What Is Photo Chroma Key?
Photo chroma key is the process of manipulating images to replace a single colored background with an image photographed at another location or digitally created using editing software.
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Chroma Key
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The process of chroma key has been around since the 1930s and has undergone many changes over the history of photography, film and video production. The process was originally referred to in film special effects as "traveling matte composite" and is now referred to as either blue or green screen. The process can be completed as either a photo chemical procedure or as a digital application using editing software. Chroma key is basically the manipulation of an image shot against a plain background; blue and green are commonly used as they are the furthest from skin tone. The background is later replaced by another background shot at a different time or produced digitally.
Photo-Chemical Process
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Chroma key began as an effect produced by shooting two sets of film. One of the negatives was then placed over the other, and the two negatives were cut or manipulated in order to produce a third image. This third image is a combination of the two original negatives, which is then printed as a single image.
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The Digital Process
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Completing the chroma key process in digital software requires an image to be shot against a colored screen. In the post-production process, after the shoot is completed everything over or under a certain brightness level is "keyed-out" and replaced by another image or color.
Digital Software
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There are a number of options when it comes to chroma key software. Application solely dedicated to chroma key are available such as TriPrism or ExpressDigital; software such as Photoshop have also produced plug-ins that are capable of completing the chroma key process.
Color Spill
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One of the major problems with chroma key is color spill: this is the reflection of color from the walls and backgrounds of the studio that tint areas of the subject and casts through the semi-transparent areas of the subject. Affected areas are usually hair, backs of shoulder, arms and legs and transparent clothing such as a wedding veil. The problem of color spill is one of the main reasons people give up on photo chroma key.
Red, Green and Blue
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Backgrounds used for chroma key have been green, blue and red in professional shoots. This is because cameras often use three individual channels to read images---red, green and blue. To complete the chroma key process, one of these channels is "keyed-out" or removed from the image to be replaced by another image. As technology has progressed, it has become easier to manipulate images digitally over many more channels than just red, green and blue, reducing the effects of color spill and color leaking over the image where the subject color matches that of the replaced background.
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