Apple Final Cut Pro 7 - Manual (Page 1226)

Using Keying to Isolate Foreground Elements
There are two different methods used for keying: chroma keying and luma keying. Chroma
keying is a method of keying on a particular hue of color. Although any color can be keyed
on, the colors most frequently used for chroma keying are blue and green. Specific hues
of blue and green with particular levels of saturation have been developed that provide
the best results, and different companies have created commercially available paints,
fabrics, and papers that use these colors.
The color you use—blue or green—depends largely on the color of your foreground
subject. If you’re trying to create a key around a blue car, you probably want to use green
as your background. Another advantage of using green, when possible, is that video
formats generally preserve more information in the green component of the signal,
resulting in slightly better keys.
Luma keying is based on a particular range of luma. Black is usually used, but you can also
key on white. While keying out a white or black background may be more convenient in
certain circumstances, it may be harder to correctly isolate your foreground subject
because of shadows and highlights, which may have black or white values close to the
luma range you’re keying out.
Regardless of the keying method you use, it’s important to start out with clips that key
well. The decisions you make before and during your shoot affect how well your footage
keys. Make sure that you:
• Choose a video format with a minimum amount of compression that is ideal for shooting
and keying
• Light the background screen and foreground subject properly
Choosing an Appropriate Video Format
Ideal video clips for keying can be captured from footage in uncompressed or minimally
compressed video formats, such as Betacam SP or Digital Betacam footage digitized with
an uncompressed video capture interface, or DVCPRO 50 footage captured digitally with
no additional compression added. Compression discards color information from a clip
and can add artifacts around high-contrast edges in the picture (such as the edges
surrounding the image to be keyed). If you use compressed video to create keying effects,
you’ll frequently lose details around the edges of the keyed image, including hair,
translucent cloth, reflections, and smoke.
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.
1226 Chapter 72 Keying, Mattes, and Masks