Chroma key compositing, or chroma keying, is a visual effects / post-production technique for compositing two images or video streams together based on color hues. The technique has been used heavily in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture and videogame industries. A color range in the footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is used in video production and post-production. No part of the subject being filmed or photographed may duplicate the color used as the backing, when using a blue screen, different weather maps are added on the parts of the image where the color is blue. If the news presenter wears blue clothes, his or her clothes will also be replaced with the background video, chroma keying is also common in the entertainment industry for visual effects in movies and videogames. Prior to the introduction of travelling mattes and optical printing, double exposure was used to introduce elements into a scene which werent present in the initial exposure and this was done using black draping where a green screen would be used today. George Albert Smith first used this approach in 1898, in 1903, The Great Train Robbery by Edwin S. Porter used double exposure to add background scenes to windows which were black when filmed on set, using a garbage matte to expose only the window areas. In 1918 Frank Williams patented a travelling matte technique, again based on using a black background and this was used in many films, such as The Invisible Man. In the 1920s, Walt Disney used a backdrop to include human actors with cartoon characters. The blue screen method was developed in the 1930s at RKO Radio Pictures, 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. In 1950, Warner Brothers employee and ex-Kodak researcher Arthur Widmer began working on a travelling matte process. He also began developing techniques, one of the first films to use them was the 1958 adaptation of the Ernest Hemingway novella, The Old Man. Petro Vlahos was awarded an Academy Award for his refinement of techniques in 1964. His technique exploits the fact that most objects in real-world scenes have a color whose blue-color component is similar in intensity to their green-color component, zbigniew Rybczyński also contributed to bluescreen technology. An optical printer with two projectors, a camera and a beam splitter, was used to combine the actor in front of a blue screen together with the background footage. In the early 1970s, American and British television networks began using green backdrops instead of blue for their newscasts, during the 1980s, minicomputers were used to control the optical printer. For the film The Empire Strikes Back, Richard Edlund created an optical printer that accelerated the process considerably
Virtual television studio with green-screen technique. The high amount of contrast between different parts of the screen is not ideal (see even lighting). Green reflections from the desk would create undesirable artifacts.
Demonstration of the creation of visual effects techniques utilizing chroma key
A live broadcast of Myx TV using green-screen chroma key. Note the lack of shadows on the screen. The whiter area near the center of the image is due to the angle this photo was taken from, and would not appear from the video camera's angle.