mutter/doc/cookbook/animations.xml

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<chapter id="animations" xmlns:xi="http://www.w3.org/2003/XInclude">
  <title>Animations</title>

  <epigraph>
    <attribution>Walt Disney</attribution>
    <para>Animation can explain whatever the mind of man can conceive.</para>
  </epigraph>

  <section id="animations-introduction">
    <title>Introduction</title>

    <para>Clutter actors have a variety of <emphasis>properties</emphasis>
    (position, size, rotation in 3D space, scale, opacity) which govern
    their visual appearance in the UI. They may also have
    <emphasis>constraints</emphasis> on how they are aligned
    and/or positioned relative to each other.</para>

    <para>The Clutter animation API provides a means of changing
    properties and constraints as a function of time: moving, scaling,
    rotating, changing opacity and colour, modifying postional
    constraints, etc.</para>

    <note><para>Clutter also makes it possible to animate non-visual
    properties if desired.</para></note>

    <section>
      <title>High level overview</title>

      <para>Here are the main concepts behind animation in Clutter:</para>

      <itemizedlist>
        <listitem>
          <para>An <emphasis>animation</emphasis> changes one or more
          properties of one or more actors over time: their rotation in
          a particular dimension (<varname>x</varname>, <varname>y</varname>,
          <varname>z</varname>), scale, size, opacity etc.</para>
        </listitem>
        <listitem>
          <para>An animation has an associated <emphasis>timeline</emphasis>.
          Think of this as analogous to the "thing" you're controlling when
          you watch a video on the internet: it's what you control with
          the play/pause button and what is measured by the bar
          showing how far through the video you are. As with the
          controls on a video player, you can play/pause/skip a Clutter
          timeline; you can also rewind it, loop it, and play it
          backwards.</para>
          <note>
            <para>If a timeline is reversed, the progress along the
            timeline is still measured the same way as it is in the forward
            direction: so if you start from the end of the timeline and run
            it backwards for 75% of its length, the progress is reported
            as 0.25 (i.e. 25% of the way from the start of the
            timeline).</para>
          </note>
        </listitem>
        <listitem>
          <para>The <emphasis>duration</emphasis> of a timeline
          (e.g. 500 milliseconds, 1 second, 10 seconds) specifies how
          long its animation will last. The timeline can be inspected
          to find out how much of it has elapsed, either as a value in
          milliseconds or as a fraction (between 0 and 1) of the total
          length of the timeline.</para>
        </listitem>
        <listitem>
          <para>An animation is divided into <emphasis>frames</emphasis>.
          The number of frames which make up the animation isn't
          constant: it depends on various factors, like how powerful
          your machine is, the state of the drivers for your hardware,
          and the load on he system. So you won't always get the same
          number of frames in an animation of a particular duration.</para>
        </listitem>
        <listitem>
          <para>The change to a property in an animation occurs over
          the course of the timeline: the start value of the property
          heads toward some target value. When it reaches the end of
          the timeline, the property should have reached the target
          value.</para>
        </listitem>
        <listitem>
          <para>Exactly how the property changes over the course of the
          timeline is governed by an <emphasis>alpha</emphasis>. This
          is the trickiest idea to explain, so it has its own section
          below.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section id="animations-introduction-alphas">
      <title>Alphas</title>

      <para>An alpha is generated for each frame of the animation.
      The alpha varies between -1.0 and 2.0, and changes during the
      course of the animation's timeline; ideally, the value should
      start at 0.0 and reach 1.0 by the end of the timeline.</para>

      <para>The alpha for any given frame of the animation is determined
      by an <emphasis>alpha function</emphasis>. Usually, the alpha
      function will return a value based on progress along the timeline.
      However, the alpha function doesn't have to respect or pay
      attention to the timeline: it can be entirely random if desired.</para>

      <para>To work out the value of a property at a given frame
      somewhere along the timeline for a given alpha:</para>

      <orderedlist>
        <listitem>
          <para>Determine the difference between the start value and
          the target end value for the property.</para>
        </listitem>
        <listitem>
          <para>Multiply the difference by the alpha for the current
          frame.</para>
        </listitem>
        <listitem>
          <para>Add the result to the start value.</para>
        </listitem>
      </orderedlist>

      <para>The shape of the plot of the alpha function over time is
      called its <emphasis>easing mode</emphasis>. Clutter provides
      various modes ranging from <constant>CLUTTER_LINEAR</constant>
      (the alpha value is equal to progress along the timeline),
      to modes based on various polynomial and exponential functions,
      to modes providing elastic and bounce shapes. See the
      ClutterAlpha documentation for examples of the shapes produced
      by these functions. There is also a good interactive demo
      of the modes on
      <ulink url="http://www.robertpenner.com/easing/easing_demo.html">Robert Penner's site</ulink>.
      </para>

      <para>Most of the time, you can use the built-in Clutter easing
      modes to get the kind of animation effect you want. However,
      in some cases you may want to provide your own alpha function.
      Here's an example (based on the quintic ease in mode from
      <filename>clutter-alpha.c</filename>):</para>

      <informalexample>
        <programlisting>
<![CDATA[
static gdouble
_alpha_ease_in_sextic (ClutterAlpha *alpha,
                       gpointer      dummy G_GNUC_UNUSED)
 {
  ClutterTimeline *timeline = clutter_alpha_get_timeline (alpha);
  gdouble p = clutter_timeline_get_progress (timeline);

  return p * p * p * p * p * p;
}
]]>
        </programlisting>
      </informalexample>

