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aece642adb
Make sure users get the idea that clutter_init()
has a return value that needs to be checked.
These were fixed via sed magic:
sed -i -s -e "s/clutter_init (.*)/\
if (& != CLUTTER_INIT_SUCCESS)\n return 1/"\
doc/*/*/*.{c,xml} doc/*/*.xml
http://bugzilla.clutter-project.org/show_bug.cgi?id=2574
459 lines
17 KiB
XML
459 lines
17 KiB
XML
<chapter id="clutter-animations">
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<chapterinfo>
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<author>
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<firstname>Matthew</firstname>
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<surname>Allum</surname>
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<affiliation>
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<address>
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<email>mallum@openedhand.com</email>
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</address>
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</affiliation>
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</author>
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<author>
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<firstname>Emmanuele</firstname>
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<surname>Bassi</surname>
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<affiliation>
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<address>
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<email>ebassi@linux.intel.com</email>
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</address>
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</affiliation>
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</author>
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</chapterinfo>
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<title>Creating Animations with Clutter</title>
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<para>With Clutter using hardware accelration for graphics rendering,
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complex and fast animations are possible. This chapter describes basic
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techniques and the utilities Clutter provides in aiding animation
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creation.</para>
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<section id="clutter-animation-basic">
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<title>Basic Animations</title>
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<para>The most basic way to create animations with Clutter is via the use
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of g_timeout_add(). This enables a callback function to be called at a
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defined interval. The callback function can then modify actors visual
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properties as to produce an animation.</para>
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<example id="clutter-timeout-example">
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<title>Simple timeout example</title>
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<para>Implement a rotating actor using 360 "frames"</para>
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<programlisting>
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struct RotationClosure {
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ClutterActor *actor;
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gdouble final_angle;
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gdouble current_angle;
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};
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static gboolean
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rotate_actor (gpointer data)
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{
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struct RotationClosure *clos = data;
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clutter_actor_set_rotation (clos->actor, clos->current_angle, 0, 0, 0);
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/* add one degree */
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clos->current_angle += 1.0
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if (clos->current_angle == clos->final_angle)
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return FALSE;
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return TRUE;
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}
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static void
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rotate_actor_cleanup (gpointer data)
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{
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struct RotationClosure *clos = data;
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g_object_unref (clos->actor);
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g_free (clos);
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}
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...
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struct RotationClosure *clos = NULL;
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clos = g_new (struct RotationClosure, 1);
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clos->actor = g_object_ref (an_actor);
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clos->final_angle = 360.0;
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clos->current_angle = 0;
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g_timeout_add_full (1000 / 360, /* 360 updates in one second */
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rotate_actor,
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clos,
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rotate_actor_cleanup);
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</programlisting>
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</example>
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<note>
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<title>Priorities</title>
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<para>%G_PRIORITY_DEFAULT should always be used as the timeouts priority
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(in case of g_timeout_add_full()) as not to intefere with Clutter's
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scheduling of repaints and input event handling.</para>
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</note>
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</section>
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<section id="clutter-animation-timelines">
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<title>Timelines</title>
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<para>Using g_timeout_add() to control an animation is complicated
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and does not work in concert with the rest of the operations Clutter
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must perform for each redraw cycle.</para>
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<para>For this reason, Clutter provides #ClutterTimeline, a class that
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allows scheduling animations with a definite duration. Timelines are
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advanced during the redraw cycle so that animations are ready to be
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performed at the right time. This also means that animations will not
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affect the event processing; it also means that if the animation is too
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complex it will be called with a longer delay, thus not blocking the
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whole UI.</para>
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<para>A Timeline is created with:</para>
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<programlisting>
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clutter_timeline_new (duration_in_milliseconds);
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</programlisting>
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<para>The duration of the timeline then be modifed via the
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#ClutterTimeline:duration property or by using
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clutter_timeline_set_duration().</para>
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<para>A timeline is started via clutter_timeline_start() and its
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playback further manipulated by the clutter_timeline_pause(),
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clutter_timeline_stop(), clutter_timeline_rewind() and
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clutter_timeline_skip() functions.</para>
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<para>By attaching a handler to the timeline's
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#ClutterTimeline::new-frame signal a timeline can then be used to
