Whats the difference between alpha key and chroma key?

Whats the difference between alpha key and chroma key?

 

Alpha key is a fourth channel of data in an image or video file (after red, green & blue). It's a numerical value describing how transparent each pixel should be.

Chroma key is calculated on-the-fly by your compositing tool based on how closely each pixel matches a chroma sample you specify, usually with some similarity and blending tolerances.

Alpha is typically generated for you by 3D apps to easily composite CGI onto live action. Chroma is generally used to put live action, shot in front of a green or blue screen, in front of a new background.

Hope this helps.
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