Adobe 25510629 - After Effects CS3 Professional, Руководство пользователя

Страница: 78 / 130

78 How to
Set Key
Set Key is the recommended means for keying, not only because it produces the best results,
but also because it’s easiest. At the click of a button, Ultra analyzes the entire frame and
generates a mathematical model that it uses to key out the background for the session.
To use this method, the clip must have at least one reference frame, which is a frame that
shows only the color screen background with nothing in the foreground. The frame can have
areas beyond the edges of the color screen, because you will mask them out later.
The reference frame does not actually have to be from the input clip that you will be
compositing. The most important thing is that the lighting, framing, and state of the
backdrop for the reference frame and the actual input clip are identical. As long as none of
these factors are different, the reference frame can come from another clip. If conditions on
the set are static and reasonably under your control, you could shoot one clip to provide the
reference frame for multiple clips with the same framing. If, however, any elements change—
for example, a wrinkle in the backdrop appears, disappears, or migrates a few inches—then
you need a new reference frame for subsequent input clips.
To use the Set Key method, assign the clip containing a reference frame as the input clip and
drag the preview slider to a reference frame. Then click the Set Key button. You don’t have to
select Enable first, because the Set Key button automatically enables the keyer.
The background should disappear to reveal the checkerboard in the Input Preview Monitor
and the current background in the Output Preview Monitor. Also, the button changes from
Set Key to Apply Points.
If the actual input clip is not loaded, assign it in place of the reference clip. Drag the preview
slider through the session with the virtual set or background assigned, looking for points
where the key is less than ideal. If you find any, then you must fine-tune the key. For more
information, see
“Fine-tuning the Keyer settings” on page 84 .
NO
TE
If you're seeing a ghost image, then you probably clicked the Set Key button with the
subject in the frame. This method works only when the frame is free of anything in the
foreground.
To resolve this problem, either drag the preview slider to a show clear reference frame
and click Set Key again, deselect Enable on the Keyer tab, or use one of the Key Point
methods. For more information, see
“Setting Key Points (Apply Points Method)”
on page 79 .
The Set Key Method works only if the framing of the camera remains constant
throughout the shot. This means no zooming in or out, no panning or tilting, no moving
boom or dolly shots, and no handheld camera. If you want these types of results, keep in
mind that you can accomplish them in Ultra by using virtual camera moves. For more
information, see
“Panning and zooming” on page 99 .