Keying is creating areas of transparency based on color or lightness values in an image. Keying is commonly performed on subjects photographed against a blue or green background, but keys can be based on any color (color or chroma keying), or on a specific range of brightness values (luma keying).
Keying allows you to composite (combine) two clips, where the video of the top, or foreground, clip is processed to key out (remove) either a color or a lightness value in areas of the video and then is combined with the bottom, or background, clip.
For example, you can take a video clip of a person standing in front of a green-screen background and replace the green with an interior scene, making it appear as though that person is standing in the room.
This type of keying is accomplished using one of two keyer effects in Final Cut Pro:
Keyer: This general-purpose chroma-keying effect is optimized for blue- or green-screen keying but can key any range of color you choose. See Use chroma keys.
Luma Keyer: This is designed to generate mattes based on the image’s lightness—you choose to remove the white or black areas and whether the gray areas should be partially transparent. See Use luma keys.
In addition to these keying effects, you may need to use a matte, a positioning effect, and color correction to ensure the foreground video looks natural when keyed over the background. For more information, see Finalize the key and Masking overview.