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<chapter id="actors">
<title>Actors</title>
<epigraph>
<attribution>Edmon Gween, actor, on his deathbed</attribution>
<para>An actor's a guy who if you ain't talkin' about him, ain't
listening.</para>
</epigraph>
<section id="actors-introduction">
<title>Introduction</title>
<para>When building a User Interface with Clutter, the visible part
of the UI — that is, what is displayed on the screen — is
commonly referred to as "the scene graph". Like every graph, a scene
graph is composed by nodes.</para>
<para>Every node on the Clutter scene graph is an
<emphasis>actor</emphasis>. Every actor has a single relationship
with the others: it can be the parent of another actor, or a child of
another actor.</para>
<note><para>The stage is an actor that can have children but cannot have
any parent.</para></note>
<para>Actors have different attributes: a position, a size, a
scale factor, a rotation angle on each axis (relative to a specific
center on the normal plane for that axis), an opacity factor.</para>
<para>The scene graph is not fixed: it can be changed, not only
by adding or removing actors, but also by changing the parent-child
relationship: it is possible, for instance, to move an entire
section of the scene graph from one parent actor to another.</para>
</section>
<section id="actors-allocation-notify">
<title>Knowing when an actor's position or size changes</title>
<section>
<title>Problem</title>
<para>You want to know when the position or the size, or
both, of an actor change, for instance to update an unrelated
actor or some internal state.</para>
</section>
<section>
<title>Solution</title>
<para>You can use the <emphasis>notify</emphasis> signal,
detailed with the coordinate or the dimension you want
to know has changed:</para>
<informalexample>
<programlisting>
g_signal_connect (actor, "notify::x",
G_CALLBACK (on_x_changed),
NULL);
g_signal_connect (actor, "notify::height",
G_CALLBACK (on_height_changed),
NULL);
g_signal_connect (actor, "notify::depth",
G_CALLBACK (on_depth_changed),
NULL);
</programlisting>
</informalexample>
<para>If you want to know if any of the coordinates or dimensions of
an actor have been changed, except for depth, you can use the
<emphasis>allocation-changed</emphasis> signal:</para>
<informalexample>
<programlisting>
g_signal_connect (actor, "allocation-changed",
G_CALLBACK (on_allocation_changed),
NULL);
</programlisting>
</informalexample>
<para>The signature for the handler of the "notify" signal is:</para>
<informalexample>
<programlisting>
void
on_notify (GObject *gobject,
GParamSpec *pspec,
gpointer user_data);
</programlisting>
</informalexample>
<para>While the signature for the handler of the "allocation-changed"
signal is:</para>
<informalexample>
<programlisting>
void
on_allocation_changed (ClutterActor *actor,
const ClutterActorBox *allocation,
ClutterAllocationFlags flags,
gpointer user_data);
</programlisting>
</informalexample>
</section>
<section>
<title>Discussion</title>
<para>Any change the position and size of an actor will cause a
change in the allocation of the actor itself. This will update the
values of the <property>x</property>, <property>y</property>,
<property>width</property> and <property>height</property>
properties as well.</para>
<para>The first technique allows a greater deal of granularity,
allowing you to know what exactly changed. Inside the callback
for the signal you can query the value of the property:</para>
<informalexample>
<programlisting>
void
on_x_changed (GObject *gobject,
GParamSpec *pspec,
gpointer user_data)
{
gint x_value = 0;
/* Round the X coordinate to the nearest pixel */
x_value = floorf (clutter_actor_get_x (CLUTTER_ACTOR (gobject))) + 0.5;
g_print ("The new X coordinate is '%d' pixels\n", x_value);
}
</programlisting>
</informalexample>
<para>The second technique is more indicated if you want to
get notification that any of the positional or dimensional
attributes changed, except for the depth:</para>
<informalexample>
<programlisting>
void
on_allocation_changed (ClutterActor *actor,
const ClutterActorBox *allocation,
ClutterAllocationFlags flags,
