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mutter-performance-source/doc/reference/clutter/clutter-animation-tutorial.xml
Emmanuele Bassi 0530405899 [clutter-docs] Update the animation tutorial
The Alpha API and usage has been changed by the recent overhaul
of the ClutterAlpha class; hence, we need to update the relative
documentation in the animation tutorial.
2009-01-28 16:47:41 +00:00

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<chapter id="clutter-animations">
<chapterinfo>
<author>
<firstname>Matthew</firstname>
<surname>Allum</surname>
<affiliation>
<address>
<email>mallum@openedhand.com</email>
</address>
</affiliation>
</author>
</chapterinfo>
<title>Creating Animations with Clutter</title>
<para>
With Clutter using hardware accelration for graphics rendering,
complex and fast animations are possible. This chapter describes basic
techniques and the utilities Clutter provides in aiding animation
creation.
</para>
<section id="clutter-animation-basic">
<title>Basic Animations</title>
<para>
The most basic way to create animations with Clutter is via the use of
g_timeout_add(). This enables a callback function to be called at a
defined interval. The callback function can then modify actors visual
properties as to produce an animation.
</para>
<example id="clutter-timeout-example">
<para>
Simple Rotation...
</para>
<programlisting>
struct RotationClosure {
ClutterActor *actor;
ClutterFixed final_angle;
ClutterFixed current_angle;
};
static gboolean
rotate_actor (gpointer data)
{
RotationClosure *clos = data;
clutter_actor_set_rotationx (clos-&gt;actor, clos-&gt;current_angle, 0, 0, 0);
/* add one degree */
clos-&gt;current_angle += COGL_FIXED_ONE;
if (clos-&gt;current_angle == clos-&gt;final_angle)
return FALSE;
return TRUE;
}
static void
rotate_actor_cleanup (gpointer data)
{
RotationClosure *clos = data;
g_object_unref (clos-&gt;actor);
g_free (clos);
}
...
RotationClosure *clos = NULL;
clos = g_new (RotationClosure, 1);
clos-&gt;actor = g_object_ref (an_actor);
clos-&gt;final_angle = CLUTTER_FLOAT_TO_FIXED (360.0);
clos-&gt;current_angle = 0;
g_timeout_add_full (1000 / 360, /* fps to interval in milliseconds */
rotate_actor,
clos,
rotate_actor_cleanup);
</programlisting>
</example>
<note><title>Priorities</title>
<para>
%G_PRIORITY_DEFAULT should always be used as the timeouts priority
(in case of g_timeout_add_full()) as not to intefere with Clutter's
scheduling of repaints and input event handling.
</para>
</note>
</section>
<section id="clutter-animation-timelines">
<title>Timelines</title>
<para>
#ClutterTimeline<!-- -->s abstract a set period of time with a set frame
rate at which to call a provided callback.
</para>
<para>
#ClutterTimeline<!-- -->s also extend the timeout sources functionality
further by:
</para>
<orderedlist>
<listitem><para>Having a set duration (in milliseconds) and a set
'frame rate' - that is, the rate at which the callback is
called</para></listitem>
<listitem><para>Passing current progress information to the
callback</para></listitem>
<listitem><para>Handling 'dropped frames' and guarenteeing the set
duration by skipping over frames if the callback cannot keep up with
the set frame rate</para></listitem>
<listitem><para>Querying the number of milliseconds elapsed between
the current and previous callback.</para></listitem>
<listitem><para>Allowing the timeline to be modified on the fly as
well as being stopped, started, looped, rewound and
reversed</para></listitem>
<listitem><para>Using a #ClutterTimeoutPool to more efficiently
schedule multiple timeout sources without incurring in potential
starvation of the main loop slices</para></listitem>
</orderedlist>
<para>
A Timeline is created with;
</para>
<programlisting>
clutter_timeline_new (n_frames, frames_per_seconds);
</programlisting>
<para>
Taking a number of frames and a frames per second, or by;
</para>
<programlisting>
clutter_timeline_new_for_duration (msecs);
</programlisting>
<para>
Which takes the duration of the timeline in milliseconds with a
default frame rate (See clutter_get_default_frame_rate()).
