Tutorial The ABCs of Chromakeys

Chromakeying is a process widely used throughout the television industry to merge (composite) one image (often a live one) with previously-shot footage or graphics and make it look perfectly natural. For instance, you see the weather forecaster on the evening news pointing to various locales on national or local maps, while advising you of what weather conditions to expect. In reality, however, he or she is actually pointing to a blank green wall while the map, animation and graphics are electronically composited with the live image of the forecaster. Chromakeying is another visual magic trick that's easy to do with video to achieve impressive results.

Originally, the motion picture industry used blue screens extensively for chromakey compositing; hence, they called the process bluescreening. Bright green has some technical advantages over blue for the process, so green has become the predominant color for chromakeying, which is frequently called - you guessed it - greenscreening. While other colors are occasionally used for specific purposes (such as when the subject is green), green is the runaway favorite and most-used color for chromakeying. Regardless of what color you use, the process basically involves keying that one specific color (or a small range of it) to make it invisible or transparent when composited with another image. Generally speaking, green is used for video work, whereas blue is favored for film work.

There are several video software packages available with chromakeying capabilities. As is usually the case, the capabilities are commensurate with the price of the software, and some of these programs can do things that are really amazing - all at a price, of course. One of my favorite programs, and the one I used for purposes of illustration here, is Adobe Ultra CS3.

Regardless of which software package you use, there are some important things that make your chromakeying look natural and believable. First and foremost, shoot your footage in crisp focus, and light it adequately and uniformly. It is also very important to light your greenscreen background uniformly and insure that it is as smooth as possible. When these elements are as good as possible, you increase your chances for successful chromakeying.

Preplanning is essential with any production, and chromakeying is no exception. First decide what image elements you want to combine, e.g., what the live or foreground image is and what the background will be. Don't forget that the background image can be either static or moving. Whether to go with a still background or a moving one depends a lot on what you're showing your audience, so it is something else that you have to consider.

It's also important to match the lighting mood between the foreground and the background. A brightly-lit foreground image shot in daylight against a dimly-lit background shot by incandescent light won't look very natural or convincing. Unless, of course, that's intentionally the look you're going for. Adobe Ultra CS3 gives you quite a bit of control for altering the lighting, shadows, color, hue, saturation, gamma, transparency and other aspects of the keyed image, so you can really fine-tune things to make them work together for a very convincing composite, even if your two image sources are somewhat disparate.