Use alpha channels
Alpha channels provide an easy way to composite one image over another. Using an application like Motion, an Apple application designed to work with Final Cut Pro, you can create still images and video clips that have alpha channels. Alpha channels can be thought of as having a built-in keyer—when you use the key effects you are creating a mask that is used in the same way a clip with an alpha channel is used for compositing.
Below is an example of a clip with an alpha channel, showing the image that fills the alpha channel.
Below is the alpha channel image—the white areas define the video areas that are composited over the background. The gray areas (which define the shadow in this example) define parts of the image that are partially transparent.
This is what the final composite looks like over a background clip.
Use a clip with an alpha channel
Select the clip that has an alpha channel in the Browser, and choose Edit > Connect to Primary Storyline (or press Q).
To see the composited clips, position the playhead within the clip that was just added above the primary storyline.
The clip is composited over the primary storyline video clip, with the alpha channel controlling the opacity of its clip.
You can also use the Opacity adjustment’s fade handles to dissolve the foreground clip on and off.