The Timeline object:

Those TimelineObjects can control one or many TrackObjects from one or many Tracks of the Timeline.

The goal of the Timeline is to offer an API suited for fast UI coding. The UI can then decide whether to listen to events/modifications taking place on the overall Timeline, or on the individual Tracks.

This allows both creating simple and complex UI while offering the same interfaces


All of the following editing actions must be done on the Timeline, TimelineObject(s) or TimelineSelections and not on the Track(s) or TrackObject(s):

  • Adding objects to the Timeline
  • Removing objects from the Timeline
  • Linking and Unlinking objects
  • Grouping and Ungrouping objects
  • Moving objects
  • Changing objects priority
  • Trimming, Sliding, Rolling, and all other actions modifying any of start duration in-point out-point priority.


  • One or many Tracks corresponding to the Project Settings. There is one Track per output Streams (Most projects will therefore have 2 Tracks, one for the Audio stream and one for the Video stream).
  • An ordered collection of TimelineObjects. Those objects can be Sources (producing content) or Operations (modifying content).
  • A list of Selections, grouping several existing TimelineObject.
    • This is a convenience for modifying several distinct TimelineObjects at the same time.
    • A TimelineObject can belong to several TimelineSelection
    • A TimelineSelection can control one or many properties to keep the modifications of those properties in sync (start-position, priority, in-point, out-point, ...)
  • The TimelineFactory, used in the Pipelines (for viewing, rendering, ...).

Timeline properties

tracks : Ordered list of Tracks controlled by the Timeline
selections : List of TimelineSelection in use.
objects : Ordered list of TimelineObject controlled by the Timeline. First ordered by start property, and then by priority.
factory : The TimelineFactory to use in Pipelines.

Track properties

objects : Ordered list of TrackObjects controlled by the Track.
stream : The Stream of the Track.

TimelineObject properties


factory : The ObjectFactory this TimelineObject corresponds to. : None is an accepted value

start : The position of the TimelineObject.
duration : The duration of the TimelineObject.
in-point : The in-point of the contents of the TimelineObject.
out-point : The out-point of the contents of the TimelineObject.
priority : The priority of the TimelineObject.
min-start : The earliest time to which we can set the start property of the TimelineObject with the trimStart method
max-duration : The maximum value we can set the duration property of the TimelienObject to

; track-objects :: The TrackObject(s) it the TimelineObject controls.
; track : the track to which the object belongs
; object : The actual TrackObject
; time-offset : The offset between the TimelineObject start position and the TrackObject start position. ALWAYS POSITIVE.
; priority-offset : The offset between the TimelineObject priority and the TrackObject priority. ALWAYS POSITIVE.

TimelineSelection properties

objects : The list of TimelineObject controlled by the TimelineSelection

TrackObject properties


parent : The TimelineObject controlling this TrackObject. All the properties below MUST NOT BE MODIFIED DIRECTLY but through the parent TimelineObject.
start : The position of the TrackObject.
duration : The duration of the TrackObject.
in-point : The in-point of the contents of the TrackObject.
out-point : The out-point of the contents of the TrackObject.
priority : The priority of the TrackObject.


Between Timeline and Track(s)

The diagram below shows the relationship between Timeline, Tracks, TimelineObject and TrackObjects.

Each TimelineObject controls at least one TrackObject in any of the Timeline's Tracks.

We can see here that the two first TimelineObject control one TrackObject per Track. But the last TimelineObject only controls a TrackObject in the first Track.

Each TrackObject of a given TimelineObject can have a relative offset

This can be seen with the 2nd TimelineObject where the TrackObject it controls in the second track doesn't start at the same position.

