cookbook: Recipe for "moving actors"

This recipe explains how to use the three animation
approaches (implicit, State, Animator) to animate movement
of actors.

Includes some guidelines about which approach to use when, with
a full code example for each approach.

The discussion section covers some subtleties around animated
movement; namely: moving actors out of their containers; anchor
points and movement; moving in the depth axis; interactions
between animated movement and constraints.

doc/cookbook/Makefile.am

@@ -62,6 +62,8 @@ VIDEO_FILES = \
 	videos/textures-crossfade-two-textures.ogv \
 	videos/animations-complex.ogv \
 	videos/animations-reuse.ogv \
+	videos/animations-moving-anchors.ogv \
+	videos/animations-moving-depth.ogv \
 	$(NULL)
 
 EXTRA_DIST = \

doc/cookbook/animations.xml

@@ -1816,4 +1816,478 @@ foo_button_pressed_cb (ClutterActor *actor,
 
   </section>
 
+  <section id="animations-moving">
+    <title>Moving actors</title>
+
+    <section>
+      <title>Problem</title>
+
+      <para>You want to animate the movement of one or more actors.
+      For example:</para>
+
+      <itemizedlist>
+        <listitem>
+          <para>To move user interface elements in response to user input
+          (e.g. keyboard control of a character in a game).</para>
+        </listitem>
+        <listitem>
+          <para>To move a group of actors "off stage" to make way
+          for another group of actors (e.g. paging through
+          thumbnails in a photo viewer).</para>
+        </listitem>
+        <listitem>
+          <para>To move an actor to a different position in the
+          interface (e.g. moving an icon for a trashed file into
+          a wastebin).</para>
+        </listitem>
+      </itemizedlist>
+
+    </section>
+
+    <section>
+      <title>Solutions</title>
+
+      <para>Animate the actors movement on one or more axes
+      (<varname>x</varname>, <varname>y</varname>,
+      <varname>z/depth</varname>) using one or more of the approaches
+      available in the Clutter API (implicit animations,
+      <type>ClutterState</type>, <type>ClutterAnimator</type>).</para>
+
+      <section>
+        <title>Solution 1: Implicit animations</title>
+
+        <para>This works well for simple movement of a single actor to
+        a single set of coordinates. Here is an example of how to animate
+        movement of a <type>ClutterActor</type> <varname>actor</varname>
+        to position <code>100.0</code> on <varname>x</varname> axis:</para>
+
+        <informalexample>
+          <programlisting>
+clutter_actor_animate (actor, CLUTTER_LINEAR, 500,
+                       "x", 100.0,
+                       NULL);
+          </programlisting>
+        </informalexample>
+
+        <para>See <link linkend="animations-moving-example-1">this
+        example</link> which demonstrates movement in each axis,
+        in response to (mouse) button presses.</para>
+
+      </section>
+
+      <section>
+        <title>Solution 2: <type>ClutterState</type></title>
+
+        <para>This suits simple, repeated movement of one or more actors
+        between sets of coordinates. Here is an example of how to
+        create two states for a <type>ClutterState</type> instance to
+        move two actors, <varname>actor1</varname> and
+        <varname>actor2</varname>:</para>
+
+        <informalexample>
+          <programlisting>
+ClutterState *transitions = clutter_state_new ();
+
+/* all state transitions take 250ms */
+clutter_state_set_duration (transitions, NULL, NULL, 250);
+
+/* create a state called move-down which moves both actors to y = 200.0 */
+clutter_state_set (transitions, NULL, "move-down",
+                   actor1, "y", CLUTTER_EASE_OUT_CUBIC, 200.0,
+                   actor2, "y", CLUTTER_EASE_OUT_CUBIC, 200.0,
+                   NULL);
+
+/* create a state called move-up which moves both actors to y = 0.0 */
+clutter_state_set (transitions, NULL, "move-up",
+                   actor1, "y", CLUTTER_EASE_OUT_CUBIC, 0.0,
+                   actor2, "y", CLUTTER_EASE_OUT_CUBIC, 0.0,
+                   NULL);
+
+/* move the actors by setting the state */
+clutter_state_set (transitions, "move-down");
+          </programlisting>
+        </informalexample>
+
+        <para>This <link linkend="animations-moving-example-2">full
+        example</link> shows how to move and simultaneously
+        scale two actors. When a button is pressed on one actor, it is
+        moved and scaled to occupy the right-hand side of the stage;
+        the other actor is simultaneously moved back to the left-hand
+        side of the stage and scaled down.</para>
+
+      </section>
+
+      <section>
+        <title>Solution 3: <type>ClutterAnimator</type></title>
+
+        <para>This is a good way to implement complex movement of
+        one or more actors between sets of coordinates.</para>
+
+        <informalexample>
+          <programlisting>
+ClutterAnimator *animator = clutter_animator_new ();
+
+/* the animation takes 500ms */
