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cookbook: Add recipe for animated scaling of an actor

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Recipe explains how to animate scaling a single actor.

Also covers scaling vs. resizing, scale center, and
scaling within layouts and containers.

The first example shows how animations around each scale
gravity look, as well as tracking the transformed position
and size of the actor and displaying those.

The second example is a simple image viewer with zoom in/out
using scaling.
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Elliot Smith 2010-11-24 12:44:35 +00:00
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commit c9d0f8b26e
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doc/cookbook/animations.xml
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@ -2709,7 +2709,20 @@ timeline_completed_cb (ClutterTimeline *timeline,
<section>
<title>Problem</title>
<para>You want to animate scaling of an actor.</para>
<para>You want to animate scaling of an actor. Example use
cases:</para>
<itemizedlist>
<listitem>
<para>To animate zooming in/out of a texture in an
image viewer application.</para>
</listitem>
<listitem>
<para>To add an animated "bounce" effect (quick scale up
followed by scale down) to a UI element
to indicate it has received focus.</para>
</listitem>
</itemizedlist>
</section>
<section>
@ -2717,18 +2730,19 @@ timeline_completed_cb (ClutterTimeline *timeline,
<para>Animate the actor's <varname>scale-x</varname> and
<varname>scale-y</varname> properties to change the scaling on
the x and y axes respectively.</para>
the <varname>x</varname> and <varname>y</varname> axes
respectively.</para>
<para>For example, to animate an actor to twice its initial scale
<para>For example, to animate an actor to twice its current scale
with implicit animations:</para>
<informalexample>
<programlisting>
<![CDATA[
/* get the actor's current scale */
gdouble scale_x;
gdouble scale_y;
/* get the actor's current scale */
clutter_actor_get_scale (actor, &scale_x, &scale_y);
/* animate to twice current scale on both axes */
@ -2742,79 +2756,263 @@ clutter_actor_animate (actor, CLUTTER_LINEAR, 1000,
<para>Alternatively, <type>ClutterAnimator</type> or
<type>ClutterState</type> can be used to animate an actor's scale
properties. See <link linkend="animations-scaling-example-1">this
example</link> for details.</para>
example</link> which uses <type>ClutterState</type> to animate
scaling.</para>
</section>
<section>
<title>Discussion</title>
<!--
<para>The scale value is a double. Values less than 1.0 will reduce the apparent size of the actor; values greater than 1.0 will increase the apparent size.</para>
<para>Scaling an actor is done through its <varname>scale-x</varname>
and <varname>scale-y</varname> properties, each of which takes
a <code>double</code> value. A value of less than
<code>1.0</code> for an axis scales an actor down on that axis,
reducing its apparent size; values greater than <code>1.0</code>
scale an actor up, increasing its apparent size.</para>
When you scale an actor, you aren't changing the actor's real size: you are applying a transform which changes its <emphasis>apparent</emphasis> size. Changing the scale will also transform the actor's position (i.e. it will appear to be at a different position within its container, although it will actually report its original position if you call clutter_actor_get_position(), clutter_actor_get_x() or clutter_actor_get_y()).
<para>Why "apparent" size? Because scaling applies a transform
to an actor which changes how it appears on the
stage, without changing its "real" size. Similarly, scaling an
actor may transform its position: it could appear to move to a
different position within its container,
although it is "really" at its original position. Run
<link linkend="animations-scaling-example-1">the
example</link> to see how size and position are
transformed by scaling.</para>
<para>It can be useful to know an actor's
<emphasis>transformed</emphasis> position and size after scaling:
for example, if you were implementing a reflowing layout manager
which used scaling as part of its allocation algorithm.
Here's an example of how to get these properties for an
actor:</para>
You can get the transformed (apparent) position and size for an actor with <function>clutter_actor_get_transformed_position()</function> and <function>clutter_actor_get_transformed_size()</function> respectively.
<informalexample>
<programlisting>
<![CDATA[
gfloat transformed_x;
gfloat transformed_y;
Because an actor is at a different apparent size when scaled, mouse clicks need translating into actor coordinates before you can use them.
gfloat transformed_width;
gfloat transformed_height;
clutter_actor_get_transformed_position (actor, &transformed_x, &transformed_y);
clutter_actor_get_transformed_size (actor, &transformed_width, &transformed_height);
]]>
</programlisting>
</informalexample>
<para>Note that you can scale an actor on both axes by the same
amount (uniform scaling), or by a different amount on each axis
(differential scaling).</para>
You can scale on both axes by the same amount (uniform scaling), or by a different amount on each axis (differential scaling).
