Two images are composited by removing the background from one image and placing it over the second image. The foreground subject is shot with a green screen behind it that is evenly lit so that there are no shadows or bright spots. During production, the green is removed from the foreground image through digital manipulation so that the subject can be placed over the background image without any visible lines. A common example is that of a weather reporter on television. The reporter stands in front of a green screen. During the video broadcast, animation replaces the green screen so that the viewer sees the forecaster in front of a map or other graphic.
Green is used because it is not likely to show up in human skin tones. Therefore, using green as a background makes it fairly easy for the artist using the compositing software to remove the areas of the foreground image that are not wanted in the final composite image.
Blue was traditionally used in the early days of filmmaking because it is complementary to skin tones. Also, film cameras were most sensitive to light in the blue spectrum. Modern video cameras respond more readily to green. Red and other colors are also sometimes used as backgrounds when the foreground image contains green or blue elements that the photographer or cinematographer wants to preserve.
Although the green screen needs to be evenly lit to reduce problems with production, the models or actors may be lit with any lighting source or combination of sources that the photographer or director wish to use. The important thing is that the green screen background should not have wide variations of color.
A wide variety of software is available for professional photographers and cinematographers, as well as entry-level software packages aimed at the consumer market. Adobe Premiere, iMovie, Windows Movie Maker, LoiLoScope and Sony's Vegas are just a few examples.