      <para>An alpha function just has to have a specified method
      signature and return a <type>gdouble</type> value when called.
      As stated above, you'd typically base the return value on the
      timeline progress; the function above shows how you get the
      timeline associated with the alpha, so you can apply the alpha
      function to it.</para>
    </section>

    <section id="animations-introduction-api">
      <title>Clutter's animation API</title>

      <para>All of the animation approaches in Clutter use the same
      basic underpinnings (as explained above), but the API provides
      varying levels of abstraction and/or ease of use on top of those
      underpinnings.</para>

      <itemizedlist>
        <listitem>
          <para><emphasis>Implicit animations</emphasis> (created using
          <function>clutter_actor_animate()</function> and related
          functions) are useful where you want to apply
          a simple or one-off animation to an actor. They enable you
          to animate one or more properties using a single easing mode;
          however, you only specify the target values for the properties
          you're animating, not the start values.</para>
        </listitem>
        <listitem>
          <para><emphasis>ClutterAnimator</emphasis> provides support
          for declarative animations (defined using <type>ClutterScript</type>).
          You can animate multiple actors with this approach, and
          have more control over the easing modes used during an
          animation: while implicit animations only allow a single
          easing mode for all properties, <type>ClutterAnimator</type>
          supports <emphasis>multiple</emphasis> easing modes for
          <emphasis>each</emphasis> property; <emphasis>key frames</emphasis>
          are used to indicate where in the animation each easing mode
          should be applied.</para>
        </listitem>
        <listitem>
          <para><emphasis>ClutterState</emphasis> enables you to describe
          <emphasis>states</emphasis>: property values across one or
          more actors, plus the easing modes used to transition to
          those values. It can also be combined with <type>ClutterAnimator</type>
          for finer grained definition of transitions if desired.</para>
          <para>States are particularly useful if you need actors to
          animate between a known set of positions/sizes/opacities etc.
          during their lifecycles (e.g. animating a list of items in
          a menu, or for animations in a picture viewer where you
          click on thumbnails to display a full view of a photograph).</para>
        </listitem>
      </itemizedlist>

      <para>The recipes in this section show when and where it is
      appropriate to use each of these approaches.</para>

    </section>

  </section>

  <section id="animations-inversion">
    <title>Inverting Animations</title>

    <section>
      <title>Problem</title>

      <para>You want to have an animation exactly mirroring another one
      that you just played.</para>
    </section>

    <section>
      <title>Solution</title>

      <para>Reverse the direction of the <type>ClutterTimeline</type>
      associated with the animation.</para>

      <para>For example, here's how to invert an implicit
      animation which moves an actor along the <varname>x</varname>
      axis. The direction of the animation is inverted when the
      movement along the <varname>x</varname> axis is completed; it is
      also inverted if the mouse button is pressed on the actor.</para>

      <para>First, set up the animation:</para>

      <informalexample>
        <programlisting>
<![CDATA[
ClutterAnimation *animation;

/*
 * animate actor to x = 300.0;
 * the implicit animation functions return a ClutterAnimation
 * which we can use to invert the timeline
 */
animation = clutter_actor_animate (actor,
                                   CLUTTER_EASE_IN_OUT_CUBIC,
                                   2000,
                                   "x", 300.0,
                                   NULL);

/* callback for when the animation completes */
g_signal_connect (animation,
                  "completed",
                  G_CALLBACK (_animation_done_cb),
                  NULL);

/*
 * callback for when the mouse button is pressed on the actor;
 * note the animation is passed as user data, so we can
 * get at the timeline
 */
g_signal_connect (actor,
                  "button-press-event",
                  G_CALLBACK (_on_click_cb),
                  animation);
]]>
        </programlisting>
      </informalexample>

      <para>Next, add a function for inverting the timeline:</para>

      <informalexample>
        <programlisting>
<![CDATA[
static void
_invert_timeline (ClutterTimeline *timeline)
{
  ClutterTimelineDirection direction = clutter_timeline_get_direction (timeline);

  if (direction == CLUTTER_TIMELINE_FORWARD)
    direction = CLUTTER_TIMELINE_BACKWARD;
  else
    direction = CLUTTER_TIMELINE_FORWARD;

  clutter_timeline_set_direction (timeline, direction);
}
]]>
        </programlisting>
      </informalexample>

      <para>Then add a function which calls <function>_invert_timeline</function>
      when the animation completes. More importantly, the callback should
      stop emission of the "completed" signal by the animation. This
      prevents the <type>ClutterAnimation</type> underlying the implicit
      animation from being unreferenced; which in turn allows it to be
      inverted:</para>

      <informalexample>
        <programlisting>
<![CDATA[
static void
_animation_done_cb (ClutterAnimation *animation,
                    gpointer          user_data)
{
  /* stop the completed signal before the ClutterAnimation is unreferenced */
  g_signal_stop_emission_by_name (animation, "completed");

  /* invert the timeline associated with the animation */
  ClutterTimeline *timeline = clutter_animation_get_timeline (animation);
  _invert_timeline (timeline);
}
]]>
        </programlisting>
      </informalexample>

      <para>Finally, the click callback function uses the same
      <function>_invert_timeline</function> function if the animation
      is playing; but if the animation is stopped, it will
      start it instead:</para>

      <informalexample>
        <programlisting>
<![CDATA[
static void
_on_click_cb (ClutterActor *actor,
              ClutterEvent *event,
              gpointer      user_data)
{
  ClutterAnimation *animation = (ClutterAnimation *)user_data;