+clutter_animator_set_duration (animator, 500);
+
+/* at the start of the animation, actor should be at 0.0,0.0;
+ * half-way through, at 100.0,100.0;
+ * by the end, actor should be at 150.0,200.0;
+ * note that you can set different easing modes for each
+ * part of the animation and for each property at each key
+ */
+clutter_animator_set (animator,
+
+                      /* keys for the start of the animation */
+                      actor, "x", CLUTTER_LINEAR, 0.0, 0.0,
+                      actor, "y", CLUTTER_LINEAR, 0.0, 0.0,
+
+                      /* keys for half-way through the animation */
+                      actor, "x", CLUTTER_EASE_OUT_CUBIC, 0.5, 100.0,
+                      actor, "y", CLUTTER_EASE_IN_CUBIC, 0.5, 100.0,
+
+                      /* keys for the end of the animation */
+                      actor, "x", CLUTTER_EASE_OUT_EXPO, 1.0, 150.0,
+                      actor, "y", CLUTTER_EASE_OUT_CUBIC, 1.0, 200.0,
+
+                      NULL);
+
+/* run the animation */
+clutter_animator_start (animator);
+          </programlisting>
+        </informalexample>
+
+        <para>The <link linkend="animations-moving-example-3">full
+        example</link> demonstrates how <type>ClutterAnimator</type>
+        can be used to programmatically animate multiple actors: in this
+        case, to simultaneously move three actors to random positions
+        along the <varname>x</varname> axis. Synchronising the
+        movement of three actors simultaneously using implicit
+        animations would be possible but awkward;
+        <type>ClutterState</type> might be another option,
+        but it wasn't really designed for this case: there are no persistent
+        states to transition between, as the actor positions are
+        generated on each key press.</para>
+
+        <note>
+          <para>If you want to apply the same movement to a group of
+          actors, rather than different movements for each actor,
+          it's often better to put the actors into a container
+          of some kind and move that instead of moving the actors
+          individually.</para>
+        </note>
+
+      </section>
+
+    </section>
+
+    <section>
+      <title>Discussion</title>
+
+      <section>
+        <title>Movement can take an actor "outside" its container</title>
+
+        <para>Actor movement in the <varname>x</varname> and
+        <varname>y</varname> axes is relative to the actor's parent
+        container. There is nothing to stop you animating an actor
+        until it falls outside the bounds of its container. This
+        could result in the actor moving "off" the interface; though it's
+        worth remembering that the actor is not unparented or destroyed
+        if this happens.</para>
+
+        <para>To ensure that an actor remains visible, its position
+        should remain within the visible area of the container. In practice,
+        this means either anywhere in the container, if no clip area
+        has been set; or within the container's clip area, if set.</para>
+
+      </section>
+
+      <section>
+        <title>Anchor points can affect movement</title>
+
+        <para>An actor's anchor point is defined as an <code>x,y</code>
+        coordinate relative to the top-left of the actor. The default
+        anchor point for an actor is in its top-left
+        corner. However, it is possible to set this to some other
+        coordinate, relative to the actor's top-left corner,
+        using the <function>clutter_anchor_set_anchor_point()</function>
+        function. For example:</para>
+
+        <informalexample>
+          <programlisting>
+/* set the actor's size to 100px x 100px */
+clutter_actor_set_size (actor, 100, 100);
+
+/* set an anchor point half-way along the top of the actor */
+clutter_actor_set_anchor_point (actor, 50.0, 0.0);
+          </programlisting>
+        </informalexample>
+
+        <para>A positive anchor point within the width/height bounds of the
+        actor is inside the actor. An anchor point outside these bounds
+        is outside the actor. You can also set a negative
+        <varname>x</varname> or <varname>y</varname> value for
+        the anchor point, which will again place the point outside
+        the actor's bounds.</para>
+
+        <para>This is important with respect to moving an actor, because
+        you are actually moving the anchor point and "dragging" the
+        actor along with it.</para>
+
+        <para>For example: you have an actor with width 50px, and you
+        set its <varname>anchor-x</varname> property to <code>25.0</code>.
+        If you move that actor on the <varname>x</varname> axis, you are
+        effectively moving a point half-way across the top of the
+        actor along the <varname>x</varname> axis (which in turn moves the
+        actor).</para>
+
+        <para>Similarly, you could set the same actor's
+        <varname>anchor-x</varname> to <code>-25.0</code>. If you then
