mutter/doc/reference/clutter-animation.sgml

<chapter id="clutter-animations">
  <chapterinfo>
    <author>
      <firstname>Matthew</firstname>
      <surname>Allum</surname>
      <affiliation>
        <address>
          <email>mallum@openedhand.com</email>
        </address>
      </affiliation>
    </author>
  </chapterinfo>

  <title>Creating Animations with Clutter</title>

  <para>

  With Clutter using hardware accelration for graphics rendering,
  complex and fast animations are possible. This chapter describes basic
  techniques and the utilitys Clutter provides in aiding animation
  creation.

  </para>

  <section id="clutter-animation-basic">
  <title>Basic Animations</title>

  <para>

   The most basic way to create animations with Clutter is via the use of
   the <code>g_timeout_add</code>. This enables a callback function to be
   called at a definefine interval. The callback function can then modify
   actors visual properties as to produce an animation.

  </para>

  <example id="clutter-timeout-example">
  <para>
  Simple Rotation...
  </para>
  <programlisting>

  FIXME

  guint g_timeout_add  (guint interval,
                        GSourceFunc function,
                        gpointer data);


  </programlisting>
  </example>

  <note><title>Prioritys</title>
  <para>

  G_PRIORITY_DEFAULT should always be used as the timeouts priority
  (in case of g_timeout_add_full) as not to intefere with Clutters
  schueduling of repaints and input event handling.

  </para>
  </note>

  </section>
  <section id="clutter-animation-timelines">
  <title>Timelines</title>
  <para>
  Clutter Timelines abstract a set period of time with a set rate at
  which to call a provided call back function.
  </para>
  <para>
  They essentially extend g_timeout like functionality further by;
  </para>
  <orderedlist>
    <listitem><para>Having a set duration (in milliseconds) and a set 'frame rate'. Essentially the rate at which the callback is called.</para></listitem>
    <listitem><para>Passing current position information to the callback.</para></listitem>
    <listitem><para>Handling 'dropped frames' in guarenteeing the set duration and skipping over frames if Clutter cannot keep up with set rates.</para></listitem>
    <listitem><para>Query the number of milliseconds elapsed between current and previous callback.</para></listitem>
    <listitem><para>Allowing the timeline to be modified on the fly as well as being stoped, started, looped, rewound, reversed.</para></listitem>
    <listitem><para>Using the ClutterTimeoutPool to more efficiently schedule multiple timeout istances.</para></listitem>
  </orderedlist>
  <para>
  A Timeline is created with;
  </para>
  <programlisting>
clutter_timeline_new (guint n_frames, guint fps); 
  </programlisting>
  <para>
  Taking a number of frames and a frames per second, or by;
  </para>
  <programlisting>
clutter_timeline_new_for_duration (guint msecs);
  </programlisting>
  <para>

  Which takes the duration of the timeline in milliseconds with a
  default frame rate (See #clutter_get_default_frame_rate())

  </para>
  <para>
 
  The speed, duration and number of frames of the timeline then be
  modifed via the objects properties and API calls. The timeline can
  be made to loop by settings it "loop" property to TRUE.

  </para>
  <para>

  The timelines is started via #clutter_timeline_start () and its
  playback further manipulated by the #clutter_timeline_pause (),
  #clutter_timeline_stop (), #clutter_timeline_rewind () ,
  #clutter_timeline_skip () calls.

  </para>
  <para>

  By attaching a handler to the timelines "new-frame" signal a timeline
  can then be used to drive an animation by altering actors visual
  properties in this callback. The callback looks like;

  </para>
  <programlisting>

  void  on_new_frame  (ClutterTimeline *timeline,
                       gint             frame_num,
                       gpointer         user_data)   
  </programlisting>
  <para>

  The new-frame signals 'frame_num' parameter is set to the timelines
  current frame number this is between 0 and the "num-frames"
  property.  This value can be used to compute the state of a
  particular animation that is dependant on the current timeline
  position. The function #clutter_timeline_get_progress () can also be
  used to get a normalised value of the timelines current position.

  </para>
  <para>

  Timelines can also be played in reverse
  #clutter_timeline_set_direction() and a one-time delay set before
  they begin playing #clutter_timeline_set_delay ().

  </para>
  <para>

  When using a timeline to control a physical simulation using
  #clutter_timeline_get_delta() allows retrieving the number of frames
  and milliseconds since the previous callback to ensure the physics
  simulation to be able to take the actual time elapsed between
  iterations into account.

