Apple Final Cut Pro 5 - Manual (Page 1176)

Chapter 19 Keying, Mattes, and Masks 389
II
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.
 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.
 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.
 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 Pro 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.
 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.