Preq: Channels
The Channel key node, your secret to processing greenscreen video. There are sample greenscreen video clips available on the net (Google, or search BlenderArtist) to use in this exercise. I discuss premultiply alpha and color spill. This also works for BlueScreen, WhiteScreen, and BlackScreen.
The second approach is using the Chroma key, which has the advantage of being able to use the eyedropper to sample the key color, the color to be "keyed" out.
The Third way is using the Difference Key.
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