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.
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Keying, Mattes, and Masks
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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.