Keying is a technique for compositing two images or frames together in which a colour (or a small colour range) from one image is removed (or made transparent), revealing another image behind it. This technique is also referred to as colour keying, colour-separation overlay, greenscreen, and bluescreen. It is commonly used for weather forecast broadcasts, wherein the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background. The meteorologist stands in front of a bluescreen, and then different weather maps are added on those parts in the image where the colour is blue. If the meteorologist wears blue clothes, their clothes will become replaced with the background video. This also works for greenscreens, since blue and green are considered the colours least like skin tone.

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