Chroma-keying is a process of creating a composite of two images by photographing the foreground subject against a solid colored background. The solid color is then removed and made transparent, allowing the background image to show through. Thus, the illusion of an object being in a place where that object or person has never been is created.
Creating the Foreground
The first step in creating a chroma-keyed image is to photograph, film or video tape the foreground subject against an evenly lit, solid-colored background. Common colors for chroma-key work are either blue or green. The size of the background will depend on the size of the foreground subject. For example, if the foreground is a plastic model, such as a car, the size of the background can be fairly small. If a person is being filmed or photographed against a chroma-key background, the size of the background will need to be larger. In some cases, the background may need to be fairly tall. For example, if a model is photographed from a low angle, the chroma-key background will need to be tall enough to fill the frame to the height of the final composite image.
Removing the Key Color
In the past, chroma key colors were chemically removed on film and the film was physically laid over a second image. With the advent of digital imaging and digital compositing, removing the chroma key is achieved by digitally selecting the color and luminance of the background. By simply deleting the colored background, the transparency for overlay on the new background is generated.
Compositing the Image
Digital composite images are created by overlaying the foreground image with its transparent background over a background image. A person photographed with even light against a chroma-key background can thus be placed in a variety of locations. A well done chroma-key composite can quickly move a person from Times Square to Trafalger Square. In some cases, the edge where the person meets the background needs to be smoothed and blended to create a believable photograph.
Amateur Chroma-Key Techniques
In many cases, a specialized chroma-key background is not necessary. With Photoshop's ability to select based on color, any evenly lot solid-colored background can often work for digital chroma-key type compositing. In some cases, with careful selection techniques, the background can be removed from a photograph that isn't a solid color. For example, the New York skyline could be carefully selected and removed and the resulting person against a transparent background could be placed against a Seattle skyline.
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