gpointer user_data)
{
g_print ("The bounding box is now: (%.2f, %.2f) (%.2f x %.2f)\n",
clutter_actor_box_get_x (allocation),
clutter_actor_box_get_y (allocation),
clutter_actor_box_get_width (allocation),
clutter_actor_box_get_height (allocation));
}
</programlisting>
</informalexample>
<para>All actors will update these properties when their size
or position change.</para>
<para>Note that the stage, on the other hand, will not notify on
position changes, so it is not possible to use the
<property>x</property> and <property>y</property>
properties to know that the platform-specific window embedding the
stage has been moved — if the platform supports a windowing
system. In order to achieve that you will have to use backend-specific
API to extract the surface used by the stage and then platform-specific
API to retrieve its coordinates.</para>
</section>
</section>
<section id="actors-paint-wrappers">
<title>Overriding the paint sequence</title>
<section>
<title>Problem</title>
<para>You want to override the way an actor paints itself
without creating a subclass.</para>
</section>
<section>
<title>Solution</title>
<para>You can use the <emphasis>paint</emphasis> signal to
invoke a callback that will be executed before the actor's
paint implementation:</para>
<informalexample>
<programlisting>
g_signal_connect (actor, "paint", G_CALLBACK (on_paint), NULL);
</programlisting>
</informalexample>
<para>You can paint something after the actor's paint implementation
by using the <function>g_signal_connect_after()</function> function
instead of <function>g_signal_connect()</function>:</para>
<informalexample>
<programlisting>
g_signal_connect_after (actor, "paint", G_CALLBACK (on_paint_after), NULL);
</programlisting>
</informalexample>
<para>The signature for the handler of the "paint" signal is:</para>
<informalexample>
<programlisting>
void on_paint (ClutterActor *actor, gpointer user_data);
</programlisting>
</informalexample>
</section>
<section>
<title>Discussion</title>
<para>The paint cycle in Clutter works its way recursively from the
stage through every child.</para>
<para>Whenever an Actor is going to be painted it will be positioned in
a new frame of reference according to the list of transformations
(scaling, rotation and additional translations). After that, the "paint"
signal will be emitted.</para>
<para>The "paint" signal is defined as <emphasis>run-last</emphasis>,
that is the signal handlers connected to it using
<function>g_signal_connetc()</function> will be called first; then the
default handler defined by the Actor's sub-class will be called;
finally, all the signal handlers connected to the signal using
<function>g_signal_connect_after()</function> will be called.</para>
<para>This allows pre- and post-default paint handlers, and it also
allows completely overriding the way an Actor draws itself by default;
for instance:</para>
<informalexample>
<programlisting>
void
on_paint (ClutterActor *actor)
{
do_my_paint (actor);
g_signal_stop_emission_by_name (actor, "paint");
}
</programlisting>
</informalexample>
<para>The code above will prevent the default paint implementation of
the actor from running.</para>
</section>
</section>
<section id="actors-opacity">
<title>Making an actor transparent by changing its opacity</title>
<section>
<title>Problem</title>
<para>You want an actor to be transparent so that other
actors are visible through it.</para>
</section>
<section>
<title>Solution</title>
<para>Change the actor's <emphasis>opacity</emphasis> so that
it is partially (or even fully) transparent:</para>
<informalexample>
<programlisting>
/* 25% transparency */
clutter_actor_set_opacity (actor, 191.25);
/* 50% transparency */
clutter_actor_set_opacity (actor, 122.5);
/* completely transparent */
clutter_actor_set_opacity (actor, 0);
</programlisting>
</informalexample>
<para>Any actor covered or overlapped by the transparent actor
should be visible through it; the Discussion section gives
some examples of how visible you can expect the covered or
overlapped actor to be.</para>
</section>
<section>
<title>Discussion</title>
<para>Opacity is a property of every <type>ClutterActor</type>.
