gstreamer/docs/random/design

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GStreamer Editing Services
--------------------------
This is a list of features and goals for the GStreamer Editing
Services.
Some features are already implemented, and some others not. When the
status is not specified, this means it is still not implemented but
might be investigated.
FUNDAMENTAL GOALS:
1) API must be easy to use for simple use-cases. Use and abuse
convenience methods.
2) API must allow as many use-cases as possible, not just the simple
ones.
* Project file load/save support (GESFormatter)
Status:
Implemented, requires API addition for all use-cases.
Problems:
Timelines can be stored in many different formats, we need to
ensure it is as easy/trivial as possible for users to load/save
those timelines.
Some timeline formats might have format-specific
sources/objects/effects which need to be handled in certain ways
and therefore provide their own classes.
The object that can save/load GESTimeline are Formatters.
Formatters can offer support for load-only/save-only formats.
There must be a list of well-known GES classes that all formatters
must be able to cope with. If a subclass of one of those classes is
present in a timeline, the formatter will do its best to store a
compatible information.
A Formatter can ask a pre-render of classes that it doesn't
understand (See Proxy section).
Formatters can provide subclasses of well-known GES classes when
filling in the timeline to offer format-specific features.
* Grouping/Linking of Multiple TrackObjects
Status:
Implemented, but doesn't have public API for controlling the
tracked objects or creating groups from TimelineObject(s)
Problems:
In order to make the usage of timelines at the Layer level as easy
as possible, we must be able to group any TrackObject together as
one TimelineObject.
The base GESTimelineObject keeps a reference to all the
GESTrackObjects it is controlling. It contains a mapping of the
position of those track objects relatively to the timeline objects.
TrackObjects will move and be modified synchronously with the
TimelineObject, and vice-versa.
TrackObjects can be 'unlocked' from the changes of its controlling
TimelineObject. In this case, it will not move and be modified
synchronously with the TimelineObject.
* Selection support (Extension from Grouping/Linking)
Problems:
In order to make user-interface faster to write, we must have a way
to create selections of user-selected TimelineObject(s) or
TrackObject(s) to move them together.
This should be able to do by creating a non-visible (maybe not even
inserted in the layer?) TimelineObject.
* Effects support
TBD (Pending proposal by Thibault)
* Source Material object
Problems:
Several TimelineSource for a same uri actually share a lot
in common. That information will mostly come from GstDiscoverer,
but could also contain extra information provided by 3rd party
modules.
Definition:
Material: n, The substance or substances out of which a thing is or
can be made.
In order to avoid duplicating that information in every single
TimelineSource, a 'Material' object needs to be made available.
A Material object contains all the information which is independent
of the usage of that material in a timeline.
A Material object can specify the TimelineSource class to use in a
Layer.
* Proxy support
Problems:
A certain content might be impossible to edit on a certain setup
due to many reasons (too complex to decode in realtime, not in
digital format, not available locally, ...).
In order to be able to store/export timelines to some formats, one
might need to have to create a pre-render of some items of the
timeline, while retaining as much information as possible.
Content here is not limited to single materials, it could very well
be a complex combination of materials/effects like a timeline or a
collection of images.
To solve this problem, we need a notion of ProxyMaterial.
It is a subclass of Material and as such provides all the same
features as Material.
It should be made easy to create one from an existing TimelineSource
(and it's associated Material(s)), with specifiable rendering
settings and output location.
The user should have the possibility to switch from Proxy materials
to original (in order to use the lower
resolution/quality/... version for the editing phase and the
original material for final rendering phase).
* Editing modes (Ripple/Roll/Slip/Slide)
Problems:
Most editing relies on heavy usage of 4 editing tools which editors
will require. Ripple/Roll happen on edit points (between two clips)
and Slip/Slide happen on a clip.
The Ripple tool allows you to modify the beginning/end of a clip
and move the neighbour accordingly. This will change the overall
timeline duration.
The Roll tool allows you to modify the position of an editing point
between two clips without modifying the inpoint of the first clip
nor the out-point of the second clip. This will not change the
overall timeline duration.
The Slip tool allows you to modify the in-point of a clip without
modifying it's duration or position in the timeline.
The Slide tool allows you to modify the position of a clip in a
timeline without modifying it's duration or it's in-point, but will
modify the out-point of the previous clip and in-point of the
following clip so as not to modify the overall timeline duration.
These tools can be used both on TimelineObjects and on
TrackObjects, we need to make sure that changes are propagated
properly.
* Coherent handling of Content in different formats
Problems:
When mixing content in different format (Aspect-Ratio, Size, color
depth, number of audio channels, ...), decisions need to be made on
whether to conform the material to a common format or not, and on
how to conform that material.
Conforming the material here means bringing it to a common format.
* Media Asset Management integration
(Track, Search, Browse, Push content) TBD
* Templates
Problem:
In order to create as quickly as possible professional-looking
timelines, we need to provide a way to create 'templates' which
users can select and have an automatic timeline 'look' used.
This will allow users to be able to quickly add their clips, set
titles and have a timeline with a professional look. This is
similar to the same feature that iMovie offers both on desktop and
iOS.
* Plugin system
Problem:
All of GES classes are made in such a way that creating new
sources, effects, templates, formatters, etc... can be easily added
either to the GES codebase itself or to applications.
But in order to provide more features without depending on GES
releases, limit those features to a single application, and in
order to provide 'closed'/3rd party features, we need to implement
a plugin system so one can add new features.
Use a registry system similar to GStreamer.