Grass Valley 1200 Operation Manual - Manual (Page 24)

2-6
Section 2— Startup
Key
A key is an effect in which parts of one picture are inserted into
another to create a composite picture. For example, keys are used
to insert captions, to place a small news scene or graphic over the
shoulder of a newscaster, or to place the image of a weather
reporter in front of a weather map. There are several types of keys,
each of which serves a different purpose.
Luminance Key
A luminance key , shown in Figure 2-3, 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 we want to insert a green logo into
some background video. We could print the logo in white on
black paper and then focus a camera on it. We can then apply the
signal from the camera to the switcher’s keyer where it will be
used as a key signal. Next we can select a green matte video to
apply to the keyer as the fill video. When the clip is set properly,
the keyer will ignore the black paper and use only the white logo
shape to cut a hole in the background video. Then it will fill the
logo-shaped hole with green matte video. This creates a green
logo inserted into the background. In actuality, this method of
keying is seldom used anymore. Instead, a graphics system
generates the key and fill video, but the same concepts apply.
Luminance keyers also include a gain control, which sets the
sharpness of the transition from one picture to another at the key’s
edge; high gain yields a sharp edge, and low gain yields a soft
edge that mixes the fill and background video. In addition, the
Model 1200 keyers include an S-shaping function that prevents
the transition at the key edge from becoming too sharp and
creating what appears to be a band or line around the key.