+        moved the actor along the <varname>x</varname> axis, you would
+        effectively be moving the point 25px left of the top of the actor
+        along that axis.</para>
+
+        <para>The video below demonstrates the effect on movement of shifting
+        the anchor point on the <varname>x</varname> axis. The
+        <emphasis>red</emphasis> rectangle has <varname>anchor-x</varname>
+        set to <code>25.0</code>; the <emphasis>green</emphasis> rectangle has
+        <varname>anchor-x</varname> set to <code>0.0</code> (the default); the
+        <emphasis>blue</emphasis> rectangle has <varname>anchor-x</varname>
+        set to <code>-25.0</code>.</para>
+
+        <inlinemediaobject>
+          <videoobject>
+            <videodata fileref="videos/animations-moving-anchors.ogv"/>
+          </videoobject>
+          <alt>
+            <para>Video showing the effect of anchor point on movement</para>
+          </alt>
+        </inlinemediaobject>
+
+        <para> A <type>ClutterAnimator</type> is
+        used to move each of the rectangles to <code>x = 225.0</code>.
+        Although the three rectangles move to the same position on the
+        <varname>x</varname> axis, it's actually the the anchor points
+        which are at the same position. These all align on the
+        <varname>x</varname> axis with the left-hand edge of the green
+        rectangle.</para>
+
+      </section>
+
+      <section>
+        <title>Actors can move in the <varname>z</varname> axis</title>
+
+        <para>The examples so far have shown how to move actors in
+        the <varname>x</varname> and <varname>y</varname> axes; but it
+        is also possible to move actors in the <varname>z</varname>
+        axis (i.e. move them closer or further away from the view point).
+        This lets you move actors under/over each other.</para>
+
+        <para>To move an actor in the <varname>z</varname> axis, animate
+        its <varname>depth</varname> property. Animating to a negative
+        depth moves the actor away from the view point; animating to a
+        positive depth moves the actor towards the view point.</para>
+
+        <para>Changing the depth of an actor also causes perspective
+        effects: the actor gets smaller and converges on the center
+        of the stage as it gets further from the view point, and
+        gets larger and diverges from the center of the stage as it gets
+        closer. This results in an apparent (but not actual) change in
+        the <code>x,y</code> position and scale of the actor.</para>
+
+        <note>
+          <para>Animating the depth of an actor is slightly different
+          from animating its x and y coordinates, as depth is relative
+          to the whole stage, not just the parent container of the
+          actor. This means that perspective effects are with
+          respect to the whole stage: so as an actor's depth
+          moves below <code>0.0</code>, it converges on the center
+          of the stage, and may even apparently move outside its
+          container (if the container stays at the same depth).</para>
+        </note>
+
+        <para>The video below demonstrates the effect of animating
+        the depth of four actors to a value of <code>-15000.0</code>.
+        Note how the actors converge on the center of the stage,
+        as well as appearing to change position and scale; also note
+        that they appear to move outside the bounds of their parent containers
+        (the four yellow <type>ClutterBoxes</type>).</para>
+
+        <inlinemediaobject>
+          <videoobject>
+            <videodata fileref="videos/animations-moving-depth.ogv"/>
+          </videoobject>
+          <alt>
+            <para>Video showing perspective effects when animating
+            actor depth</para>
+          </alt>
+        </inlinemediaobject>
+
+      </section>
+
+      <section>
+        <title>Movement is affected by constraints</title>
+
+        <para>An actor can have its x,y position constrained by
+        the position of other actors through <type>ClutterBindConstraints</type>.
+        This can affect movement in two ways:</para>
+
+        <orderedlist>
+          <listitem>
+            <para>If an actor has its <varname>x</varname> and/or
+            <varname>y</varname> properties
+            bound or aligned to another actor's, you can't animate
+            those properties.</para>
+
+            <para>In effect this means that the bound actor can't be
+            moved on a bound axis directly, but can only be moved by
+            animating the constraint's properties.</para>
+          </listitem>
+
+          <listitem>
+            <para>If you move an actor which has other actors bound to
+            it, the bound actors will also move. For example, if
+            the actor has several other actors whose <varname>x</varname>
+            properties are bound to its <varname>x</varname> property,
+            moving the actor on the <varname>x</varname> axis will also