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drive an animation by altering an actor's visual properties. The
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callback looks like:</para>
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<programlisting>
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void
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on_new_frame (ClutterTimeline *timeline,
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gint elapsed_msecs,
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gpointer user_data)
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{
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}
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</programlisting>
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<para>The <literal>elapsed_msecs</literal> parameter is set to the amount
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of time elapsed since the beginning of the timeline, and its value is
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always between 0 and the #ClutterTimeline:duration value.</para>
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<para>The function clutter_timeline_get_progress() can also be used to
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get a normalised value of the timeline's current position between 0 and
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1.</para>
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<para>Timelines can also be played in reverse by setting the direction
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using clutter_timeline_set_direction(), and can also have a one-time
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delay set before they begin playing by using the function
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clutter_timeline_set_delay().</para>
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<para>Timelines can also control a pyshical simulation; the
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clutter_timeline_get_delta() function allows retrieving the number of
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milliseconds elapsed since the previous callback to ensure the physics
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engine to be able to take the actual time elapsed between iterations
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into account.</para>
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<example id="clutter-timeline-example">
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<title>Using a Timeline to drive an animation</title>
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<para>Rewrite the example above with a #ClutterTimeline instead of
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using g_timeout_add()</para>
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<programlisting>
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#include <clutter/clutter.h>
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static void
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on_new_frame (ClutterTimeline *timeline,
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gint elapsed_msecs,
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ClutterActor *actor)
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{
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gdouble angle = 360 * clutter_timeline_get_progress (timeline);
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clutter_actor_set_rotation (actor, CLUTTER_Z_AXIS,
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angle,
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clutter_actor_get_width (actor) / 2,
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clutter_actor_get_height (actor) / 2,
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0);
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}
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...
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ClutterTimeline *timeline;
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timeline = clutter_timeline_new (1000); /* one second */
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clutter_timeline_set_loop (timeline, TRUE);
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g_signal_connect (timeline, "new-frame", G_CALLBACK (on_new_frame), actor);
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clutter_timeline_start (timeline);
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</programlisting>
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</example>
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<note><para>Multiple timelines can be sequenced in order by using a
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#ClutterScore. See the #ClutterScore documentation for more details on
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using this.</para></note>
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</section>
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<section id="clutter-animation-behaviours">
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<title>Behaviours</title>
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<para>With a large application containing many animations, the use of
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just timelines can become unwieldy and difficult to manage, with much
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code duplication in the #ClutterTimeline::new-frame handlers that can
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require over-complex code changes for minor animation modifications. To
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ease these problems the #ClutterAlpha and #ClutterBehaviour classes were
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created.</para>
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<para>#ClutterAlpha and #ClutterBehaviour attempt to generalise the
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#ClutterTimeline::new-frame function by defining common actions (or
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behaviours) that can be quickly modified, applied to multiple actors or
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mixed on a single actor.</para>
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<para>A #ClutterAlpha is a 'function of time' (and does not refer to the
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alpha channel of a RGBA color). It is created by referencing a source
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timeline and an "easing mode" whichproduces a value between -1.0 and 2.0
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depending on the progress of the timeline. Clutter provides various
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easing modes, as described by the #ClutterAnimationMode enumeration. It is
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also possible to register new animation modes using the function
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clutter_alpha_register_func() or to provide a custom #ClutterAlphaFunc for
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a specific #ClutterAlpha instance.</para>
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<para>A #ClutterBehaviour is created with a #ClutterAlpha and a set of
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parameters for whatever the behaviour modifies in an actor. The value of
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a #ClutterAlpha during the animation is then mapped to a value for the
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behaviour parameters and then applied on the actors referenced by the
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#ClutterBehaviour. With the #ClutterAlpha's underlying timeline playing
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the produced value will change and the behaviour will animate an
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actor.</para>
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<para>A #ClutterBehaviour is effectively 'driven' by a supplied
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#ClutterAlpha and when then applied to an actor it will modify a visual
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property or feature of the actor dependant on the Alpha's value. For
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example, a path-based behaviour applied to an actor will alter its
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position along a #ClutterPath, depending on the current alpha value over
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time. The actual progress of the motion will depend on the chosen "easing
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mode".</para>
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<para> Multiple behaviours can of course be applied to an actor as well
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as a single behaviour being applied to multiple actors. The separation
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of timelines, alphas and behaviours allows for a single timeline to drive
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many behaviours each potentially using different alpha functions.
