Composition and Layering
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In a collection of tracks, the first track has the lowest priority (priority 0) and each subsequent track has a priority one level higher. At each priority level, the source clips in that track hide the source clips in the tracks below it, unless that layer also contains a transition. Thus you can imagine DES making several passes when it renders.
First, it renders track 0. There is nothing "under" Track 0, so empty regions are rendered as a solid black image. Transitions in this layer occur between the black image and track 0 or vice versa. DES lays track 1 on top of track 0, generating any transitions between the two tracks. The result is the composite of the two tracks. Next, it places track 2 onto this composite. Transitions at this layer occur between the composite and track 2. The process continues until the last (highest-priority) track is put down.
When several tracks are composited together, they behave like a single track (called a virtual track). The composition object encapsulates this behavior, making complex transitions possible. For example, one video clip can wipe to a second clip, while the composite (both clips plus the wipe) fades to a third clip.
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