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"Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing 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 the newscasting, motion picture and videogame industries. A color range in the top layer is made transparent, revealing another image behind... This technique is also referred to as color keying, or by various terms for specific color-related variants such as green screen, and blue screen, since green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors." (from Wikipedia https://en.wikipedia.org/wiki/Chroma_key )
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