</para>
<para>
The speed, duration and number of frames of the timeline then be
modifed via the objects properties and API calls. The timeline can
be made to loop by setting its "loop" property to %TRUE.
</para>
<para>
The timelines is started via clutter_timeline_start() and its
playback further manipulated by the clutter_timeline_pause(),
clutter_timeline_stop(), clutter_timeline_rewind() and
clutter_timeline_skip() calls.
</para>
<para>
By attaching a handler to the timeline's #ClutterTimeline::new-frame
signal a timeline can then be used to drive an animation by altering
an actor's visual properties in this callback. The callback looks like:
</para>
<programlisting>
void
on_new_frame (ClutterTimeline *timeline,
gint frame_num,
gpointer user_data)
{
}
</programlisting>
<para>
The <literal>frame_num</literal> parameter is set to the timeline's
current frame number (which is between 1 and the "num-frames" property).
This value can be used to compute the state of a particular animation
that is dependant on the frame numer. The clutter_timeline_get_progress()
function can also be used to get a normalised value of the timeline's
current position between 0 and 1.
</para>
<para>
Timelines can also be played in reverse by setting the direction using
clutter_timeline_set_direction(), and can also have a one-time delay set
before they begin playing by using clutter_timeline_set_delay().
</para>
<para>
Timelines can also control a pyshical simulation; the
clutter_timeline_get_delta() function allows retrieving the number of
frames and milliseconds elapsed since the previous callback to ensure
the physics engine to be able to take the actual time elapsed between
iterations into account.
</para>
<example id="clutter-timeline-example">
<para>
The following example demonstrates rotating an actor with a timeline.
</para>
<programlisting>
#include &lt;clutter/clutter.h&gt;
void
on_new_frame (ClutterTimeline *timeline,
gint frame_num,
gpointer data)
{
ClutterActor *actor = CLUTTER_ACTOR(data);
clutter_actor_set_rotation (actor, CLUTTER_Z_AXIS,
(gdouble) frame_num,
clutter_actor_get_width (actor) / 2,
clutter_actor_get_height (actor) / 2,
0);
}
int
main (int argc, char *argv[])
{
ClutterTimeline *timeline;
ClutterActor *stage, *actor;
GdkPixbuf *pixbuf;
clutter_init (&amp;argc, &amp;argv);
stage = clutter_stage_get_default ();
pixbuf = gdk_pixbuf_new_from_file ("an-image.png", NULL);
actor = clutter_texture_new_from_pixbuf (pixbuf);
clutter_container_add_actor (CLUTTER_CONTAINER (stage), actor);
clutter_actor_set_position (actor, 100, 100);
timeline = clutter_timeline_new_for (360, 60); /* one degree per frame */
clutter_timeline_set_loop (timeline, TRUE);
g_signal_connect (timeline, "new-frame", G_CALLBACK (on_new_frame), actor);
clutter_actor_show_all (stage);
clutter_timeline_start (timeline);
clutter_main();
return 0;
}
</programlisting>
</example>
<note><para>
Multiple timelines can be sequenced in order by means of the
#ClutterScore. See the #ClutterScore documentation for more details on
using this.
</para></note>
</section>
<section id="clutter-animation-behaviours">
<title>Behaviours</title>
<para>
With a large application containing many animations, the use of just
timelines can become unwieldy and difficult to manage with much code
duplication in the new-frame handlers that can require over complex
code changes for minor animation modifications. To ease these
problems the #ClutterAlpha and #ClutterBehaviour classes were created.
</para>
<para>
#ClutterAlpha and #ClutterBehaviour attempt to generalise the
new-frame function by defining common actions or behaviours that can
be quickly modified, applied to multiple actors or mixed on a single
actor.
