Blackmagic Design DaVinci Resolve Advanced Panel - Manual (Page 211)

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Compositing Using the Alpha Output
The Alpha Output is an optional output bar that you can turn on in the node graph, that appears
underneath the Output bar. The Alpha Output bar is useful for creating true transparency in a clip in
order to composite it with other clips on lower video tracks.
There are many ways of using the Alpha Output, but let’s focus on two examples. In the first one, we’ll
use the Alpha Output to composite a greenscreen clip with a background plate.
In preparation for this composite, the background plate is on track V1 of the Timeline, and the greenscreen
clip has been superimposed on track V2.
To create a chroma-key composite :
1. Right-click anywhere within the node tree working area and choose Add Alpha Output
from the contextual menu to add the Alpha Output bar at the left of the node graph.
An Alpha Output bar appears underneath the Output bar.
A greenscreen clip, ready for compositing
A superimposed greenscreen clip on track V2 of the Timeline, above a background plate
on track V1, ready for compositing.