  </para>

  <example id="clutter-timeline-example">
  <para>
  The following example demonstrates rotating an actor with a timeline.
  </para>
  <programlisting>
#include &lt;clutter/clutter.h&gt;

void
on_new_frame (ClutterTimeline *timeline, 
	      gint             frame_num, 
	      gpointer         data)
{
  ClutterActor *actor = CLUTTER_ACTOR(data);

  clutter_actor_set_rotation (actor, (gdouble)frame_num, 
                              clutter_actor_get_width (actor)/2,
			      clutter_actor_get_height (actor)/2);
}

int
main (int argc, char *argv[])
{
  ClutterTimeline *timeline;

  ClutterActor    *stage, *actor;
  GdkPixbuf       *pixbuf;

  clutter_init (&amp;argc, &amp;argv);

  stage = clutter_stage_get_default ();

  pixbuf = gdk_pixbuf_new_from_file ("an-image.png", NULL);

  actor  = clutter_texture_new_from_pixbuf (pixbuf);

  clutter_container_add_actor (CLUTTER_CONTAINER (stage), actor);

  clutter_actor_set_position (actor, 100, 100);

  timeline = clutter_timeline_new (360, 60);    /* num frames, fps */
  g_object_set(timeline, "loop", TRUE, NULL);   /* have it loop */

  g_signal_connect (timeline, "new-frame", G_CALLBACK (on_new_frame), actor);

  clutter_actor_show_all (stage);

  clutter_timeline_start (timeline);

  clutter_main();

  return 0;
}
  </programlisting>
  </example>

  <note><para>
  Multiple timelines can be sequenced in order by means of the
  #ClutterScore. See the #ClutterScore documentation for more details on
  using this.
  </para></note>

  </section>
  <section id="clutter-animation-behaviours">
  <title>Timelines</title>
  <para>

  With a large application containing many animations, the use of just
  timelines can become unweldy and difficult to manage with much code
  duplication in the new-frame handlers that can require over complex
  code changes for minor animation modifications. To ease these
  problems the #ClutterAlpha and #ClutterBehaviour classes were created.

  </para>
  <para>

  #ClutterAlpha and #ClutterBehaviour attempt to generalise the
  new-frame function by defining common actions or behaviours that can
  be quickly modified, applied to multiple actors or mixed on a single
  actor.

  </para>
  <para>

  A ClutterAlpha is simply a 'function of time' (not pixel alpha!). It
  is created by referencing a source timeline and a function which
  produces a value between 0 and %CLUTTER_ALPHA_MAX dependant on the
  timeline position. Various prebuilt alpha functions are included
  with Clutter these include

  </para>
  <para>

   %CLUTTER_ALPHA_RAMP_INC
   %CLUTTER_ALPHA_RAMP_DEC
   %CLUTTER_ALPHA_RAMP
   %CLUTTER_ALPHA_SINE
   %CLUTTER_ALPHA_SINE_INC
   %CLUTTER_ALPHA_SINE_DEC
   %CLUTTER_ALPHA_SINE_HALF
   %CLUTTER_ALPHA_SQUARE
   %CLUTTER_ALPHA_SMOOTHSTEP_INC
   %CLUTTER_ALPHA_SMOOTHSTEP_DEC
   %CLUTTER_ALPHA_EXP_INC
   %CLUTTER_ALPHA_EXP_DEC

  </para>
  <para>

  A Behaviour is created with a #ClutterAlpha and a set of limits for
  whatever the behaviour modifys actor wise. The current #ClutterAlpha
  value is then mapped to a value between these limits and this value
  set on any applied actors. With the #ClutterAlpha's underlying
  timeline playing the produced value will change and the behaviour
  will animate the actor.

  </para>
  <para>

  A #ClutterBehaviour is effectively 'driven' by a supplied #ClutterAlpha and
  when then applied to an actor it will modify a visual property or
  feature of the actor dependant on the Alpha's value. For example a
  path based behaviour applied to an actor will alter its position
  along the path dependant on the current alpha value over time. The
  actual motion will depend on the chosen #ClutterAlphaFunc - a
  #CLUTTER_ALPHA_RAMP_INC making it to move at constant speed along the
  path, a #CLUTTER_ALPHA_SINE making it alternate from one end of the
  path to the other with non constant speed.

  </para>
  <para>

  Multiple behaviours can of course be applied to an actor as well as
  a single behaviour being applied to multiple actors.  The separation
  of timelines, alphas and behaviours allows for a single timeline to
  drive many behaviours each potentially using different alpha
  functions. Behaviour parameters can also be changed on the fly.

