Apple Motion 2 - Manual (Page 760)

760 Chapter 10 Using Filters
Keying Filters
Keying filters are used to isolate a subject that is surrounded by an area of fairly
uniform color or brightness, by generating an alpha channel based on the shape of
that area. Keying is most commonly performed on subjects that were shot against a
blue- or green-colored background, but keys can also be based on brightness values.
A common example of color keying is the meteorologist on TV. This person is always
seen standing in front of an animated map of the weather, but in reality he or she is
standing in front of a big greenscreen. As part of the broadcast, the greenscreen is
keyed out, and replaced by the map. You can do the same thing in Motion.
Important: Keying a foreground subject is not always easy, and it takes time and
patience to learn how to use the parameters in each filter to achieve the best effect.
Even so, most keys are pulled using more than one tool to do the job. Good
compositing artists usually combine one or more masked keying filters, Matte Choker
filters, a Spill Suppressor filter, and one or more garbage and holdout masks to isolate a
single subject. For more information on techniques you can use to improve a key, see
“
Performing Multiple Keys on a Single Subject ” on page 872.
Blue Green Screen
The Blue Green Screen filter is intended for subjects that were shot in front of a
bluescreen or greenscreen. This filter renders the blue or green areas of the image
transparent, allowing one or more background images to show through.
Parameters in the Inspector
Key: Selects one of three keying modes: blue, green, or blue/green difference.
Color Level: Sets the percentage of color purity being keyed. Values range from 0
to 100.
Tolerance: Sets the color tolerance for your key. Values range from 1 to 100.
Edge Thin: Sets the amount of thinning done at the edges of the matte. A positive
value erodes the matte edge. Values range from -100 to 100.
Invert Matte: Toggles whether or not the matte is inverted.
Dashboard controls
The Dashboard contains the same controls as the Inspector.
Color Key
This filter can pull a key based on any color you select. Bear in mind that if the color is
too close to that of the subject you’re trying to isolate, parts of your foreground subject
may disappear as well. The resulting effect renders the selected color transparent,
allowing a background image to show through.
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