My interest in Luma Keying was sparked when one of my students had a problem during the Green Screen assignment I give each semester in my Video Production for Photographers class at Santa Monica College. The student, as students will, chose a situation where using a green screen or any other color screen was impossible because the subject was an octopus in a public aquarium with no access to the back. She did luck out in that the background went black.
Here is why. The way sampling in video is noted is Luma (brightness). Chroma (color). Chroma (color) often noted as 4.4.4, 4.2.0 or some other variation of 4 and the two other numbers. This means almost always when compressing video into a camera codec, the full range of luma, which is the first number, is kept intact and only the color information is compressed. So she starts out with a good amount of luma (brightness). Her next bit of joy comes from the fact that not only are there several chroma (color) keyers in Final Cut Pro but there is also a luma keyer as well. Simply go to Browser>Video Filters>Key>Luma Key. The adjustments are simple, too. Decide Key Out Darker or Lighter and find the best Threshold and Tolerance settings. Each shot will probably be a little different and you will have a decent key. All keying is a black art and seldom is applying one filter going to solve all the issues. There often is a mixture of tracks, mattes and filters needed to get a really good keying done. Also see my post on Double Your Pleasure.