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Creating Groups, Compositions, and Tracks

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Microsoft DirectShow 9.0

Creating Groups, Compositions, and Tracks

Groups, compositions, and tracks are the intermediate layers between the timeline and the source clips. They are distinguished by the type of object they can contain.

  • Tracks contain source objects.
  • Compositions contain tracks and other compositions, but not source objects.
  • Groups are top-level compositions. Groups contain compositions and tracks, but compositions cannot contain groups.
  • A virtual track is any object that can reside inside a composition or a group. This includes tracks and compositions.

These objects expose the following interfaces:

Interface Exposed By
IAMTimelineTrack Tracks
IAMTimelineVirtualTrack Tracks, Compositions
IAMTimelineComp Compositions, Groups
IAMTimelineGroup Groups

These interfaces contain the methods for adding objects to the timeline.

For example, the following code inserts a new track into a group. As shown in the previous table, a group is considered a kind of composition, and a track is a kind of virtual track. Therefore, to insert the track into the group, you must query the group for its IAMTimelineComp interface and call the IAMTimelineComp::VTrackInsBefore method.

IAMTimelineGroup    *pGroup;
// Create a new group (not shown). 

IAMTimelineComp     *pComp = NULL;
IAMTimelineObj      *pTrackObj = NULL;

pTL->CreateEmptyNode(&pTrackObj, TIMELINE_MAJOR_TYPE_TRACK);
pGroup->QueryInterface(IID_IAMTimelineComp, (void **)&pComp);
pComp->VTrackInsBefore(pTrackObj, 0);

The second parameter to VTrackInsBefore specifies the priority of the virtual track. Priority levels start at zero. If you specify the value –1, the virtual track is inserted at the end of the priority list. If you specify a value where there is already a virtual track, everything from that point on moves down the list by one priority level. Do not insert a virtual track at a priority greater than the number of virtual tracks, because it will cause undefined behavior.

To delete an object permanently from the timeline, call IAMTimelineObj::RemoveAll on the object. This method removes the object and all its children. To remove an object for the purpose of reinserting it elsewhere in the timeline, call IAMTimelineObj::Remove instead. Unlike RemoveAll, this method does not delete the object's children. To remove everything from the timeline, call IAMTimeline::ClearAllGroups.