gstreamer/docs/design/effects.txt

Effects
-------

Summary
-------
1. Basic ideas
2. Problems
3. Propositions  to solve those problems
  A. The registry
  B. Effects configurability
  C. Keyframes
4. Use-cases
5. API draft

The goal of this proposal is to design a simple way to handle effects through an
API which would allow developers to handle any use-cases.

1. Basic ideas
----------------

  * GESTrackEffects are subclasses of GESTrackOperation

  * You can add effects on any clip or layer

  * You can add effects over several clips and control them as a unique effect.

  * Effects are configurable and those properties can change during time

  * We must be able to handle third-party effect providers, like the
    gnome-video-effects standard.

  * We must be able to implement complex effects. This means effects that are
    more than adding GstElement-s to the timeline. It can also mean effects
    that apply both video and audio changes.

2. Problems
----------
  * We must be able to provide a list of effects available on the system at
    runtime.

  * We must be able to configure effects through an API in GES
    withtout having to access the GstElements properties directly.

  * We should also expose the GstElement-s contained in an effect so
    it is possible for people to control their properties as they wish.

  * We must be able to implement and handle complexe effects directly in GES

  * We must be able to configure effects through time -> Keyframes without
    duplicating code from GStreamer


3. Propositions to solve those problems
---------------------------------------

A. The registry => Still to design

  We could implement a GESRegistry which would actually
  retrieve elements (effects) from the GSTRegistry and any other mean
  such as gnome-video-effects to let us get all the effects that are present
  on the system....
  This way the developers could have the list of all the effects
  that are installed on the system pretty easily.

B. Effects configurability

  The idea to be able to configure effects through a simple API in GES would
  be to add an API in GESTrackObject to access the gst-object properties that
  user would like to configure.
  We would also have a method to set those properties easily.

  We should also find a way to handle that in the case of systems such as
  gnome-effects

C. Keyframes

  We may want to handle this use-case directly in GES and for any kind of
  time related configuration? FIXME
    => Special specifications for that?

4. Use-cases
-----------

  UC-1. The user wants to add an effect to an entire clip => GESTimelineObject
        new API

  UC-2. The developer wants to allow users to configure effects => New
  GESTrackOperation API

  UC-3. The user wants to add an effect on a specific portion of a clip, we
  should allow him to specify a portion of the clip where the effect should be
  applied.

  UC-4. We want to implement an effect which isn't only composed by a bin, but
  is more complexe than that (ex: "effect '24'") => we have the
  GESTrackOperation which is the base class (abstract) for this kind of
  implementation. This class should implement vmethods to get/set configurable
  properties.

  UC-5. A developer wants to implement effect which handle music and video at
        the same time, Would the solution be to implement a GESTimelineEffect
        to handle this special usecase? FIXME

  UC-6. The developers wants to configure each elements of an effect the way
  he wants
  with a full control over it.

  UC-7. Developers want to expose all effects present on the system to the
        end-user

5. API draft
------------


  A. GESTrackObject new API

      signals:
      -------
        * property-changed: emited when a usefull property of a GstElement
        contained in the GESTrackObject changes

      /**
      * ges_track_object_list_children_properties:
      *
      * @object: The origin #GESTrackObject
      *
      * A convenience method that lists all the usefull configurable properties
      * of the GstElement-s contained in @object.
      *
      * Returns: an array of GParamSpec of the configurable properties of the
      * GstElement-s contained in @object or %NULL if a problem occurred.
      */
      GParamSpec **
      ges_track_object_list_children_properties (GESTrackObject *object);

      -> Usecases: Let user know all the property he can configure.

      /**
      * ges_track_object_set_child_property:
      *
      * @object: The origin #GESTrackObject
      * @property_name: The name of the property
      * @value: the value
      *
      * Sets a property of a GstElement contained in @object.
      *
      */
      void ges_track_object_set_child_property (GESTrackObject *object,
                                              const gchar *property_name,
                                              GValue * value);
        -> Usecases:
          + Let user configure effects easily (UC-3)

      /**
      * ges_track_object_get_child_property:
      *
      * @object: The origin #GESTrackObject
      * @property_name: The name of the property
      * @value: return location for the property value
      *
      * Gets a property of a GstElement contained in @object.
      */
      void ges_track_object_get_child_property (GESTrackObject *object,
                                              const gchar *property_name,
                                              GValue * value);