</para>
<para>
A ClutterAlpha is simply a 'function of time' (not a pixel alpha channel!).
It is created by referencing a source timeline and an "easing mode" which
produces a value between -1 and 2 depending on the progress of the
timeline. Clutter provides various easing modes, as described by
the #ClutterAnimationMode enumeration. It is also possible to register
a new animation mode using clutter_alpha_register_func() or to provide
a custom #ClutterAlphaFunc for a specific #ClutterAlpha instance.
</para>
<para>
A Behaviour is created with a #ClutterAlpha and a set of limits for
whatever the behaviour modifies in an actor. The current #ClutterAlpha
value is then mapped to a value between these limits and this value
set on any applied actors. With the #ClutterAlpha's underlying
timeline playing the produced value will change and the behaviour
will animate the actor.
</para>
<para>
A #ClutterBehaviour is effectively 'driven' by a supplied #ClutterAlpha
and when then applied to an actor it will modify a visual property or
feature of the actor dependant on the Alpha's value. For example a
path based behaviour applied to an actor will alter its position
along the path dependant on the current alpha value over time. The
actual motion will depend on the chosen "easing mode".
</para>
<para>
Multiple behaviours can of course be applied to an actor as well as
a single behaviour being applied to multiple actors. The separation
of timelines, alphas and behaviours allows for a single timeline to
drive many behaviours each potentially using different alpha
functions. Behaviour parameters can also be changed on the fly.
</para>
<para>
<figure id="behaviour-path-alpha">
<title>Effects of alpha functions on a path</title>
<graphic fileref="path-alpha-func.png" format="PNG"/>
<blockquote>
The actors position between the path's end points directly correlates
to the #ClutterAlpha's current alpha value driving the behaviour. With
the #ClutterAlpha's animation mode set to %CLUTTER_LINEAR the actor
will follow the path at a constant velocity, but when changing to
%CLUTTER_EASE_SINE_IN_OUT the actor initially accelerates before quickly
decelerating.
</blockquote>
</figure>
</para>
<para>
The behaviours included in Clutter are
</para>
<para>
<variablelist>
<varlistentry>
<term>#ClutterBehaviourDepth</term>
<listitem><simpara>Changes the depth of actors</simpara></listitem>
</varlistentry>
<varlistentry>
<term>#ClutterBehaviourEllipse</term>
<listitem><simpara>Moves actors along an ellipsis</simpara></listitem>
</varlistentry>
<varlistentry>
<term>#ClutterBehaviourOpacity</term>
<listitem><simpara>Changes the opacity of actors</simpara></listitem>
</varlistentry>
<varlistentry>
<term>#ClutterBehaviourPath</term>
<listitem><simpara>Moves actors along a path</simpara></listitem>
</varlistentry>
<varlistentry>
<term>#ClutterBehaviourRotate</term>
<listitem><simpara>Rotates actors along an axis</simpara></listitem>
</varlistentry>
<varlistentry>
<term>#ClutterBehaviourScale</term>
<listitem><simpara>Changes the scaling factors of
actors</simpara></listitem>
</varlistentry>
</variablelist>
</para>
<example id="clutter-behaviour-example">
<para>
The following example demonstrates an ellipse behaviour in action.
