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THE A-B-Cs OF CHROMA KEYS
Without a doubt, you see chroma keys used countless times – probably every time you watch the news on TV – and perhaps you aren’t even aware of it. Chroma keying is a process widely used through- out the television industry to merge (composite) one image – often a live one – with previously shot foot- age 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 compos- ited with the live image of the forecaster. Chroma keying is another visual magic trick that’s easy to do with video to achieve impressive results. BLUE vs. GREEN Originally, blue screens were used extensively for chroma key compositing in the motion picture industry; hence, the process became known as blue- screening. Bright green has some technical advan- tages over blue for the process, so green has be- come the predominant color used for chroma keying, which is frequently called – you guessed it – green- screening. 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 chroma keying. 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 an- other image. Generally speaking, green is used for video work whereas blue is favored for film work. SHEDDING SOME LIGHT ON CHROMA KEYS There are several video software packages available with chroma keying capabilities. As is us- ually the case, the capabilities are commensurate
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by Tom Benford
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. But regardless of which software package you use, there are some important things to do that make your chroma keying 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 green-screen background uniformly lit and insure that it is as smooth as possible. When these elements are as good as possible you increase your chances for successful chroma keying. SOUNDS LIKE A PLAN Pre-planning is essential with any production, and chroma keying is no exception. First decide what image elements you want to combine; e.g., what is the live or foreground image and what will the background be. And don’t forget that the background image can either be 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 has to be considered as well. 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.