Blackmagicdesign ATEM Television Studio Installation And Operation Manual page 82

Operating your ATEM Switcher
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Background - a full screen image, often a camera source.
Fill - the graphic you plan to display on top of your background video. Notice that the black parts of the
graphic remain intact because the key signal is used to cut out the fill signal. The fill signal is often provided
by a graphics system.
Key - a grayscale image that defines the region in the image that will be removed so that the fill signal can
be correctly stacked on top of the background. The key signal is often provided by a graphics system.
Pre-multiplied Key
A modern graphics system or character generator that offers fill and key outputs will most likely provide
what is known as a pre-multiplied or shaped key. A pre-multiplied key is a special combination of the fill
and key signal where the fill signal has been pre-multiplied with the key signal over a black background.
Photoshop generated images that contain an alpha channel are pre-multiplied.
ATEM switchers have an auto key adjustment for pre-multiplied keys so that when the pre-multiplied key
setting is enabled, the clip and gain parameters are automatically set by the system.
When using a Photoshop generated image, generate graphics over a black background layer and place
all content on the upper layers. You also need to make sure you have created an alpha channel in your
Photoshop document that the ATEM can use for blending the graphic over the live video. Then when saved
as a Targa image file, or downloaded direct to the media pool, you can select pre-multiplied in the keyer
to get a good key. Photoshop documents are pre-multiplied by nature, so you should always use the pre-
multiplied settings on the ATEM switcher when keying them.