</para>
<programlisting>
#include &lt;clutter/clutter.h&gt;
int
main (int argc, char *argv[])
{
ClutterTimeline *timeline;
ClutterBehaviour *behave;
ClutterAlpha *alpha;
ClutterActor *stage, *actor;
GdkPixbuf *pixbuf;
clutter_init (&amp;argc, &amp;argv);
stage = clutter_stage_get_default ();
pixbuf = gdk_pixbuf_new_from_file ("ohpowers.png", NULL);
actor = clutter_texture_new_from_pixbuf (pixbuf);
clutter_container_add_actor (CLUTTER_CONTAINER (stage), actor);
timeline = clutter_timeline_new_for_duration (4000); /* milliseconds */
clutter_timeline_set_loop (timeline, TRUE);
/* Set an alpha func to power the behaviour */
alpha = clutter_alpha_new_full (timeline, CLUTTER_EASE_SINE_IN_OUT);
behave = clutter_behaviour_ellipse_new (alpha,
200, /* center x */
200, /* center y */
400, /* width */
300, /* height */
CLUTTER_ROTATE_CW, /* direction */
0.0, /* initial angle */
360.0); /* final angle */
clutter_behaviour_apply (behave, actor);
clutter_actor_show_all (stage);
clutter_timeline_start (timeline);
clutter_main();
/* clean up */
g_object_unref (behave);
g_object_unref (timeline);
return 0;
}
</programlisting>
</example>
<note>Behaviour parameters can be changed whilst a animation is running</note>
<para>
There can be many #ClutterAlpha's attached to a single timeline. There
can be many behaviours for a #ClutterAlpha. There can be many behaviours
applied to an actor. A #ClutterScore can be used to chain many behaviour
together.
</para>
<warning><para>Combining behaviours that effect the same actor properties
(i.e two separate paths) will cause unexpected results. The values
will not be merged in any way with only the last applied behaviour taking
precedence.</para></warning>
<para>
Tips for implementing a new behaviour can be found <link
linkend="creating-your-own-behaviours">here</link>.
</para>
</section>
<section id="clutter-animation-implicit">
<title>Implicit Animations</title>
<para>Using behaviours for simple animations of a single actor may
be too complicated, in terms of memory management and bookkeeping
of the object instances. For this reason, Clutter also provides a
simple animation API for implicit animations using properties of
an actor: clutter_actor_animate().</para>
<para>The clutter_actor_animate() family of functions will create
and use an implicit #ClutterAnimation instance, which will then
handle the animation of one or more #ClutterActor properties between
a range of values.</para>
<example id="clutter-actor-animate-example">
<para>
The following example demonstrates how to use the
clutter_actor_animate() method to tween an actor
between the current position and a new set of coordinates.
The animation takes 200 milliseconds to complete and
uses a linear speed.
</para>
<programlisting>
clutter_actor_animate (actor, CLUTTER_LINEAR, 200
"x", 200,
"y", 200,
NULL);
</programlisting>
</example>
<para>The clutter_actor_animate() method returns a #ClutterAnimation
instance that can be used to start, stop and modify the animation
while it's running. The #ClutterAnimation::completed signal will
be emitted when the animation has been completed.</para>
<warning><para>When the animation is complete it will be automatically
unreferenced, and disposed if nothing else is holding a reference
on it.</para></warning>
<para>Calling clutter_actor_animate() multiple times on an
actor which is being animated will cause the animation to be updated
with the new values.</para>
<example id="clutter-actor-animate-multi-example">
<para>
The following example demonstrates how to animate an actor
inside the signal handler for a button press event. If the
user presses the button on a new position while the animation
is running, the animation will be restarted with the new
final values updated.
</para>
<programlisting>
static gboolean
on_button_press (ClutterActor *actor,
ClutterButtonEvent *event,
gpointer user_data)
{
clutter_actor_animate (actor, CLUTTER_EASE_SINE_OUT, 500,
"x", event-&gt;x,
"y", event-&gt;y,
NULL);
return TRUE;
}
</programlisting>
</example>
</section>
<section id="clutter-animation-conclusion">
<title>Conclusion</title>
<para>
Clutter provides a number of utility classes to aid animations and
complex animations can be produced by combining the various features
provided.
</para>
<para>
Of course animations can becreated outside of the Clutter animation
framework, as the framework is not expected to cover every kind of
possible animation scenario.
</para>
<para>
The animation functionality in Clutter is primarily suited to
building animations with a set or finite running time - i.e transitions
and the like. For animations involving variable input (such as touchscreen
handling) physical simulations may be more suited.
</para>
</section>
</chapter>