Composition and Layering

Microsoft DirectShow 9.0

Composition and Layering

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.