<para>Use <function>clutter_actor_is_scaled()</function> to determine
whether scaling has been applied to an actor: this function returns
<code>FALSE</code> if both <varname>scale-x</varname> and
<varname>scale-y</varname> are <code>1.0</code>; otherwise, it
returns <code>TRUE</code>.</para>
clutter_actor_is_scaled() tells you whether scaling has been applied to the actor: it returns FALSE if both scale-x and scale-y are 1.0.
<section>
<title>Scaling vs. resizing</title>
<para>Scaling changes the <emphasis>apparent</emphasis> size
of an actor, while leaving its real size unchanged. By contrast,
resizing changes the <emphasis>real</emphasis> size of the actor,
by modifying its <varname>width</varname> and
<varname>height</varname> properties.</para>
<para>Resizing and scaling produce the same visual
effect, as both make an actor appear to be larger or
smaller. Therefore, for most purposes, they are interchangeable
if you just want to change an actor's apparent size.</para>
<para>So why would you scale an actor rather than resize it?</para>
Scaling a container scales all actors inside the container.
<itemizedlist>
<listitem>
<para>If you've scaled an actor, you can easily reset it
to its original size, by setting its
scale back to <code>1.0</code> on both axes. By contrast,
to reset a resized actor to its original size,
you would have to track the original size manually: the
actor doesn't make its original size accessible.</para>
</listitem>
-->
<listitem>
<para>Scaling can easily change the apparent size
of multiple actors inside a container. For example, say you
wanted to shrink multiple actors inside a container
to half their original size. There are two ways you
could do this:</para>
<orderedlist>
<listitem>
<para>The hard way would be to resize
each actor individually. You couldn't just resize the container,
as resizing a container doesn't resize its children: usually
they will be clipped so that they are either partially or
wholly hidden.</para>
</listitem>
<listitem>
<para>The easy way would be to set the container's scale
to half its initial value: the actors
in the container would retain their original sizes, but would
appear at half size.</para>
</listitem>
</orderedlist>
</listitem>
</itemizedlist>
</section>
<section>
<title>Scaling, layouts and containers</title>
<para>It is possible to scale actors inside containers. For
example, if you were using a <type>ClutterBox</type>
which has a <type>ClutterBoxLayout</type> layout manager,
you could scale the children of that layout.</para>
<para>However, you should remain aware that layout managers
don't take account of the scale of their children, only their
size. So if you scale up an actor inside a layout manager,
it may overlap other actors in the layout: the size allocated
by the layout manager doesn't increase as an actor's scale
increases.</para>
<para>Similarly, scaling an actor down doesn't reduce the space
it will be allocated by a layout.</para>
</section>
<section>
<title>Setting the scale center</title>
<!--
You can change the center of the scaling, using either gravity or actor-relative coordinates. Note that setting the scale gravity on an actor sets the scale-center-x and scale-center-y values behind the scenes.
<para>An actor's scale center is the point around which
scaling occurs: when you scale the actor, it will "shrink"
into (if scale &lt; 1.0) or "expand" out of (if scale &gt; 1.0)
its scale center.</para>
<para>You can change an actor's scale center using
either gravity (a named position on the actor; for example, the
middle of the top edge of the actor is
<constant>CLUTTER_GRAVITY_NORTH</constant>); or
x,y coordinates relative to the actor's anchor point (by default,
the anchor point for an actor is at <code>0,0</code>).</para>
<para>Setting scale gravity has the same consequences as
setting both the <varname>scale-center-x</varname> and
<varname>scale-center-y</varname> properties for an actor.
For example, <constant>CLUTTER_GRAVITY_NORTH_EAST</constant>
sets the scale center to <code>&lt;width of the actor&gt;, 0</code>,
relative to the actor's anchor point (defaults to the top-right
corner of the actor). However, the advantage of scale
gravities is that they change with the actor: so if the
actor is resized, you don't have to manually reset the scale
center. This means that <constant>CLUTTER_GRAVITY_NORTH_EAST</constant>
will always represent the top-right corner of the actor,
regardless of how it is scaled or resized. The same is true
of each of the other scale gravities.</para>
<para>If you're animating an actor's scale but want a different
scale center, set it before the animation begins. One way to
do this is to leave the actor's scale unchanged, but with
a different scale center:</para>
When you scale the actor, it will "shrink" into (if scale < 1.0) or "expand" out of (if scale > 1.0) the center.