      /**
      * ges_track_object_get_material:
      *
      * @object: The origin #GESTrackObject
      *
      * This is a convenience method to get the #GESMaterial
      * from which @object has been made.
      *
      * Returns: The material from which @object has been made or %NULL
      * if @object has been made by another mean
      */
      GESMaterial *ges_track_object_get_material (GESTrackObject *object);

  B. GESTimelineObject new API

      signals:
      -------
        * effect-added: emited when an effect is added
        * effect-removed: emited when an effect is removed
        * effect-priority-changed: emited when an effect priority-changed

      /**
      * ges_timeline_object_add_effect:
      *
      * @object: The origin #GESTimelineObject
      * @effect_material: The #GESEffect from which to create the effect
      * @position: The top position you want to give to the effect,
      * -1 if you want it to be added at the end of effects.
      *
      * Adds a new effect corresponding to @effect_material to the
      * #GESTimelineObject
      *
      * Returns: The newly created #GESTrackEffect, or %NULL if there was an
      * error.
      */
      GESTrackEffect *ges_timeline_object_add_effect (GESTimelineObject *object,
                                                  GESEffect *effect_material,
                                                  gint position);

      /**
      * ges_timeline_object_get_effects:
      *
      * @object: The origin #GESTimelineObject
      *
      * Returns: a #GList of the #GESTrackEffect that are applied on
      * @object order by ascendant priorities.
      * The refcount of the objects will be increased. The user will have to
      * unref each #GESTrackOperation and free the #GList.
      */
      GList *
      ges_timeline_object_get_effects (GESTimelineObject *object);
        -> Usecases:
          + First step to allow the configuration of effects (UC-3)

      /**
      * ges_timeline_object_set_top_effect_position:
      *
      * @object: The origin #GESTimelineObject
      * @effect: The #GESTrackEffect to move
      * @newposition: the new position at which to move the @effect
      *
      * Returns: %TRUE if @effect was successfuly moved, %FALSE otherwize.
      */
      gboolean
      ges_timeline_object_set_top_effect_position (GESTimelineObject *object,
            GESTrackEffect *effect, guint newposition);

  C - The GESTrackEffect API:

      /**
      * ges_track_effect_new_from_material:
      *
      * @effect_material: The #GESEffect from which to create this
      * #GESTrackEffect
      *
      * Creates a new #GESTrackEffect from a #GESEffect
      *
      * Returns: a newly created #GESTrackEffect, or %NULL if something went
      * wrong.
      */
    GESTrackEffect *ges_track_effect_new_from_material(GESTrackEffect *effect,
                                                   GESEffect *effect_material);

      /**
      * ges_track_effect_new_from_bin_desc:
      *
      * @bin_dec: The gst-launch like bin description of the effect
      *
      * Creates a new #GESTrackEffect from the description of the bin. This is
      * a convenience method for testing puposes.
      *
      * Returns: a newly created #GESTrackEffect, or %NULL if something went
      * wrong.
      */
    GESTrackEffect *ges_track_effect_new_from_bin_desc(GESTrackEffect *effect,
                                                       const gchar *bin_desc);

  D - The GESTimelineEffect API:

  The GESTimelineEffect basically doesn't have anything else but what
  GESTimelineObject has.

    -> Usecases: The user wants to control multiple effects in sync. The user
                 wants to add an effect to the whole timeline. The user wants
                 to had an effect to a segment of the timeline without caring
                 bout what clip it is applied on.

  E - The GESEffect:

  The GESEffect class is a subclass of GESMaterial, it is used to describe 
  effects independently of the usage which is made of it in the timeline.

  A GESEffect can specify a GESTrackOperation class to use in a 
  TimelineObject.

  All important properties are inherited from GESMaterial such as:
      * Name
      * Description
      * Tags
      * ...

  We should also be able to list properties of the effect from the GESMaterial.
  
=================
  TODO GESRegistry API:
      This should be a singleton since we don't want an app to instanciate more
      than one registry. It must be able to get effects from various sources.
      We should also make sure any custom effect is detected.

      /**
      * ges_registry_get_default:
      *
      * Returns a newly created #GESEffectRegistry or the existing one
      * increasing
      * its refcount
      */
      GESEffectRegistry *
      ges_registry_get_default (void);
        -> Usecases:
          + Have a registry of all effects that are on the système (UC-8)

      /**
      * ges_effect_registry_get_effect_list:
      *
      * @self: The origin #GESEffectRegistry
      *
      * Returns a #GList of #GESEffectDescriptors. The
      */
      GList *
      ges_registry_get_effect_list (GESEffectRegistry *self);
        -> Usecases:
          + Get all effects descriptors that are on the système (UC-8)