There's nothing quite as magical as being able to juxtapose images, scenes, and people into the composition you are creating. It is the key, pardon me, of creativity. While we associate keying with our modern film and digital composting, 'chroma key', 'color keying', 'blue screen' is actually a reliably old process dating to the very earliest movie and television productions.
In the 1930's two techniques were created simultaneously that are fundamental and widely used for creating virtual video and animation, the use of 'mattes' and 'keying'. The matte then as now is simply creating a scene to be projected 'on top of' the subject, 'behind' the subject. Concepts such as cut outs, literally painting a background onto glass that was placed before the camera during the video shoot and in our world of After Effects creating a 'matte layer', 'mask matte', create the illusion of this imagery as the true background of the video. This allows your actor to be in African jungles, atop great mountains, and standing before stampeding buffalo.
The companion technique is the 'blue screen', the keying. Whatever your subject is doing, he / she must be cleanly separated from their true back to be realistically placed against the matte layer. The actor was photographed against a blue background. This blue or green background was chosen primarily because of the contrast to human skin color. These colors made it easier to separate the subject from the background. Subtle techniques of special film and filters, using black and white negative images were used to extract the subject as carefully as possible from the 'blue screen'. As this technique was developed, much simpler more commonplace applications became popular. The 'meteorologist screen' uses the same technique to allow the weatherman to stand before his weather maps when he shares the weather news with us. He is filmed before a blue screen, then placed on a weather map layer.
In it's most sophisticated stage, the older forms of keying required several steps of film development, processing the matte and blue screen extraction, synchronization of the two to produce a seamless product. Fortunately for us, After Effects has a category of keying effects that allow us to create very reliable results in only minutes. One of the most highly recommended is the KeyLight tool from the Foundry:
There are many great tutorials available for keying basics and KeyLight in particular. One of my favorite is by After Effects guru Chris Meyer:
You will find the basics of keying easy to embrace, and the tools available to apply these techniques allow you to focus on your project, imagination, and possibilities instead. In my next article on keying, I will focus more on the tool, the steps, and the considerations that help you sharpen your technique and create a more reliable key.