If you want to key text (or anything else) where you remove part of the image/video and show the video underneath, there are three ways to do it. Each has its benefits and challenges.
First, there’s luma (or luminance) keying. This is the simplest way to key. You have two choices. Remove all the black or remove all the white from an image. The downside is by removing all the black, you can’t use black to separate the foreground, for example, white text from the background, like a white shirt or white guitar.
Second, we have chroma. This type of key is much more versatile. You generally remove blue (more often used in film) or green. As long as the key image doesn’t contain both of those colors, you should be able to get a good key. There’s a limitation here, too. Chromakey removes one hue and doesn’t do a good job with translucency. Still, it’s a cheap way to go about doing things.
The third is a linear key (or alpha channel). This system normally required dedicated hardware and isn’t available on all systems, but it IS the most flexible. With alpha, you can use all colors because it a separate video channel to indicate transparency. This channel also allows for translucency as well.
To watch how I do it with the ATEM television studio HD, ProPresenter 7, and a BMD decklink duo2 card, watch the video above.