Apple Final Cut Express 4 - Manual (Page 840)

840 Part IX Effects and Color Correction
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.
Shooting Footage That Keys Well
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. 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.