It is a float on a scale from 0 (invisible) to 255 (completely
opaque). Actors with <code>0 < opacity < 255</code> will
have a varying amount of solidity on the stage, so other actors
may be visible through them.</para>
<para>For example, below are 4 yellow rectangles overlapping
a white rectangle on a blue stage:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/actors-opacity.png" />
</imageobject>
<alt>
<para>The effect of different opacities levels on
an actor's appearance</para>
</alt>
</mediaobject>
</screenshot>
<para>The rectangles have the following opacities:</para>
<itemizedlist>
<listitem>
<para>top-left: <code>255</code> (0% transparency)</para>
</listitem>
<listitem>
<para>top-right: <code>191.25</code> (25% transparency)</para>
</listitem>
<listitem>
<para>bottom-right: <code>122.5</code> (50% transparency)</para>
</listitem>
<listitem>
<para>bottom-left: <code>61.25</code> (75% transparency)</para>
</listitem>
</itemizedlist>
<para>Notice how both the stage and the white rectangle are
visible through the yellow rectangles.</para>
<para>As opacity is a property of every actor, it can
be animated like any other GObject property, using any of
the approaches in the animation API.</para>
<para>The following sections cover some other considerations
when working with actor opacity.</para>
<section>
<title>Container and color opacity</title>
<para>If a container has its opacity set, any children of the
container have their opacity combined with their parent's opacity.
For example, if a parent has an opacity of <code>122.5</code>
(50% transparent) and the child also has an opacity of
<code>122.5</code>, the child's <emphasis>effective</emphasis>
opacity is 25% (<code>opacity = 61.25</code>, and it is
75% transparent).</para>
<para>To demonstrate the visual effect of this, here are
three rectangles with the same color but different opacity settings,
inside parents which also have different opacity settings:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/actors-opacity-container-affects-opacity.png" />
</imageobject>
<alt>
<para>How a container's opacity affects the opacity of
its children</para>
</alt>
</mediaobject>
</screenshot>
<itemizedlist>
<listitem>
<para>The left-hand rectangle has <code>opacity = 255</code>
and is in a <type>ClutterGroup</type> with
<code>opacity = 255</code>. This means it is fully opaque.</para>
</listitem>
<listitem>
<para>The middle rectangle has <code>opacity = 255</code>
and is in a <type>ClutterGroup</type> with
<code>opacity = 122.5</code>. Notice that the parent opacity
makes the rectangle appear darker, as the stage colour is showing
through from behind.</para>
</listitem>
<listitem>
<para>The right-hand rectangle has <code>opacity = 122.5</code>
and is in a <type>ClutterGroup</type> with
<code>opacity = 122.5</code>. Notice that the rectangle appears
to be even darker, as the stage colour is showing
through both the rectangle and its parent.</para>
</listitem>
</itemizedlist>
<para>Similarly, <type>ClutterColor</type> also contains an
<varname>alpha</varname> property which governs the transparency
of the color. Where an actor can have a color set (e.g.
<type>ClutterRectangle</type>) the alpha value of the color also
affects the transparency of the actor, for example:</para>
<informalexample>
<programlisting>
<![CDATA[
/* color with 50% transparency */
ClutterColor half_transparent_color = { 255, 0, 0, 122.5 };
ClutterRectangle *actor = clutter_rectangle_new ();
/* set actor's transparency to 50% */
clutter_actor_set_opacity (actor, 122.5);
/* rectangle will be 25% opaque/75% transparent */
clutter_rectangle_set_color (CLUTTER_RECTANGLE (actor),
&half_transparent_color);
]]>
</programlisting>
</informalexample>
</section>
<section>
<title>Depth and depth order</title>
<para>Each actor has two more aspects which affect its
apparent opacity:</para>
<itemizedlist>
<listitem>
<para>An actor's <emphasis>depth</emphasis> can have an
effect if the stage has fog (a depth cueing effect) turned on.
As an actor's depth increases, the actor apparently "recedes" from
view and gradually blends into the colour of the stage. This
produces an effect similar to making the actor transparent.