+            move the bound actors on that axis.</para>
+
+            <para>Similarly, if some actor is the source for
+            alignment constraints on other actors, moving the source
+            will cause those other actors to move, so that they remain in
+            alignment with it.</para>
+          </listitem>
+        </orderedlist>
+
+        <para>For example, consider two actors bound by constraints
+        as follows:</para>
+
+        <informalexample>
+          <programlisting>
+/* the source actor for the constraint */
+ClutterActor *source;
+
+/* the actor bound by the constraint */
+ClutterActor *target;
+
+/* a constraint to be added to target */
+ClutterConstraint *constraint;
+
+/* ...initialize actors etc... */
+
+/* create a constraint for binding the x position of some actor to the
+ * x position of source
+ */
+constraint = clutter_bind_constraint_new (source, CLUTTER_BIND_X, 0.0);
+
+/* add the constraint to target with a name */
+clutter_actor_add_constraint_with_name (target, "bind-x", constraint);
+          </programlisting>
+        </informalexample>
+
+        <para>Animating <varname>source</varname> on the <varname>x</varname>
+        axis also animates <varname>target</varname> on the same axis:</para>
+
+        <informalexample>
+          <programlisting>
+clutter_actor_animate (source, CLUTTER_LINEAR, 500,
+                       "x", 250.0,
+                       NULL);
+          </programlisting>
+        </informalexample>
+
+        <para>...while this has no effect, as it would violate
+        <varname>constraint</varname> (it's best not to animate
+        <varname>target's</varname> <varname>x</varname> property
+        directly):</para>
+
+        <informalexample>
+          <programlisting>
+clutter_actor_animate (target, CLUTTER_LINEAR, 500,
+                       "x", 250.0,
+                       NULL);
+          </programlisting>
+        </informalexample>
+
+        <para>But the constraint's properties can be animated, to change
+        how <varname>source</varname> and <varname>target</varname>
+        are bound; which in turn moves <varname>target</varname>:</para>
+
+        <informalexample>
+          <programlisting>
+clutter_actor_animate (target, CLUTTER_LINEAR, 500,
+                       "@constraints.bind-x.offset", 250.0,
+                       NULL);
+          </programlisting>
+        </informalexample>
+
+        <para>Note the <code>@constraints.&lt;constraint name&gt;.&lt;constraint property&gt;</code>
+        syntax (which is why we needed to use
+        <function>clutter_actor_add_constraint_with_name()</function>,
+        so that the constraint can be accessed through the actor).
+        We are still animating <varname>target</varname>, but really
+        we're indirectly animating a property of one of its constraints.</para>
+
+        <para>Another alternative would be to directly animate
+        the constraint's properties through <type>ClutterState</type>
+        or <type>ClutterAnimator</type>, rather than using
+        pseudo-properties on the actor animation:</para>
+
+        <informalexample>
+          <programlisting>
+ClutterAnimator *animator = clutter_animator_new ();
+clutter_animator_set_duration (animator, 500);
+
+clutter_animator_set (animator,
+                      constraint, "offset", CLUTTER_LINEAR, 0.0, 0.0,
+                      constraint, "offset", CLUTTER_LINEAR, 1.0, 250.0,
+                      NULL);
+
+clutter_animator_start (animator);
+          </programlisting>
+        </informalexample>
+
+        <para>This could be useful if you need to animate
+        multiple constraints between multiple values simultaneously.</para>
+
+      </section>
+
+    </section>
+
+    <section id="animations-moving-examples">
+      <title>Full examples</title>
+
+      <example id="animations-moving-example-1">
+        <title>Simple movement using implicit animations</title>
+        <programlisting>
+<xi:include href="examples/animations-moving-implicit.c" parse="text">
+  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
+</xi:include>
+        </programlisting>
+      </example>
+
+      <example id="animations-moving-example-2">
+        <title>Using <type>ClutterState</type> to repeatedly move
+        (and scale) two actors</title>
+        <programlisting>
+<xi:include href="examples/animations-moving-state.c" parse="text">
+  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
+</xi:include>
+        </programlisting>
+      </example>
+
+      <example id="animations-moving-example-3">
+        <title>Using <type>ClutterAnimator</type> to randomly move
+        three actors along the <varname>x</varname> axis</title>
+        <programlisting>
+<xi:include href="examples/animations-moving-animator.c" parse="text">
+  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
+</xi:include>
+        </programlisting>
+      </example>
+
+    </section>
+
+  </section>
+
 </chapter>