Apple MA261Z/A User Manual - Page 809

IX If you must apply compression during capture, you can still pull good keys from clips with as much as a 2:1 compression ratio, but ideal source footage should be uncompressed. DV footage, which is compressed with a 5:1 ratio as it's recorded, is less than ideal. This is because of compression artifacts that, while invisible during ordinary playback, become apparent around the edges of your foreground subject when you start to key. However, this doesn't mean that you can't key with DV footage. With a high-quality DV camera and good lighting, it's possible to pull a reasonable key using DV clips, but you cannot expect the kind of subtleties around the edges of a keyed subject that you can get with uncompressed or minimally compressed footage. For example, while you may be able to preserve smoke, reflections, or wisps of hair when keying uncompressed footage, with equivalent DV footage this probably won't be possible. On the other hand, if your foreground subject has slicked back hair and a crisp suit, and if there are no translucent areas to worry about, you may be able to pull a perfectly acceptable key. Use Proper Lighting The lighting you use when shooting blue or green screen footage plays a crucial role in determining whether or not you'll be able to key out the background easily. Â Blue or green background: Should be evenly lit, with no exceptionally bright areas (hot spots) or shadows. Whatever material you use for the background screen should be smooth, with no bumps or wrinkles. Â Video signal: Should have a minimum of film or video grain, since the "noisiness" grain introduces can make it more difficult to pull a good key. Video can get grainy in low-light situations, so the lighting on your background screen should be bright enough that you don't have to turn up the gain of your videocamera. Â Lighting of foreground subject: Should have close to a 1:1 ratio to the lighting of the background screen. This avoids overexposing or underexposing the background screen when the foreground subject is correctly lit. Once your background blue or green screen is properly lit, you should concentrate on lighting the foreground subject to match the scene into which you're going to composite it. It's especially important to make sure that the contrast between the shadows and highlights of your subject's lighting is correct. While you can use the Final Cut Express HD color correction filters to easily adjust the color and overall brightness of your subject, contrast is not so easy to change. This is not to say that you need to light your foreground subject flatly. Just make sure that the direction, quality, and contrast of the lighting you use works for the scene your subject will inhabit. Â Distance between foreground subject and background screen: It's a good idea to have some distance between the foreground subject and the background screen, to reduce the amount of colored light bouncing off the background blue or green screen and "spilling" on the foreground subject. In general, position your subject 5 to 10 feet away from the background screen. Chapter 55 Keying, Mattes, and Masks 809