See the <type>ClutterStage</type> documentation for
more details about fog.</para>
<para>Depth also needs to be considered if you want
one actor to be visible through another: the actor you want
to see through a transparent actor must be "deeper" than (or at
the same depth as) the transparent actor.</para>
</listitem>
<listitem>
<para>The <emphasis>depth order</emphasis> governs how
actors within a <type>ClutterContainer</type> implementation
are placed with respect to each other.</para>
<note>
<para>Depth ordering is not the same thing as depth: depth
ordering records relationships between actors at the same
depth.</para>
</note>
<para>If you have two overlapping actors <code>actorA</code> and
<code>actorB</code> in a container, and you want <code>actorA</code>
(opaque) to be visible through <code>actorB</code> (transparent),
you should ensure that <code>actorB</code> is "above" <code>actorA</code>
in the depth ordering. You could do this as follows:</para>
<informalexample>
<programlisting>
/*
* raise actorB so it is above actorA in the depth order;
* NB actorA and actorB both need to be in the same container
* for this to work
*/
clutter_actor_raise (actorB, actorA);
</programlisting>
</informalexample>
<para><function>clutter_actor_raise()</function>,
<function>clutter_actor_lower()</function> and related
<type>ClutterActor</type> functions set
depth ordering on actors; see also <type>ClutterContainer</type>'s
<function>clutter_container_raise_child()</function> and
<function>clutter_container_lower_child()</function>
functions.</para>
</listitem>
</itemizedlist>
</section>
</section>
</section>
<section id="actors-non-rectangular">
<title>Creating an actor with a non-rectangular shape</title>
<section>
<title>Problem</title>
<para>You want to create a <type>ClutterActor</type> subclass,
but don't want it to be rectangular; for example, you want a
star-shaped actor.</para>
</section>
<section>
<title>Solution</title>
<para>Use Cogl primitives to draw the actor.</para>
<para>Below is an example of the pick and paint implementations for a
star-shaped <type>StarActor</type> class (an extension of
<type>ClutterActor</type>).</para>
<para>Like <type>ClutterRectangle</type>, it has a private
struct internally, which contains a <type>ClutterColor</type>
denoting the color it should be painted. This is used to set the Cogl
source color.</para>
<informalexample>
<programlisting>
<![CDATA[
static void
star_actor_paint (ClutterActor *actor)
{
ClutterActorBox allocation = { 0, };
gfloat width, height;
guint tmp_alpha;
/* priv is a private internal struct */
ClutterColor color = STAR_ACTOR (actor)->priv->color;
clutter_actor_get_allocation_box (actor, &allocation);
clutter_actor_box_get_size (&allocation, &width, &height);
tmp_alpha = clutter_actor_get_paint_opacity (actor)
* color.alpha
/ 255;
cogl_path_new ();
cogl_set_source_color4ub (color.red,
color.green,
color.blue,
tmp_alpha);
/* create and store a path describing a star */
cogl_path_move_to (width * 0.5, 0);
cogl_path_line_to (width, height * 0.75);
cogl_path_line_to (0, height * 0.75);
cogl_path_move_to (width * 0.5, height);
cogl_path_line_to (0, height * 0.25);
cogl_path_line_to (width, height * 0.25);
cogl_path_line_to (width * 0.5, height);
cogl_path_fill ();
}
static void
star_actor_pick (ClutterActor *actor,
const ClutterColor *pick_color)
{
if (!clutter_actor_should_pick_paint (actor))
return;
ClutterActorBox allocation = { 0, };
gfloat width, height;
clutter_actor_get_allocation_box (actor, &allocation);
clutter_actor_box_get_size (&allocation, &width, &height);
cogl_path_new ();
cogl_set_source_color4ub (pick_color->red,
pick_color->green,
pick_color->blue,
pick_color->alpha);
/* create and store a path describing a star */
cogl_path_move_to (width * 0.5, 0);
cogl_path_line_to (width, height * 0.75);
cogl_path_line_to (0, height * 0.75);
cogl_path_move_to (width * 0.5, height);
cogl_path_line_to (0, height * 0.25);
cogl_path_line_to (width, height * 0.25);
cogl_path_line_to (width * 0.5, height);
cogl_path_fill ();
}
]]>
</programlisting>
</informalexample>
<para>If you need more information about how to implement your own