  ClutterTimeline *timeline = clutter_animation_get_timeline (animation);

  if (clutter_timeline_is_playing (timeline))
    {
      _invert_timeline (timeline);
    }
  else
    {
      clutter_timeline_start (timeline);
    }
}
]]>
        </programlisting>
      </informalexample>

    </section>

    <section>
      <title>Discussion</title>

      <para>If you are using <type>ClutterAnimator</type> rather than
      implicit animations, <function>clutter_animator_get_timeline()</function>
      enables you to get the underlying timeline; you could then use
      the techniques shown above to invert it.</para>

      <para><type>ClutterState</type> enables a different approach
      to "inverting" an animation: rather than having a single animation
      which you invert, you would define two or more
      <emphasis>keys</emphasis> for an actor (or set of actors) and
      transition between them.</para>

      <para>For the example above, you would define two keys:
      one for the actor's initial position; and a second for the actor
      at <code>x = 300.0</code>. You would also define the
      transition between them: 2000 milliseconds with a
      <constant>CLUTTER_EASE_IN_OUT_CUBIC</constant> easing mode.</para>

      <para>With the states defined, you would then use
      <function>clutter_state_set_state()</function> inside callbacks to
      animate the actor between the two <varname>x</varname> positions.
      Behind the scenes, <type>ClutterState</type> would handle the
      animations and timelines for you.</para>

    </section>

  </section>

  <section id="animations-fading">
    <title>Fading an actor out of or into view</title>

    <section>
      <title>Problem</title>

      <para>You want to animate an actor so that it fades out of or into
      view.</para>
    </section>

    <section>
      <title>Solution</title>

      <para>Animate the actor's opacity property.</para>

      <para>You can do this using any of the approaches provided
      by the animation API. Here's how to fade out an actor (until it's
      completely transparent) using implicit animations:</para>

      <informalexample>
        <programlisting>
<![CDATA[
/* fade out actor over 4000 milliseconds */
clutter_actor_animate (actor,
                       CLUTTER_EASE_OUT_CUBIC,
                       4000,
                       "opacity", 0,
                       NULL);
]]>
        </programlisting>
      </informalexample>

      <para>Here's an example of a rectangle fading out using this
      animation:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-fading-out.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing an actor fading out using implicit
          animations</para>
        </alt>
      </inlinemediaobject>

      <para><constant>CLUTTER_EASE_OUT_CUBIC</constant> is one of the
      Clutter easing modes; see
      <link linkend="animations-introduction-alphas">the introduction</link>
      for more details about what these are and how to choose one.</para>

      <para>Here's an example of the transitions you could use to
      fade an actor in and out using <type>ClutterState</type>:</para>

      <informalexample>
        <programlisting>
<![CDATA[
ClutterState *transitions = clutter_state_new ();

/* all transitions last for 2000 milliseconds */
clutter_state_set_duration (transitions, NULL, NULL, 2000);

/* transition from any state to "fade-out" state */
clutter_state_set (transitions,
                   NULL,        /* from state (NULL means "any") */
                   "fade-out",  /* to state */
                   actor, "opacity", CLUTTER_EASE_OUT_QUAD, 0,
                   NULL);

/* transition from any state to "fade-in" state */
clutter_state_set (transitions, NULL, "fade-in",
                   actor, "opacity", CLUTTER_EASE_OUT_QUAD, 255,
                   NULL);

/* put the actor into the "fade-out" state with no animation */
clutter_state_warp_to_state (transitions, "fade-out");
]]>
        </programlisting>
      </informalexample>

      <para>You would then trigger an animated state change as events
      occur in the application (e.g. mouse button clicks):</para>

      <informalexample>
        <programlisting>
<![CDATA[
clutter_state_set_state (transitions, "fade-in");
]]>
        </programlisting>
      </informalexample>

      <para>Here's an example of this animation fading in then out again:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-fading-in-then-out.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing an actor fading in then out using
          <type>ClutterState</type></para>
        </alt>
      </inlinemediaobject>

      <note>
        <para><type>ClutterState</type> is most useful where you
        need to animate an actor backwards and forwards between multiple
        states (e.g. fade an actor in and out of view). Where you just
        want to fade an actor in or out once,
        <function>clutter_actor_animate()</function> is adequate.</para>
      </note>

    </section>

    <section>
      <title>Discussion</title>

      <para>Reducing an actor's transparency to zero does not make it
      inactive: the actor will still be reactive even if it's not
      visible (responding to key events, mouse clicks etc.).
      To make it really "disappear", you could use
      <function>clutter_actor_hide()</function> once you'd made the actor
      fully transparent.</para>

    </section>

  </section>

  <section id="animations-rotating">
    <title>Rotating an actor</title>

    <section>
      <title>Problem</title>

      <para>You want to animate rotation of an actor. Some example cases
      where you might want to do this:</para>

      <itemizedlist>
        <listitem>
          <para>To rotate an image so it's the right way up for
          viewing.</para>
        </listitem>
        <listitem>
          <para>To make actors more or less prominent, rotating them
          towards or away from the view point.</para>
        </listitem>
        <listitem>
          <para>To turn an actor "around" and display different UI
          elements "behind" it.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Solution</title>

      <para>Animate one of the <varname>rotation-angle-(x|y|z)</varname>
      properties of the actor.</para>

