieve an effect similar to the effect of the invisibility cloak in the Harry Potter films. The actor can thus be filmed with an object of the same color against a chroma key background, so that he is replaced by the background and magically disappears. Problems arise when the depicted object / subject has to have the same color as the background, as in the movie Superman , in which the actor wears a blue outfit and therefore cannot be shot in front of a blue screen. In the Spider-Man movie from 2002, the scenes with Spider-Man and the green goblin had to be filmed in front of different backgrounds, since the costume of Spider-Man was blue and red and that of the goblin was green. So both blue and green screens were used in a film.
Uniform illumination
The greatest challenge when installing blue or green screens is uniform illumination and thus avoiding shadows. The aim is to keep the color palette that is being digitally replaced as small as possible. A shadow would appear like a darker color to the camera and may not be captured for replacement. This can occasionally occur during low budget productions or live broadcasts when manual repair is not immediately possible. Above all, the material that is used influences the quality of the recordings as well as the simplicity of the even illumination. Glossy surfaces are less promising than matt. Glossy surfaces have reflective zones that make these areas appear paler, while others appear darker. A matt surface diffuses the reflected light and has a uniform color palette. With the aim of obtaining the purest “ key ” for shooting with a green screen, it is necessary to create a value difference between the subject / object and the green screen. In some cases, shadows are also used for special effects. Areas of the blue or green screen that are covered with a shadow can be replaced by darker versions of the desired background image. Even changing the focal length of the camera (s) affects the result of “chroma keying”.
Individual evidence
- ↑ New ZDF studio has to prove itself in everyday life. Retrieved February 11, 2017 .