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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 because green and blue backgrounds differ most distinctly in hue from most human skin colors." (from Wikipedia https://en.wikipedia.org/wiki/Chroma_key )
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