  </para>

  <para>
  
  <figure id="behaviour-path-alpha">
    <title>Effects of alpha functions on a path</title>
    <graphic fileref="path-alpha-func.png" format="PNG"/>
    <blockquote>
    The actors position between the path's end points directly correlates
    to the #ClutterAlpha's current alpha value driving the behaviour. With
    the #ClutterAlpha's function set to %CLUTTER_ALPHA_RAMP_INC the actor
    will follow the path at a constant velocity, but when changing to
    %CLUTTER_ALPHA_SINE_INC the actor initially accelerates before quickly
    decelerating.
    </blockquote>
  </figure>

  </para>
  <para>

  The behaviours included with clutter are

  </para>
  <para>

  #ClutterBehaviourBspline
  #ClutterBehaviourDepth
  #ClutterBehaviourEllipse
  #ClutterBehaviourOpacity
  #ClutterBehaviourPath
  #ClutterBehaviourRotate
  #ClutterBehaviourScale

  </para>

  <example id="clutter-timeline-example">
  <para>
  The following example demonstrates an ellipse behaviour in action.
  </para>
  <programlisting>
#include &lt;clutter/clutter.h&gt;

int
main (int argc, char *argv[])
{
  ClutterTimeline  *timeline;
  ClutterBehaviour *behave;
  ClutterAlpha     *alpha;
  ClutterActor     *stage, *actor;
  GdkPixbuf        *pixbuf;

  clutter_init (&amp;argc, &amp;argv);

  stage = clutter_stage_get_default ();

  pixbuf = gdk_pixbuf_new_from_file ("ohpowers.png", NULL);

  actor  = clutter_texture_new_from_pixbuf (pixbuf);

  clutter_container_add_actor (CLUTTER_CONTAINER (stage), actor);

  timeline = clutter_timeline_new (100, 26);    /* num frames, fps */
  g_object_set(timeline, "loop", TRUE, NULL);   /* have it loop */

  /* Set an alpha func to power behaviour */
  alpha = clutter_alpha_new_full (timeline,
                                  CLUTTER_ALPHA_SINE,
                                  NULL, NULL);

  behave = clutter_behaviour_ellipse_new (alpha, 
					  200,   /* center x */
					  200,   /* center y */
					  400,   /* width */
					  300,   /* height */
					  CLUTTER_ROTATE_CW, /* direction */
					  0.0,   /* angle begin */
					  360.0); /* angle end */

  clutter_behaviour_apply (behave, actor);

  clutter_actor_show_all (stage);

  clutter_timeline_start (timeline);

  clutter_main();

  return 0;
}

  </programlisting>
  </example>

  <note>Behaviour parameters can be changed whilst a animation is running</note>

 <para>
  There can be many ClutterAlpha's attached to a single timeline. There can be
  many Behaviours for a ClutterAlpha There can be many Behaviours applied to an
  actor. A ClutterScore can be used to chain many behaviour togeather
  </para>

<warn>combining behaviours that effect the same actor properties
(i.e two seperate paths) will cause unexpected results. The values
will not be merged in any way with essentially a the last applied
behaviour taking precedence.</warn>

   <para>
   FIXME: actually move subclassing behaviours here?
   </para>

  </section>
  <section id="clutter-animation-effects">
  <title>Effects</title>

  <para>
  
   ClutterEffect's provide a simplified abstraction for firing simple
   transitions from code. ClutterEffects are created from
   ClutterEffectTemplate s which are an abstraction of a timeline and
   an alpha. An effect template can be created with:

  </para>
  <programlisting>
ClutterEffectTemplate *etemplate;

etemplate = clutter_effect_template_new_for_duration (
                                           2000, CLUTTER_ALPHA_RAMP_INC);
  </programlisting>
  <para>

  This will create an effect template lasting 2000 milliseconds (2
  seconds) and use an alpha function of CLUTTER_ALPHA_RAMP_INC, there
  are other more advanced forms for creating effect templates from
  existing timelines, as well as attaching a callback to be called
  with user_data when the effecttemplate is destroyed.

  </para>
  <para>

  When we have an effect-template we can create a temporary behaviour
  animating an actor simply by issuing:

  </para>
  <programlisting>
clutter_actor_move (etemplate, actor, 23, 42, NULL, NULL);
  </programlisting>
  <para>
and the actor will move to the coordintes 23, 42 in 2 seconds, if we at the
same time issued:
  </para>
  <programlisting>
clutter_actor_fade (etemplate, actor, 0x0, NULL, NULL);
  </programlisting>
  <para>
The actor would fade out at the same time.
  </para>
  <para>

  Clutter effects return a timeline, you can stop an effect from
  immediatly happening by calling clutter_timeline_stop () on the
  returned timeline. This returned timeline can also be used to then
  use effects in the ClutterScore etc.

  </para>
  </section>
  <section id="clutter-animation-conclusion">
  <title>Conclusion</title>
  <para>

  Clutter provides a number of utility classes to aid animations and
  complex animations can be produced by combining the various features
  provided.

  </para>
  <para>

  Of course animations can be created outside of Clutter Utilities,
  they are not expected to cover every kind of possible animation
  scenario.

  </para>
  <para>

  The animation functionality in clutter is primarily suited to building animations with a set or finite running time - i.e transitions and the like. For animations involving variable input (such as touchscreen handling) physical simulations may be more suited.
  
  </para> 

  </section>
</chapter>