      <para>The most "obvious" (and probably most commonly used) rotation is
      in the <emphasis>z axis</emphasis> (parallel
      to the 2D surface of the UI). The other rotation axes
      (<emphasis>x</emphasis> and <emphasis>y</emphasis>)
      are less obvious, as they rotate the actor in the depth dimension,
      "away from" or "towards" the view point.</para>

      <para>Examples of each type of rotation are given below. While the
      examples use <link linkend="animations-introduction-api">implicit
      animations</link>, it is also possible to use
      <type>ClutterAnimator</type> and <type>ClutterState</type> to animate
      rotations: see <link linkend="animations-rotating-example">the
      full example at the end of this recipe</link> for some
      <type>ClutterState</type> code.</para>

      <note>
        <para>I've added an inaccurate (but hopefully useful) metaphor to
        each rotation axis ("wheel", "letter box", "door"), to make it
        easier to remember the effect you get from animating in that axis
        (and when the rotation center is inside the actor).</para>
      </note>

      <para><emphasis>Rotating on the z axis</emphasis> ("wheel")</para>

      <informalexample>
        <programlisting>
<![CDATA[
clutter_actor_animate (actor,
                   CLUTTER_LINEAR,            /* easing mode */
                   1000,                      /* duration in milliseconds */
                   "rotation-angle-z", 90.0,  /* target rotation angle in degrees */
                   NULL);
]]>
        </programlisting>
      </informalexample>

      <para>The above code animating a texture:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-rotating-z-90.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing an actor rotating to 90 degrees on the
          z axis</para>
        </alt>
      </inlinemediaobject>

      <para>By default, the center of the rotation is derived from
      the anchor point of the actor; unless you've changed the anchor
      point, the default is the top-left corner of the actor. See the
      Discussion section below for more about setting the rotation center.</para>

      <note>
        <para>An animated rotation moves an actor <emphasis>to</emphasis>
        the specified rotation angle; it <emphasis>does not</emphasis>
        increment or decrement the actor's current rotation angle by
        the amount specified.</para>
      </note>

      <para><emphasis>Rotating on the <code>x</code> axis</emphasis>
      ("letter box")</para>

      <informalexample>
        <programlisting>
<![CDATA[
clutter_actor_animate (actor,
                   CLUTTER_LINEAR,
                   1000,
                   "rotation-angle-x", -45.0,
                   NULL);
]]>
        </programlisting>
      </informalexample>

      <para>The above code animating a texture:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-rotating-x-minus-45.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing an actor rotating to -45 degrees on the
          x axis</para>
        </alt>
      </inlinemediaobject>

      <para>Notice how the texture rotates away from the view point,
      and also how perspective effects are applied (as the actor is rotating
      "into" the depth dimension).</para>

      <para><emphasis>Rotating on the <code>y</code> axis</emphasis>
      ("door")</para>

      <informalexample>
        <programlisting>
<![CDATA[
clutter_actor_animate (actor,
                   CLUTTER_LINEAR,
                   1000,
                   "rotation-angle-y", 45.0,
                   NULL);
]]>
        </programlisting>
      </informalexample>

      <para>The above code animating a texture:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-rotating-y-45.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing an actor rotating to 45 degrees on the
          y axis</para>
        </alt>
      </inlinemediaobject>

      <para>Again, the rotation is into the depth dimension, so
      you get perspective effects.</para>

    </section>

    <section>
      <title>Discussion</title>

      <para>It can sometimes be difficult to predict exactly
      how a particular rotation animation will appear when applied.
      Often the only way to find out is to experiment. However,
      the sections below outline some of the most common factors which
      affect animated rotations, with the aim of minimising the
      experimentation you need to do.</para>

      <section>
        <title>Setting the rotation center for an animation</title>

        <para>The examples in the previous section used the default
        center of rotation for each axis. However, it is possible to
        change the rotation center for an axis, in turn changing
        the appearance of the animation.</para>

        <note>
          <para>Rotation center coordinates are relative to the
          actor's coordinates, not to the coordinates of the actor's
          container or the stage.</para>
        </note>

        <section>
          <title>Setting a rotation center inside an actor</title>

          <para>You can set the center for rotation on the x or y axes
          like this:</para>

          <informalexample>
            <programlisting>
<![CDATA[
/*
* only required for y axis rotation;
* here set to the mid point of the actor's y axis
*/
gfloat x_center = clutter_actor_get_height (actor) * 0.5;

/*
* only required for x axis rotation;
* here set to the mid point of the actor's x axis
*/
gfloat y_center = clutter_actor_get_width (actor) * 0.5;

/*
* depth for the rotation center: positive numbers
* are closer to the view point, negative ones
* are further away
*/
gfloat z_center = 0.0;

/* set rotation center */
clutter_actor_set_rotation (actor,
                          CLUTTER_X_AXIS,  /* or CLUTTER_Y_AXIS */
                          0.0,             /* set the rotation to this angle */
                          x_center,
                          y_center,
                          z_center);
]]>
            </programlisting>
          </informalexample>

          <para>Because z axis rotations are more common, Clutter
          provides some convenience functions to set the rotation
          center for this axis:</para>

          <informalexample>
            <programlisting>
<![CDATA[
clutter_actor_set_z_rotation_from_gravity (actor,
                                         0.0,
                                         CLUTTER_GRAVITY_CENTER);
]]>
            </programlisting>
          </informalexample>

          <para><constant>CLUTTER_GRAVITY_CENTER</constant> makes the
          center of the actor the rotation center for
          the z axis. See the <type>ClutterGravity</type> enumeration for
          acceptable values for this parameter.</para>

