mutter/doc/cookbook/animations.xml

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<chapter id="animations">
  <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>
      <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>
      <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>

</chapter>