Relationship between Timeline and Track

Between containers, TimelineObject and ObjectFactory

The following diagram shows the relationship between:

  • Containers
    • Timeline
    • Track
  • Objects
    • At the Timeline level : TimelineObject
    • At the Track level : TrackObject
  • ObjectFactory
    • At the Timeline level : ObjectFactory
    • At the Track level : Streams(s) of an ObjectFactory

Relationship between TimelineObject, Containers and ObjectFactory

With the diagram above, the use-case of adding a source file to a Timeline becomes trivial:

  • Pick the ObjectFactory corresponding to your source file
  • Add it to the Timeline
    • The Timeline creates a TimelineObject compatible with the given ObjectFactory type

      Timelines can therefore reject incompatible ObjectFactory like Live devices

    • The timeline looks for what Streams the ObjectFactory can consume/provide

    • For each of the Streams that the ObjectFactory handles:

      • Find a Track with a compatible Stream

        The user can of course choose his own Stream<=>Track mapping

      • Create a TrackObject for that ObjectFactory Stream

      • Add the TrackObject to the Track

      • Link the TrackObject to the newly created TimelineObject

Between TrackObject and GStreamer

The following diagram shows:

  • On the left, the class hierarchy for TrackObject
  • On the right, the class hierarchy for the various GNonLin GStreamer elements.

The links between TrackObject(s) and GnlObject(s) show their relationship. Relationship between TrackObject(s) and the GStreamer element they control

Use cases

Unlinking two TrackObjects

Unlinking two TrackObjects coming from the same ObjectFactory

We have a TimelineObject 'X' controling two TrackObject 'A' and 'B' coming from a common ObjectFactory 'O'. This is the most common case when adding a Audio+Video File to the Timeline.

We want to handle the TrackObject(s) separately. Maybe to offset them, maybe to remove one of the TrackObject, ...

  • We ask to unlink a certain TrackObject (B) from its controlling TimelineObject (X).
  • The TimelineObject (X) looks for the ObjectFactory (O) from which the TrackObject (B) was created. In this case there's only one ObjectFactory, but there could be several in the case of LinkedSources.
  • It creates a new empty TimelineObject (Y) for the selected ObjectFactory (O). That new TimelineObject is a complete clone of (X) except for the list of TrackObject(s) it controls.
  • It removes the TrackObject (B) from the list of objects it's tracking. This means that:
    • (B) temporarily has no controlling TimelineObject.
    • The TrackObject (B) has NOT been removed from the Track to which it belonged.
  • It adds the TrackObject (B) to the list of objects controlled by the new TimelineObject (Y).

Linking Two TrackObjects

That come from the same ObjectFactory

Linking two TrackObjects coming from the same ObjectFactory

This is the case when we had two TrackObjects originally belonging to the same TimelineObject, but which we unlinked.

  • We ask to link a TrackObject (B) to a TimelineObject (X).
  • The TimelineObject (X) compares the originating ObjectFactory of the TrackObject (B) and sees they are the same.
  • The TimelineObject (X) unsets the TrackObject (B) from its current parent TimelineObject (Y)
    • If (Y) is no longer controlling any TrackObjects, we remove it from the Timeline.
  • The TimelineObject (X) adds the TrackObject (B) to its list of controlled TrackObject.

That come from different ObjectFactory

Linking two TrackObjects coming from different ObjectFactory

This is for the more generic case of linking two TrackObjects. A common example is when we recorded Audio and Video on separate devices/files.

  • We ask to link TrackkObject (B) to TrackObject (A)
  • We see they belong to originated from completely different ObjectFactory, requiring a new TimelineObject to control the two.
  • Since we don't want to lose the original relationships between TrackObject and ObjectFactory, we create a LinkedtimelineObject (XY), wrapping the two previously existing TimelineObject (X and Y).
    • Possibility : Since all TimelineObject have a counterpart ObjectFactory, we could automatically create a LinkedObjectFactory for the LinkedTimelineObject (XY) we just created
      • This allows infinit reuse of objects created in the Timeline.

Remaining issues

  • Does core need to provide some help for the UI's marker/keyframe handling ?
  • How do we 'properly' handle the different kinds of linkage in LinkedTimelineObject
    • Some people might just want to have synchronized 'start' positions, but independent priorities for the TrackObjects.
    • Some people might want to have the priorities synchronized, but freely move the positions of the TrackObjects.

The results of the search are