          <note>
            <para>Setting the rotation center for the z axis using gravity
            is recommended, as Clutter will automatically recompute the
            rotation center if the actor's size changes. For the x and y
            axes, you have to do this computation yourself if you
            want an actor's center of rotation to stay in the same place
            if it is resized.</para>
          </note>

          <para>Rotation on the x axis around an actor's center:</para>

          <inlinemediaobject>
            <videoobject>
              <videodata fileref="videos/animations-rotating-x-centered.ogv"/>
            </videoobject>
            <alt>
              <para>Video showing an actor rotating around its center
              on the x axis</para>
            </alt>
          </inlinemediaobject>

          <para>Rotation on the y axis around an actor's center:</para>

          <inlinemediaobject>
            <videoobject>
              <videodata fileref="videos/animations-rotating-y-centered.ogv"/>
            </videoobject>
            <alt>
              <para>Video showing an actor rotating around its center
              on the y axis</para>
            </alt>
          </inlinemediaobject>

          <para>Rotation on the z axis around an actor's center:</para>

          <inlinemediaobject>
            <videoobject>
              <videodata fileref="videos/animations-rotating-z-centered.ogv"/>
            </videoobject>
            <alt>
              <para>Video showing an actor rotating around its center
              on the z axis</para>
            </alt>
          </inlinemediaobject>

        </section>

        <section>
          <title>Setting the rotation center outside an actor</title>

          <para>Rather than rotating the actor around a point inside
          itself, the rotation center can be moved to a position
          outside the actor. (In the case of the z axis,
          any rotation center setting is outside the actor as its depth
          is 0.) When animated, the actor will describe an arc around the
          rotation center, as if it's swinging from an invisible thread.</para>

          <para>The same code as shown above can be used to set the
          rotation center: just set the rotation center coordinates to
          negative numbers (outside the actor). However, you can't use the
          gravity functions if the rotation center falls outside an actor.</para>

          <para>For example, here's a rotation to -180 degrees in the x
          axis, with the y rotation center set to -96 (the same as the height
          of the actor):</para>

          <inlinemediaobject>
            <videoobject>
              <videodata fileref="videos/animations-rotating-x-minus-180-with-y-minus-96.ogv"/>
            </videoobject>
            <alt>
              <para>Video showing an actor rotating to -180 degrees on
              the x axis with y rotation center set to -96</para>
            </alt>
          </inlinemediaobject>

          <para>Similarly, moving the z rotation center (for a rotation
          in the x or y axis) will cause the actor to swing "into" or "out
          of" the UI. Its final apparent size may be different, as it could
          reach a different depth in the UI by the end of the
          animation.</para>

          <para>For example, here's a rotation to -180 in the x axis,
          with the z rotation center set to -96 (the same as the height
          of the actor):</para>

          <inlinemediaobject>
            <videoobject>
              <videodata fileref="videos/animations-rotating-x-minus-180-with-z-minus-96.ogv"/>
            </videoobject>
            <alt>
              <para>Video showing an actor rotating to -180 degrees on
              the x axis with z rotation center set to -96</para>
            </alt>
          </inlinemediaobject>

          <para>The apparent final size of the actor is reduced, as it
          has rotated away from the view point.</para>

        </section>

      </section>

      <section>
        <title>Direction of rotation</title>

        <para>The apparent direction of an animated rotation depends on
        two things:</para>

        <orderedlist>
          <listitem>
            <para>Whether the angle of rotation is positive or negative.</para>
          </listitem>
          <listitem>
            <para>The rotation of the container(s) the actor is inside.</para>
          </listitem>
        </orderedlist>

        <para>In the case of the sign of the rotation, here's what
        happens for each axis and rotation angle sign (positive or
        negative).</para>

        <informaltable>
          <thead>
            <tr>
              <th>Axis</th>
              <th>Sign of rotation angle</th>
              <th>Effect on actor</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td>z</td>
              <td>+</td>
              <td>
                Clockwise spin about the <code>x,y</code> center of
                rotation.
              </td>
            </tr>
            <tr>
              <td>z</td>
              <td>-</td>
              <td>
                Anti-clockwise spin about the <code>x,y</code>
                center of rotation.
              </td>
            </tr>
            <tr>
              <td>x</td>
              <td>+</td>
              <td>
                The top swings away from the view point and the
                bottom swings towards it. If y rotation center == 0,
                the top is fixed; if y rotation center == the actor's
                height, the bottom is fixed.
              </td>
            </tr>
            <tr>
              <td>x</td>
              <td>-</td>
              <td>
                The bottom swings away from the view point and the
                top swings towards it. If y rotation center == 0,
                the top is fixed; if y rotation center == the actor's
                height, the bottom is fixed.
              </td>
            </tr>
            <tr>
              <td>y</td>
              <td>+</td>
              <td>
                The right-hand side swings away from the view point and
                the left-hand side swings towards it. When x rotation
                center == 0, the left-hand side if fixed; when x
                rotation center == the actor's width, the right-hand
                side is fixed.
              </td>
            </tr>
            <tr>
              <td>y</td>
              <td>-</td>
              <td>
                The right-hand side swings towards the view point and
                the left-hand side swings away from it. When x rotation
                center == 0, the left-hand side if fixed; when x
                rotation center == the actor's width, the right-hand
                side is fixed.
              </td>
            </tr>
          </tbody>
        </informaltable>

