Clip-s are the core objects of a Layer.
Each clip may exist in a single layer but may control several TrackElement-s that span several Track-s. A clip will ensure that all its children share the same start and duration in their tracks, which will match the start and duration of the clip itself. Therefore, changing the timing of the clip will change the timing of the children, and a change in the timing of a child will change the timing of the clip and subsequently all its siblings. As such, a clip can be treated as a singular object in its layer.
For most uses of a Timeline, it is often sufficient to only interact with Clip-s directly, which will take care of creating and organising the elements of the timeline's tracks.
In more detail, clips will usually have some *core* TrackElement children, which are created by the clip when it is added to a layer in a timeline. The type and form of these core children will depend on the clip's subclass. You can use is_core to determine whether a track element is considered such a core track element. Note, if a core track element is part of a clip, it will always be treated as a core *child* of the clip. You can connect to the child_added signal to be notified of their creation.
When a child is added to a clip, the timeline will select its tracks using select_tracks_for_object. Note that it may be the case that the child will still have no set track after this process. For example, if the timeline does not have a track of the corresponding track_type. A clip can safely contain such children, which may have their track set later, although they will play no functioning role in the timeline in the meantime.
If a clip may create track elements with various track_type(s), such as a UriClip, but you only want it to create a subset of these types, you should set the supported_formats of the clip to the subset of types. This should be done *before* adding the clip to a layer.
If a clip will produce several core elements of the same track_type, you should connect to the timeline's select_tracks_for_object signal to coordinate which tracks each element should land in. Note, no two core children within a clip can share the same Track, so you should not select the same track for two separate core children. Provided you stick to this rule, it is still safe to select several tracks for the same core child, the core child will be copied into the additional tracks. You can manually add the child to more tracks later using add_child_to_track. If you do not wish to use a core child, you can always select no track.
The in_point of the clip will control the in_point of its core children to be the same value if their has_internal_source is set to true .
The max_duration of the clip is the minimum max_duration of its core children. If you set its value to anything other than its current value, this will also set the max_duration of all its core children to the same value if their has_internal_source is set to true. As a special case, whilst a clip does not yet have any core children, its max_duration may be set to indicate what its value will be once they are created.
Some subclasses (SourceClip and BaseEffectClip) may also allow their objects to have additional non-core BaseEffect-s elements as children. These are additional effects that are applied to the output data of the core elements. They can be added to the clip using add_top_effect, which will take care of adding the effect to the timeline's tracks. The new effect will be placed between the clip's core track elements and its other effects. As such, the newly added effect will be applied to any source data **before** the other existing effects. You can change the ordering of effects using set_top_effect_index.
Tracks are selected for top effects in the same way as core children. If you add a top effect to a clip before it is part of a timeline, and later add the clip to a timeline, the track selection for the top effects will occur just after the track selection for the core children. If you add a top effect to a clip that is already part of a timeline, the track selection will occur immediately. Since a top effect must be applied on top of a core child, if you use select_tracks_for_object, you should ensure that the added effects are destined for a Track that already contains a core child.
In addition, if the core child in the track is not active, then neither can any of its effects be active. Therefore, if a core child is made in-active, all of the additional effects in the same track will also become in-active. Similarly, if an effect is set to be active, then the core child will also become active, but other effects will be left alone. Finally, if an active effect is added to the track of an in-active core child, it will become in-active as well. Note, in contrast, setting a core child to be active, or an effect to be in-active will *not* change the other children in the same track.
### Time Effects
Some effects also change the timing of their data (see BaseEffect for what counts as a time effect). Note that a BaseEffectClip will refuse time effects, but a Source will allow them.
When added to a clip, time effects may adjust the timing of other children in the same track. Similarly, when changing the order of effects, making them (in)-active, setting their time property values or removing time effects. These can cause the duration_limit to change in value. However, if such an operation would ever cause the duration to shrink such that a clip's Source is totally overlapped in the timeline, the operation would be prevented. Note that the same can happen when adding non-time effects with a finite max_duration .
Therefore, when working with time effects, you should -- more so than usual -- not assume that setting the properties of the clip's children will succeed. In particular, you should use set_child_property_full when setting the time properties.
If you wish to preserve the *internal* duration of a source in a clip during these time effect operations, you can do something like the following.
```c void do_time_effect_change (GESClip * clip) { GList *tmp, *children; GESTrackElement *source; GstClockTime source_outpoint; GstClockTime new_end; GError *error = NULL;
// choose some active source in a track to preserve the internal // duration of source = ges_clip_get_track_element (clip, NULL, GES_TYPE_SOURCE);
// note its current internal end time source_outpoint = ges_clip_get_internal_time_from_timeline_time ( clip, source, GES_TIMELINE_ELEMENT_END (clip), NULL);
// handle invalid out-point
// stop the children's control sources from clamping when their // out-point changes with a change in the time effects children = ges_container_get_children (GES_CONTAINER (clip), FALSE);
for (tmp = children; tmp; tmp = tmp->next) ges_track_element_set_auto_clamp_control_source (tmp->data, FALSE);
// add time effect, or set their children properties, or move them around ... // user can make sure that if a time effect changes one
source, we should // also change the time effect for another source. E.g. if // "GstVideorate:GESClip:rate" is set to 2.0,
we also set "GstPitch:GESClip:rate" to // 2.0
// Note the duration of the clip may have already changed if the // duration-limit of the clip dropped below its current value
new_end = ges_clip_get_timeline_time_from_internal_time ( clip, source, source_outpoint, &error); // handle error
if (!ges_timeline_elemnet_edit_full (GES_TIMELINE_ELEMENT (clip), -1, GES_EDIT_MODE_TRIM, GES_EDGE_END, new_end, &error)) // handle error
for (tmp = children; tmp; tmp = tmp->next) ges_track_element_set_auto_clamp_control_source (tmp->data, TRUE);
g_list_free_full (children, gst_object_unref); gst_object_unref (source); } ```