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Behaviour parameters can also be changed on the fly.</para>
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<para>
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<figure id="behaviour-path-alpha">
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<title>Effects of alpha functions on a path</title>
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<graphic fileref="path-alpha-func.png" format="PNG"/>
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<blockquote>The actors position between the path's end points
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directly correlates to the #ClutterAlpha's current alpha value
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driving the behaviour. With the #ClutterAlpha's animation mode
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set to %CLUTTER_LINEAR the actor will follow the path at a constant
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velocity, but when changing to %CLUTTER_EASE_IN_OUT_SINE the actor
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initially accelerates before quickly decelerating.</blockquote>
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</figure>
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</para>
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<para>The behaviours included in Clutter are:</para>
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<para>
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<variablelist>
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<varlistentry>
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<term>#ClutterBehaviourDepth</term>
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<listitem><simpara>Changes the depth of actors</simpara></listitem>
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</varlistentry>
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<varlistentry>
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<term>#ClutterBehaviourEllipse</term>
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<listitem><simpara>Moves actors along an elliptical path</simpara></listitem>
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</varlistentry>
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<varlistentry>
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<term>#ClutterBehaviourOpacity</term>
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<listitem><simpara>Changes the opacity of actors</simpara></listitem>
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</varlistentry>
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<varlistentry>
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<term>#ClutterBehaviourPath</term>
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<listitem><simpara>Moves actors along a path</simpara></listitem>
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</varlistentry>
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<varlistentry>
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<term>#ClutterBehaviourRotate</term>
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<listitem><simpara>Rotates actors along an axis</simpara></listitem>
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</varlistentry>
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<varlistentry>
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<term>#ClutterBehaviourScale</term>
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<listitem><simpara>Changes the scaling factors of actors</simpara></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<example id="clutter-behaviour-example">
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<title>Using a #ClutterBehaviour</title>
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<para>The following example demonstrates an ellipse behaviour in
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action.</para>
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<programlisting>
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#include <clutter/clutter.h>
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int
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main (int argc, char *argv[])
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{
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ClutterTimeline *timeline;
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ClutterBehaviour *behave;
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ClutterAlpha *alpha;
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ClutterActor *stage, *actor;
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if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
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return 1;
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stage = clutter_stage_get_default ();
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actor = clutter_texture_new_from_file ("ohpowers.png", NULL);
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clutter_container_add_actor (CLUTTER_CONTAINER (stage), actor);
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/* set up the animation to be 4 seconds long */
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timeline = clutter_timeline_new (4000);
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clutter_timeline_set_loop (timeline, TRUE);
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/* set up a sinusoidal easing mode to power the behaviour; the
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* alpha will take a reference on the timeline so we can safely
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* release the reference we hold
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*/
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alpha = clutter_alpha_new_full (timeline, CLUTTER_EASE_IN_OUT_SINE);
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g_object_unref (timeline);
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/* the behaviour will own the alpha by sinking its floating
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* reference (if needed)
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*/
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behave = clutter_behaviour_ellipse_new (alpha,
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200, /* center x */
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200, /* center y */
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400, /* width */
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300, /* height */
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CLUTTER_ROTATE_CW, /* direction */
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0.0, /* initial angle */
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360.0); /* final angle */
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clutter_behaviour_apply (behave, actor);
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clutter_actor_show_all (stage);
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clutter_timeline_start (timeline);
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clutter_main();
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/* clean up; behaviours are top-level objects */
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g_object_unref (behave);
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return 0;
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}
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</programlisting>
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</example>
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<note><para>The parameters of a #ClutterBehaviour can be changed whilst
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a animation is running.</para></note>
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<para>There can be many #ClutterAlpha's attached to a single timeline.