<type>ClutterActor</type>, see the Clutter reference
manual.</para>
<para>Note that the code in these two functions is virtually identical:
the Discussion section suggests how to remove this redundancy.</para>
</section>
<section>
<title>Discussion</title>
<para>The above is one approach to creating a non-rectangular
actor. But it's also possible to get a similar effect by
subclassing an existing actor (like <type>ClutterRectangle</type>)
and giving it a non-rectangular appearance. You could do this by
making the underlying rectangle transparent and then drawing on
top of it (e.g. using Cairo or Cogl).</para>
<para>However, if you then made such an actor reactive, events
like mouse button presses would be triggered from anywhere on
the underlying rectangle. This is true even if the visible part
of the actor only partially fills the rectangle (underneath, it's
still a rectangle).</para>
<para>The advantage of using Cogl paths is that the reactive area
of the actor is defined by the Cogl path. So if you have a
star-shaped actor, only clicks (or other events) directly on the
star will have any effect on it.</para>
<section>
<title>Cogl path coordinates</title>
<para>In the example shown, <function>cogl_path_move_to()</function>
and <function>cogl_path_line_to()</function> are used. These
take absolute <code>x</code> and <code>y</code> coordinates as
arguments, relative to the GL 'modelview' transform matrix; in
the case of an actor's <function>paint</function> implementation,
relative to the bounding box for the actor. So if an actor has
width and height of 50 pixels, and you used
<function>cogl_move_to (25, 25)</function> in its
<function>paint</function> implementation, the "pen"
moves to the centre of the actor, regardless of where the actor
is positioned on the stage. Similarly, using
<function>cogl_path_line_to()</function> creates a line segment
from the current pen position to the absolute coordinates
(<code>x</code>, <code>y</code>) specified.</para>
<para>The Cogl API also provides various "rel" variants of the path
functions (e.g. <function>cogl_path_rel_line_to()</function>), which
create path segments relative to the current pen position (i.e.
<code>pen_x + x</code>, <code>pen_y + y</code>).</para>
<para>It's important to note that the path isn't drawn until you
call <function>cogl_path_stroke()</function> (to draw the path segments)
or <function>cogl_path_fill()</function> (to fill the area enclosed by
the path). The path is cleared once it's been drawn.
Using the <function>*_preserve</function> variants of these functions draws
the path and retains it (so it could be drawn again).</para>
</section>
<section>
<title>Other Cogl primitives</title>
<para>Note that the Cogl primitives API provides other types of path
segment beyond straight lines that we didn't use here, including:</para>
<itemizedlist>
<listitem>
<para>Bezier curves (<function>cogl_path_curve_to()</function>)</para>
</listitem>
<listitem>
<para>Arcs (<function>cogl_path_arc()</function>)</para>
</listitem>
<listitem>
<para>Polygons (<function>cogl_path_polygon()</function>)</para>
</listitem>
<listitem>
<para>Rectangles (<function>cogl_path_rectangle()</function>)</para>
</listitem>
<listitem>
<para>Rectangles with rounded corners
(<function>cogl_path_round_rectangle()</function>)</para>
</listitem>
<listitem>
<para>Ellipses (<function>cogl_path_ellipse()</function>)</para>
</listitem>
</itemizedlist>
<para>If you need more flexibility than is available in the Cogl path
API, you can make direct use of the <type>CoglVertexBuffer</type>
API instead. This is a lower-level API, but could potentially
be used to draw more complex shapes.</para>
</section>
<section>
<title>Using <type>ClutterPath</type> to store the path</title>
<para>The disadvantage of the code above is that the paths are stored in two
places: once for <function>pick</function>, and once for
<function>paint</function>. It would make sense to store the
path in one place and reference it from both of these functions to
prevent duplication.</para>
<para>Clutter provides a <type>ClutterPath</type> API for storing
generic path descriptions. It can be used to describe paths