        <para>If an actor's container is rotated, this may affect the
        appearance of rotation animations applied to the actor. In
        particular, if an actor's container has been rotated
        by 180 degrees in one axis, the direction of that actor's
        rotation may appear reversed.</para>

        <para>For example, the video below shows an actor being animated
        to 90 degrees on the z axis, then back to 0 degrees;
        the actor's container is then rotated by 180 degrees in the y
        axis; then the same rotation 90 degree rotation is applied
        to the actor again. Note that the first time the animation
        is applied, the rotation is clockwise; but the second time (as
        the actor is effectively "reversed"), it is anti-clockwise.</para>

        <inlinemediaobject>
          <videoobject>
            <videodata fileref="videos/animations-rotating-container-reverses-direction.ogv"/>
          </videoobject>
          <alt>
            <para>Video showing how an actor's apparent rotation is
            affected by the rotation of its parent</para>
          </alt>
        </inlinemediaobject>

      </section>

      <section>
        <title>Apparent vs. actual rotation</title>

        <para>There is a difference between an actor's <emphasis>apparent</emphasis>
        rotation (how much an actor appears to be rotating, from the
        perspective of someone looking at the UI) and its
        <emphasis>actual</emphasis> rotation (how much that actor is
        really rotating).</para>

        <para>For example, if you rotate an actor and its container
        simultaneously, each by 90 degrees in the same direction, the
        actor will appear to have rotated by 180 degrees by the end
        of the animation. However, calling the
        <function>clutter_actor_get_rotation()</function> function
        for that axis on the actor still returns a rotation of 90
        degrees.</para>
      </section>

      <section>
        <title>Orientation of rotation axes</title>

        <para>The rotation axes remain fixed in the same place on
        the actor regardless of its rotation, even though from the viewer's
        perspective they may appear to move.</para>

        <para>For example, when rotation in the z axis is 0 degrees,
        the actor's x axis is horizontal (across the UI) from both the
        actor's and the viewer's perspective. However, if you rotate the
        actor by 90 degrees in the z axis, the x axis is now vertical from
        <emphasis>the viewer's</emphasis> perspective, but still horizontal
        across the actor from <emphasis>the actor's</emphasis>
        perspective.</para>
      </section>

    </section>

    <section>
      <title>Full example</title>

      <example id="animations-rotating-example">
        <title>Rotating an actor around x, y, and z axes using
        <type>ClutterState</type></title>
        <programlisting>
<xi:include href="examples/animations-rotating.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>
    </section>

  </section>

  <section id="animations-complex">
    <title>Creating complex animations with
    <type>ClutterAnimator</type></title>

    <section>
      <title>Problem</title>

      <para>You want to create a complex animation involving one or more
      actors. The animation will consist of a sequence of transitions
      over multiple properties on each actor.</para>

      <para>An example might be moving several actors between points,
      with different types of movement for each part of the path, while
      transforming each actor (e.g. scaling or rotating it).</para>

    </section>

    <section>
      <title>Solution</title>

      <para>Use a <type>ClutterAnimator</type> to define the animation.</para>

      <para>Because there are many complex animations you
      <emphasis>could</emphasis> implement, the example below does
      this:</para>

      <inlinemediaobject>
        <videoobject>
          <videodata fileref="videos/animations-complex.ogv"/>
        </videoobject>
        <alt>
          <para>Video showing a complex animation of an actor
          using <type>ClutterAnimator</type></para>
        </alt>
      </inlinemediaobject>

      <para>Although this uses a single actor, the animation is complex
      enough to make it difficult to implement with implicit animations
      or <type>ClutterState</type> (see
      <link linkend="animations-complex-why-clutteranimator">the Discussion
      section</link> for reasons why).</para>

      <para>Here is a JSON definition of the stage, actors, and
      the <type>ClutterAnimator</type> for this
      animation:</para>

      <example id="animations-complex-example-1">
        <title>JSON definition of a complex animation using
        <type>ClutterAnimator</type></title>
        <programlisting>
<xi:include href="examples/animations-complex.json" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

      <note>
        <para>The core to understanding this example is understanding
        how to define keys for a <type>CutterAnimator</type>. As
        this is an involved topic, further explanation
        is given in <link linkend="animations-complex-discussion-keys">the
        Discussion section</link>.</para>
      </note>

      <para>The program for loading this JSON definition from a file
      is as follows:</para>

      <example id="animations-complex-example-2">
        <title>Simple program for loading a JSON script;
        any key press starts the animation</title>
        <programlisting>
<xi:include href="examples/animations-complex.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

      <note>
        <para>It is also possible to use the <type>ClutterAnimator</type>
        C API to define keys for an animation, but this will
        typically be much more verbose than the JSON equivalent.</para>

        <para>One other advantage of JSON is that it is much simpler
        to tweak and test an animation, as you don't have to recompile
        the application each time you edit it (you just load
        the new JSON file).</para>
      </note>

    </section>

    <section id="animations-complex-discussion">
      <title>Discussion</title>

      <para>You can think of <type>ClutterAnimator</type>
      as a way to give directions to actors. For example,
      you could give a real (human) actor a direction like "move
      downstage; when you get there, stop and
      rotate 90 degrees to your right". In code,
      this might equate to a transition in the <varname>x</varname>
      and <varname>y</varname> properties of the actor, followed by a
      rotation in one axis.</para>

      <note>
        <para><type>ClutterAnimator</type> can give
        "directions" to any type of GObject, but we concentrate
        on animating <type>ClutterActors</type> in this section.</para>
      </note>