<informalexample>
<programlisting>
<![CDATA[
gdouble scale_x;
gdouble scale_y;
You can't really change the scaling center as part of the animation: you should do it before scaling an actor with an animation.
/* get the actor's current scale */
clutter_actor_get_scale (actor, &scale_x, &scale_y);
Once you've scaled an actor, it's not a great idea to change the scale center: if you do, it probably won't do what you expect.
/* set scale center using x,y coordinates, leaving scale unchanged;
* the actor's size here is assumed to be 200x200
*/
clutter_actor_set_scale_full (actor,
scale_x,
scale_y,
100.0, /* center x */
100.0 /* center y */);
/* set scale center using gravity, leaving scale unchanged */
clutter_actor_set_scale_with_gravity (actor,
scale_x,
scale_y,
CLUTTER_GRAVITY_CENTER);
]]>
</programlisting>
</informalexample>
<para>Another approach is to set scale center properties
via GObject, which doesn't require you to figure out the
actor's scale first:</para>
animations-scaling.c shows all the scale gravities
<informalexample>
<programlisting>
<![CDATA[
/* set scale center using x,y coordinates */
g_object_set (actor,
"scale-center-x", 100.0, /* center x */
"scale-center-y", 100.0, /* center y */
NULL);
/* set scale center using gravity */
g_object_set (actor,
"scale-gravity", CLUTTER_GRAVITY_CENTER,
NULL);
]]>
</programlisting>
</informalexample>
<para>Once the scale center is set, you can animate the
scaling as per usual.</para>
<para>It is even possible to animate the
<varname>scale-center-*</varname> properties, which can
produce interesting, though slightly
unpredictable, effects. It's usually better to change the
scale center before the animation starts.</para>
What if you set scale centers and scale gravity? which gets precedence - I think scale center
<para><link linkend="animations-scaling-example-1">The
example</link> cycles through the available scale gravities,
showing the effect on the animation of each of the scale
centers.</para>
<para>The <link linkend="animations-scaling-example-2">second
example</link> shows how to combine scaling in and out on a
texture, in response to mouse button presses. In this case,
the scale gravity remains at <constant>CLUTTER_GRAVITY_NORTH_WEST</constant>
(i.e. at the anchor point of the actor). However, the anchor
point is moved to the coordinates of each double click on button 1
(usually the left mouse button) or button 3 (usually the right
mouse button); which in turn automatically moves the scale center
before the texture is scaled. As a result, the texture
"expands" or "contracts" around the clicked point,
while the point remains still.</para>
<warning>
<para>One final caveat about scale centers: if an actor is
already scaled, the scale center coordinates are relative to
the <emphasis>real size</emphasis> of the actor, rather than
its <emphasis>transformed</emphasis> size. This can result in
a "jumping" effect if you change the scale center on
a scaled actor.</para>
<para>For example, you might set the scale gravity of an actor
to <constant>CLUTTER_GRAVITY_WEST</constant>, then
scale the actor to <code>0.5</code> on both axes. Later, you
change the actor's scale gravity to
<constant>CLUTTER_GRAVITY_EAST</constant>. The effect of this
is to "jump" the actor to the right, so its right-hand edge
is aligned with where it was at scale <code>1.0</code>.</para>
For example, you have a square actor size 200x200 at x=100, y=100.
You scale it to half scale, setting the scale center to x=100, y=0 (middle of the top of the square)
(the scale center is relative to the actor)
The center of the top of the square stays where it is
The part of the line either side of the center "shrinks" in towards the center of the line
The top of the square stays still; the rest of the square "shrinks" up towards the top
The square now appears at half its original size, and with an _transformed_ position of x=150, y=0
Note that the scale center is relative to the actor's actual size, not its transformed size.
-->
<para>If this isn't desirable, you can just retain the scale
center on a scaled actor, and only change it when the actor
is unscaled.</para>
</warning>
</section>
</section>
@ -2828,6 +3026,17 @@ Note that the scale center is relative to the actor's actual size, not its trans
<programlisting>
<xi:include href="examples/animations-scaling.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="animations-scaling-example-2">
<title>Animated scaling (up and down) of a texture in response
to button presses. Call with the path to an image as the
first argument.</title>
<programlisting>
<xi:include href="examples/animations-scaling-zoom.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>