Creating Animations with Clutter

<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>

  Clutter has a powerful and flexible framework for animating
  actors. The basis of which is the #ClutterTimeline class which
  reprents a period of time in frames. A #ClutterTimeline takes two
  parameters, a total number of frames and a frame rate (in frames per
  second). Once created, a signal ("new-frame") can be attached and
  then on starting (clutter_timeline_start()) the signal callback wil
  be called every time a new frame is reached. With the callback also
  receiving the current frame number this information can be used to
  modify actor properties and thus produce an animation.
  </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_rotate_z (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>
  <para>

  Timelines will 'drop' frames if it appears the application cannot
  keep up with the requested framerate. The first and last frames are
  guaranteed to be called however. Read the #ClutterTimeline
  documentation for more information on how they can be manipulated.

  </para>
  <para>

  Timelines on there own are useful for simple animations but can be
  come very unweldy for more complex multiple actor animations. Also
  they can lead to much code duplication. The #ClutterAlpha and
  #ClutterBehaviour classes build on timelines to offer further
  animation functionality and avoid these problems.

  </para>

  <para>

  A #ClutterAlpha is a 'function if time' (note, not pixel alpha!). It is
  created by passing both a #ClutterTimelime and a
  #ClutterAlphaFunc. The Alpha then produces a value between 0 and
  CLUTTER_ALPHA_MAX. This value is dependant on both the position of
  the Alpha's supplied timeline and the supplied function used by the
  Alpha.

  </para>

  <para>

  Clutter comes with many predefined #ClutterAlphaFunc's including:
  #CLUTTER_ALPHA_RAMP_INC - A rising alpha value over time,
  #CLUTTER_ALPHA_RAMP_DEC - A decreasing alpha value over time,
  #CLUTTER_ALPHA_SINE, A sinewave etc.

  </para>
  <para>

  A #ClutterBehaviour is then '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>

  <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>
  <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.

  </para>

  <para>

  Properties of the behaviour, alpha and timeline can be changed on
  the fly making animations. Experiment!

  </para>
  <para>

  ClutterEffects provide a simpler (but more limited) layer around the above.
  FIXME.

  </para>

</chapter>