      <para>Each direction like this has an implicit
      timeline, spanning the length of time the direction should
      take to fulfil (you set the length of the timeline through
      the <varname>duration</varname> property of the
      <type>ClutterAnimator</type>). But within that timeline, you may
      change the proportion of time spent on each action: "move
      downstage quickly, then slowly rotate 90 degrees
      to your right". The direction is the same, but we've
      specified how much of the timeline should be devoted to each
      action.</para>

      <para>In <type>ClutterAnimator</type>, this concept is
      captured by <emphasis>key frames</emphasis>. A
      key frame represents a point somewhere along the timeline,
      with one or more target property values for one or more actors.
      A <type>ClutterAnimator</type> manages the transitions
      between property values for each object, ensuring that
      the target values are reached when the associated key frame
      is reached.</para>

      <para>To change the amount of time a transition
      should take, you change the percentage of the timeline
      between key frames. Using our real stage directions as an
      example, you might define the key frames like this:</para>

      <itemizedlist>
        <listitem>
          <para><emphasis>0.2 (after 20% of the timeline):</emphasis>
          arrive downstage</para>
        </listitem>
        <listitem>
          <para><emphasis>1.0 (by the end of the timeline):</emphasis>
          achieve a 90 degree rotation to the right</para>
        </listitem>
      </itemizedlist>

      <para>See
      <link linkend="animations-complex-discussion-keys">this
      section</link> for more details about keys and key frames.</para>

      <para>Finally, a direction might be further refined with
      a description of the kind of movement to use:
      rather than saying "move downstage quickly, then
      slowly rotate 90 degrees to your right" a director could say:
      "start off slowly, but build up to a run;
      run downstage quickly; then stop and start rotating
      slowly to your right, gradually speeding up, turn a little more, then slow
      down gradually; you should end up rotated 90 degrees to your right"
      (this granularity of description is closer to what you might
      see in dance notation like
      <ulink href="http://en.wikipedia.org/wiki/Labanotation">Laban</ulink>;
      though of course you can't animate human opacity, scale, dimensions
      etc...).</para>

      <para><type>ClutterAnimator</type> gives you this level of
      granularity. Each transition to a property value between
      key frames can have a separate <emphasis>easing mode</emphasis>:
      for example, starting off slowly and building to a constant
      speed equates to an "ease in" mode; starting slowly, speeding
      up, maintaining a constant speed, then gradually slowing down
      equates to "ease in and ease out".</para>

      <para>To summarise: creating a complex animation means deciding:</para>

      <itemizedlist>
        <listitem>
          <para>Which properties need to change on which actors?</para>
        </listitem>
        <listitem>
          <para>What target value should each property transition to?</para>
        </listitem>
        <listitem>
          <para>How quickly (by which key frame) should the property
          reach the target value?</para>
        </listitem>
        <listitem>
          <para>What "shape" (easing mode) should the change to
          the target value follow?</para>
        </listitem>
      </itemizedlist>

      <section id="animations-complex-discussion-keys">
        <title>Understanding keys and key frames</title>

        <para>A <type>ClutterAnimator</type> maintains a list of
        <varname>properties</varname> objects, each being a unique pair
        of <varname>object</varname> (an object to be animated) +
        <varname>name</varname> (name of the property
        to be animated on that object).</para>

        <para>Each <varname>properties</varname> object in turn has a
        list of keys, with each key having three elements:</para>

        <itemizedlist>
          <listitem>
            <para>The <emphasis>key frame</emphasis>, expressed as a fraction
            (between 0.0 and 1.0) of the duration of the animation. At this
            point, the named property should reach a target value.</para>
          </listitem>
          <listitem>
            <para>The <emphasis>easing mode</emphasis> to use to transition
            the property to that value.</para>
          </listitem>
          <listitem>
            <para>The <emphasis>target value</emphasis> the property
            should transition to.</para>
          </listitem>
        </itemizedlist>

        <para>For example:</para>

        <informalexample>
          <programlisting>
{
  "object" : "rectangle",
  "name" : "x",
  "ease-in" : true,
  "keys" : [
    [ 0.0, "linear", 0.0 ],
    [ 0.1, "easeInCubic", 150.0 ],
    [ 0.8, "linear", 150.0 ],
    [ 1.0, "easeInCubic", 0.0 ]
  ]
}
          </programlisting>
        </informalexample>

        <para>defines a sequence of transitions for the <varname>x</varname>
        property (position on the x axis) of the <code>rectangle</code>
        object, as follows:</para>

        <orderedlist>
          <listitem>
            <para><emphasis>[ 0.0, "linear", 0.0 ]</emphasis>:
            At the start of the animation, <code>x</code> should be
            0.0; <code>linear</code> is used as the easing mode, as there
            is no transition here.</para>
          </listitem>

          <listitem>
            <para><emphasis>[ 0.1, "easeInCubic", 150.0 ]</emphasis>:
            By 10% of the way through the animation,
            <code>x</code> should reach a value of <code>150.0</code>.
            This moves the rectangle horizontally across the stage.</para>

            <para>The <code>easeInCubic</code> easing mode means that
            the transition to the new value starts slow and speeds up.
            This makes the movement look more "natural".</para>
          </listitem>

          <listitem>
            <para><emphasis>[ 0.8, "linear", 150.0 ]</emphasis>:
            From 10% of the way through the animation to 80%
            of the way through, the <code>x</code> value remains at
            <code>150.0</code>. This makes the rectangle stay still
            on the x axis throughout this period.</para>