which translate to Cogl or Cairo paths, and can also be used to
describe animation paths.</para>
<para>We can use a <type>ClutterPath</type> instance stored
inside the actor to define the path for <function>pick</function> and
<function>paint</function>; then, inside those functions, we
translate the <type>ClutterPath</type> into Cogl path function calls
(NB <type>ClutterPath</type> is effectively a declarative method
for defining a path, while the Cogl path API is imperative).</para>
<para>First we add a <varname>path</varname> member to the private
struct for the <type>StarActor</type> class (using standard
GObject mechanisms). The <function>init</function> implementation for
<type>StarActor</type> creates an empty path:</para>
<informalexample>
<programlisting>
static void
star_actor_init (StarActor *self)
{
self->priv = STAR_ACTOR_GET_PRIVATE (self);
self->priv->path = clutter_path_new ();
clutter_actor_set_reactive (CLUTTER_ACTOR (self), TRUE);
}
</programlisting>
</informalexample>
<para>One consideration is that the path coordinates need to
fit inside the actor's bounding box. So as the actor's allocation
changes, <varname>path</varname> also needs to change. We can do this
by implementing <function>allocate</function> for the
<type>StarActor</type> class:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
star_actor_allocate (ClutterActor *actor,
const ClutterActorBox *box,
ClutterAllocationFlags flags)
{
ClutterPath *path = STAR_ACTOR (actor)->priv->path;
gfloat width, height;
clutter_actor_box_get_size (box, &width, &height);
/* create and store a path describing a star */
clutter_path_clear (path);
clutter_path_add_move_to (path, width * 0.5, 0);
clutter_path_add_line_to (path, width, height * 0.75);
clutter_path_add_line_to (path, 0, height * 0.75);
clutter_path_add_move_to (path, width * 0.5, height);
clutter_path_add_line_to (path, 0, height * 0.25);
clutter_path_add_line_to (path, width, height * 0.25);
clutter_path_add_line_to (path, width * 0.5, height);
CLUTTER_ACTOR_CLASS (star_actor_parent_class)->allocate (actor, box, flags);
}
]]>
</programlisting>
</informalexample>
<para>This clears then adds segments to the
<type>ClutterPath</type> stored with the
<type>StarActor</type> instance. The positioning and
lengths of the segments are relative to the size of the actor when
its allocation changes.</para>
<para>The <function>pick</function> and <function>paint</function>
functions now reference the <type>ClutterPath</type> (only the
<function>pick</function> is shown below); and
to turn the path into drawing operations, we implement a
<function>star_actor_convert_clutter_path_node()</function> function
which takes a <type>ClutterPathNode</type> and converts it
into its Cogl equivalent:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
star_actor_convert_clutter_path_node (const ClutterPathNode *node,
gpointer data)
{
g_return_if_fail (node != NULL);
ClutterKnot knot;
switch (node->type)
{
case CLUTTER_PATH_MOVE_TO:
knot = node->points[0];
cogl_path_move_to (knot.x, knot.y);
break;
case CLUTTER_PATH_LINE_TO:
knot = node->points[0];
cogl_path_line_to (knot.x, knot.y);
break;
default:
break;
}
}
static void
star_actor_pick (ClutterActor *actor,
const ClutterColor *pick_color)
{
if (!clutter_actor_should_pick_paint (actor))
return;
ClutterActorBox allocation = { 0, };
gfloat width, height;
ClutterPath *path = STAR_ACTOR (actor)->priv->path;
clutter_actor_get_allocation_box (actor, &allocation);
clutter_actor_box_get_size (&allocation, &width, &height);
cogl_path_new ();
cogl_set_source_color4ub (pick_color->red,
pick_color->green,
pick_color->blue,
pick_color->alpha);
clutter_path_foreach (path, star_actor_convert_clutter_path_node, NULL);
cogl_path_fill ();
}
]]>
</programlisting>
</informalexample>
<note>
<para>The conversion function only covers
<type>ClutterPathNode</type> types encountered in this
actor.</para>
</note>
<para>Instead of converting to Cogl path operations, another alternative
would be to use the <function>clutter_path_to_cairo_path()</function>
function to write directly from the <type>ClutterPath</type>
onto a Cairo context.</para>
</section>
</section>
</section>
</chapter>