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There can be many behaviours for a #ClutterAlpha. There can be many
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behaviours applied to an actor. A #ClutterScore can be used to chain
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many behaviours together.</para>
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<warning><para>Combining behaviours that effect the same actor properties
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(i.e two separate paths) will cause unexpected results. The values will
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not be merged in any way with only the last applied behaviour taking
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precedence.</para></warning>
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<note><para>Tips for implementing a new behaviour can be found <link
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linkend="creating-your-own-behaviours">here</link>.</para></note>
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</section>
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<section id="clutter-animation-implicit">
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<title>Implicit Animations</title>
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<para>Using behaviours for simple animations of a single actor may
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be too complicated, in terms of memory management and bookkeeping
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of the object instances. For this reason, Clutter also provides a
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simple animation API for implicit animations using properties of
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an actor: clutter_actor_animate().</para>
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<para>The clutter_actor_animate() family of functions will create
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and use an implicit #ClutterAnimation instance, which will then
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handle the animation of one or more #ClutterActor properties between
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a range of values.</para>
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<example id="clutter-actor-animate-example">
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<title>Using clutter_actor_animate()</title>
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<para>The following example demonstrates how to use the
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clutter_actor_animate() method to tween an actor between the current
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position and a new set of coordinates. The animation takes 200
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milliseconds to complete and uses a linear progression.</para>
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<programlisting>
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clutter_actor_animate (actor, CLUTTER_LINEAR, 200,
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"x", 200.0f,
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"y", 200.0f,
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NULL);
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</programlisting>
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</example>
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<para>The clutter_actor_animate() method returns a #ClutterAnimation
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instance that can be used to start, stop and modify the animation
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while it's running. The #ClutterAnimation::completed signal will
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be emitted when the animation has been completed.</para>
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<warning><para>When the animation is complete it will be automatically
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unreferenced, and disposed if nothing else is holding a reference
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on it.</para></warning>
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<example id="clutter-actor-animate-multi-example">
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<title>Animating inside an event handler</title>
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<para>The following example demonstrates how to animate an actor
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inside the signal handler for a button press event. If the user
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presses the button on a new position while the animation is running,
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the animation will be restarted with the new final values
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updated.</para>
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<programlisting>
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static gboolean
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on_button_press (ClutterActor *actor,
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ClutterEvent *event,
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gpointer user_data)
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{
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gfloat event_x, event_y;
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clutter_event_get_coords (event, &event_x, &event_y);
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clutter_actor_animate (actor, CLUTTER_EASE_SINE_OUT, 500,
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"x", event_x,
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"y", event_y,
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NULL);
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return TRUE;
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}
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</programlisting>
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</example>
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<para>Calling clutter_actor_animate() multiple times on an
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actor which is being animated will cause the animation to be updated
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with the new values.</para>
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<para>If you need to chain up multiple animations created using
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clutter_actor_animate() you should connect to the
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#ClutterAnimation::completed signal using g_signal_connect_after()
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to have the guarantee that the current #ClutterAnimation has been
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detached from the actor. The documentation for clutter_actor_animate()
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has further examples.</para>
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</section>
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<section id="clutter-animation-conclusion">
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<title>Conclusion</title>
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<para>Clutter provides a number of utility classes to aid animations
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and complex animations can be produced by combining the various features
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provided.</para>
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<para>Of course animations can becreated outside of the Clutter animation
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framework, as the framework is not expected to cover every kind of
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possible animation scenario.</para>
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<para>The animation functionality in Clutter is primarily suited to
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building animations with a set or finite running time - i.e transitions
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between states. For animations involving variable input (such as
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touchscreen handling) physical simulations may be more suited.</para>
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</section>
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</chapter>
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