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Section 3 — Switcher Concepts
Luminance Key
A luminance key, shown in Figure 3-3, uses the brightness
information in one picture, called the key source to cut a hole in
another picture, called the background. A third picture, called the
fill, is inserted into the hole to fill it. A clip control allows the
operator to set the level of key source brightness that will cut the
hole so that any part of the picture that is lower in brightness than
the clip setting is ignored and will not cut the hole. The operator
can also invert the key so that dark areas of the source cut the hole
instead of bright areas.
To use an example, suppose you want to insert a green logo into
some background video. You could print the logo in white on
black paper and then focus a camera on it. You can then apply the
signal from the camera to the switcher’s keyer where it will be
used as a key signal. Next, you can select a green matte video to
apply to the keyer as the fill video.
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Key Source
Fill Video
Background with Key and Fill Inserted
Figure 3-3. Luminance Key Example