Chroma key compositing, or chroma keying, is a special effects/post-production technique for compositing (layering) two images or video streams together based on color hues (chroma range). The technique has been used heavily in many fields to remove a background from the subject of a photo or video – particularly live news and weather casting, motion picture, and videogame industries. A color range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production. Chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors. No part of the subject being filmed or photographed may duplicate the color used as the backing.
It is commonly used for weather forecast broadcasts; wherein a news presenter is usually seen standing in front of a large CGI map during live television newscasts, though it is a large blue or green background. 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.
Photomontage is a mechanical equivalent to chroma key photography. The foreground of the new work is cut and trimmed from one photograph and pasted on top of a second, background photograph, line drawing, or painting.
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