            <para>It's important to specify interim key frames if
            in a later key frame you intend to change the value again
            (as is done for the <code>x</code> value here). Otherwise
            you can get premature transitions to a value over longer
            periods than you intended. By specifying the interim
            key frames where the value remains constant, you ensure
            that it doesn't change before you want it to.</para>
          </listitem>

          <listitem>
            <para><emphasis>[ 1.0, "easeInCubic", 0.0 ]</emphasis>:
            From 80% of the way through the animation to the end,
            the <code>x</code> value should transition back to
            <code>0.0</code>. This moves the actor back to its
            starting position on the x axis. Again, an <code>easeInCubic</code>
            easing mode is used to make the transition appear more natural.</para>
          </listitem>
        </orderedlist>

        <para>There are two more properties you can set for each
        object/property pair:</para>

        <orderedlist>
          <listitem>
            <para>Set <varname>ease-in</varname> to <code>true</code> to
            animate to the target value at the first key frame. If
            <varname>ease-in</varname> is false, the animation will
            "jump" to the target value instead (if the target value is
            different from the current value).</para>
          </listitem>

          <listitem>
            <para>Set <varname>interpolation</varname> to either
            <code>"linear"</code> (the default) or <code>"cubic"</code>.
            This sets how <type>ClutterAnimator</type> transitions between
            key frames; in effect, it further modulates any easing modes
            set on individual keys: if set to <code>"cubic"</code>, you
            get a slightly more natural and gentle transition between
            key frames than you do if set to <code>"linear"</code>.</para>
          </listitem>
        </orderedlist>

      </section>

      <section id="animations-complex-why-clutteranimator">
        <title>Why <type>ClutterAnimator</type>?</title>

        <para>Why use <type>ClutterAnimator</type> and not the other
        <link linkend="animations-introduction-api">Clutter animation
        approaches</link> for complex animations?</para>

        <itemizedlist>
          <listitem>
            <para><emphasis>Implicit animations</emphasis> can animate
            properties on a single actor; however, you can only specify a
            single transition for each property. Also, it's not possible
            to describe complex movement along a path in a single implicit
            animation: you would have to chain several animations together
            to do that.</para>

            <para>To animate multiple actors, you'd also need multiple
            implicit animations, one for each actor. These animations would
            also need to be synchronized (for example, by sharing a
            single timeline).</para>

            <para>So it would be possible, but more difficult than
            an implementation using <type>ClutterAnimator</type>.</para>
          </listitem>

          <listitem>
            <para><emphasis><type>ClutterState</type></emphasis> can
            be used for complex animations: each state can describe
            transitions for multiple actors and multiple properties.
            However, to make continuous movement (as in the example),
            you would need to write a state for each movement between a
            pair of points; then add a callback so that when each state
            is reached, the animation moves onto the next state. This
            adds some code (a handler for the <code>completed</code>
            signal emitted by the <type>ClutterState</type> to set
            the next state). This could work OK for a few states,
            but doesn't scale as well as <type>ClutterAnimator</type>
            if you have many transitions.</para>

            <note>
              <para><type>ClutterState</type> and
              <type>ClutterAnimator</type> are not mutually exclusive. If
              you generally need to transition between several known states
              (e.g. hiding/revealing menus which stay in the same place,
              moving between two UI layouts), but want to create a
              complex animation between states, you can use
              <type>ClutterAnimators</type> to define the transitions: see
              the documentation for
              <function>clutter_state_set_animator()</function> for
              details.</para>
            </note>

          </listitem>
        </itemizedlist>

        <para><type>ClutterAnimator</type> is a good fit for complex
        animations, and probably the best fit for the most complex:
        it is the simplest way to encode a sequence of transitions
        for a list of object/property pairs which can be treated
        as a single animation. This is largely because
        <type>ClutterAnimator</type> is effectively managing the
        chaining together of the individual transitions into a whole.</para>

        <para>One other feature of <type>ClutterAnimator</type> which
        isn't demonstrated here is how it enables transitions to overlap.
        For example, let's say you wanted an actor
        to move along a complex path (e.g. described by five pairs of
        x,y coordinates); but during that movement, you
        wanted the actor to continuously transition to a scale of
        4.0 on both the x and y axes.</para>

        <para>To achieve this with <type>ClutterState</type>, you would
        need to set up five transitions (one to move to each pair of
        x,y coordinates); plus a callback to chain the state transitions
        together; and within each transition, you'd have to figure out a
        percentage of the scaling to apply, so that the actor
        was at a scale of 4.0 on reaching the final state.</para>

        <para>With <type>ClutterAnimator</type>, you can treat the
        movement between the coordinates and the scaling separately
        within the same animation, but overlap their key frames. This
        makes coding overlapping animations of different properties
        much more straightforward. See
        <link linkend="animations-complex-example-3">this JSON
        definition</link> for an example of how to do this.</para>

      </section>

    </section>

    <section>
      <title>Full example</title>

      <example id="animations-complex-example-3">
        <title>Running multiple transition sequences with
        different key frames in parallel using
        <type>ClutterAnimator</type></title>

        <note>
          <para>This JSON file can be loaded with the same code
          as used for <link linkend="animations-complex-example-2">this
          example</link>, by passing the JSON file name on the command line:</para>

          <screen>
            <prompt>$</prompt> <command>./animations-complex animations-complex-overlapping.json</command>
          </screen>
        </note>

        <programlisting>
<xi:include href="examples/animations-complex-overlapping.json" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>
    </section>

  </section>

</chapter>