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docs: Add recipe about implementing a ClutterEffect

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Add a recipe showing how to implement two simple
effects, based on ClutterEffect: an always gray background,
and a border with configurable width and color.

Also explains the necessity to queue a redraw on
the associated actor if the effect's properties change,
and shows how to implement that.

The example gives the GObject code for both effects,
as well as an example application showing how to use them.
The example also demonstrates how to disable/enable an effect,
making the border round an actor togglable.
This commit is contained in:
Elliot Smith 2011-03-16 17:44:44 +00:00
parent 5732b1184f
commit b5af8fbfac
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2
doc/cookbook/Makefile.am
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@ -37,6 +37,8 @@ HTML_FILES = $(top_builddir)/doc/cookbook/html/*.html
CSS_FILES = $(top_builddir)/doc/cookbook/html/*.css
IMAGE_FILES = \
$(srcdir)/images/clutter-logo.png \
$(srcdir)/images/effects-basic.png \
$(srcdir)/images/effects-basic-background.png \
$(srcdir)/images/effects-built-in.png \
$(srcdir)/images/effects-custom-deform.png \
$(srcdir)/images/effects-custom-deform-back-material.png \

537
doc/cookbook/effects.xml
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@ -245,6 +245,543 @@ clutter_actor_add_effect (texture, effect);
</section>
<section id="effects-basic">
<title>Changing an actor's paint sequence using
<type>ClutterEffect</type></title>
<section>
<title>Problem</title>
<para>You want to paint on top of or under an actor in a generic
way, without editing the actor's <function>paint()</function>
implementation. Example use cases are:</para>
<itemizedlist>
<listitem>
<para>Adding a border on top of an actor.</para>
</listitem>
<listitem>
<para>Drawing a background for an actor.</para>
</listitem>
</itemizedlist>
<para>A quick way to achieve the same thing (though not
readily portable between actors) is to connect a callback
before or after an actor's <emphasis>paint</emphasis> signal.
See <link linkend="actors-paint-wrappers">this recipe</link> for
more details. However, using a <type>ClutterEffect</type>
implementation, as explained in this recipe, is the preferred
approach.</para>
</section>
<section>
<title>Solution</title>
<para>Create a subclass of the <type>ClutterEffect</type> abstract
class; then implement the <function>pre_paint()</function> and/or
<function>post_paint()</function> virtual functions. When the
effect is applied to an actor, these functions will paint
before and after the actor's own <function>paint()</function>
implementation.</para>
<note>
<para>For this solution, we implement a simple
<type>CbBackgroundEffect</type> which draws a gray rectangle
under an actor. The full source is in
<link linkend="effects-basic-example-cbbackgroundeffect">this
section</link>. To keep it simple, the effect has no properties
and isn't configurable (the background is always gray); see the
<link linkend="effects-basic-example-cbbordereffect">border
effect</link> for a more detailed implementation with GObject
trimmings.</para>
</note>
<para>First, create a <type>ClutterEffect</type> subclass. This
requires the trappings of a GObject class; in particular,
it needs a private struct to hold the effect's state. This
should include any <type>CoglMaterials</type>,
<type>CoglColors</type> or other private member variables
you intend to use to draw the effect.</para>
<para>In the case of the background effect, we have a background
<type>CoglMaterial</type> and a <type>CoglColor</type> for that
material:</para>
<informalexample>
<programlisting>
struct _CbBackgroundEffectPrivate
{
CoglMaterial *background;
CoglColor *color;
};
</programlisting>
</informalexample>
<para>In the <function>init()</function> function for objects of
your class, create any Cogl resources which you need to draw the
effect. In the case of the background effect,
we need to create the <type>CoglMaterial</type> and
<type>CoglColor</type> for the private struct:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
cb_background_effect_init (CbBackgroundEffect *self)
{
/* get the private struct for the object
CbBackgroundEffectPrivate *priv;
priv = self->priv = CB_BACKGROUND_EFFECT_GET_PRIVATE (self);
/* create the background material */
priv->background = cogl_material_new ();
/* gray color for filling the background material */
priv->color = cogl_color_new ();
cogl_color_init_from_4ub (priv->color, 122, 122, 122, 255);
/* set the color on the material; NB this isn't configurable
* for this effect, and is always gray
*/
cogl_material_set_color (priv->background, priv->color);
}
]]>
</programlisting>
</informalexample>
<para>Optionally, you can create GObject properties for
the class, if you want a configurable effect: see
<link linkend="effects-basic-discussion-properties">this
section</link> for details.</para>
<para>The <function>dispose()</function> function for your effect
should clean up any Cogl resources:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
cb_background_effect_dispose (GObject *gobject)
{
CbBackgroundEffectPrivate *priv = CB_BACKGROUND_EFFECT (gobject)->priv;
if (priv->background != COGL_INVALID_HANDLE)
{
cogl_handle_unref (priv->background);
priv->background = COGL_INVALID_HANDLE;
}
if (priv->color != NULL)
{
cogl_color_free (priv->color);
priv->color = NULL;
}
G_OBJECT_CLASS (cb_background_effect_parent_class)->dispose (gobject);
}
]]>
</programlisting>
</informalexample>
<para>Now, the important part: implement <function>pre_paint()</function>
and/or <function>post_paint()</function>, using Cogl to draw on the
material(s) set up for the effect. In these functions, you should also
check whether the effect is enabled before drawing.</para>
<para>For the background effect, we implement <function>pre_paint()</function>,
to draw a gray rectangle under the actor:</para>
<informalexample>
<programlisting>
<![CDATA[
/* note that if pre_paint() returns FALSE
* any post_paint() defined for the effect will not be called
*/
static gboolean
cb_background_effect_pre_paint (ClutterEffect *self)
{
ClutterActor *actor;
gfloat width;
gfloat height;
CbBackgroundEffectPrivate *priv;
ClutterActorMeta *meta = CLUTTER_ACTOR_META (self);
/* check that the effect is enabled before applying it */
if (!clutter_actor_meta_get_enabled (meta))
return TRUE;
priv = CB_BACKGROUND_EFFECT (self)->priv;
/* get the associated actor's dimensions */
actor = clutter_actor_meta_get_actor (CLUTTER_ACTOR_META (self));
clutter_actor_get_size (actor, &width, &height);
/* draw a Cogl rectangle in the background using the default color */
cogl_set_source (priv->background);
/* the rectangle has the same dimensions as the actor */
cogl_rectangle (0, 0, width, height);
return TRUE;
}
]]>
</programlisting>
</informalexample>
<para>Now, in the <function>init()</function> function for the
effect <emphasis>class</emphasis>, assign your implementations to the
virtual methods of the <type>ClutterEffect</type> abstract class:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
cb_background_effect_class_init (CbBackgroundEffectClass *klass)
{
ClutterEffectClass *effect_class = CLUTTER_EFFECT_CLASS (klass);
GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
effect_class->pre_paint = cb_background_effect_pre_paint;
gobject_class->dispose = cb_background_effect_dispose;
g_type_class_add_private (klass, sizeof (CbBackgroundEffectPrivate));
}
]]>
</programlisting>
</informalexample>
<para>If you intend to make your effect reusable, provide
a public constructor (as is done for the example effects in this
recipe):</para>
<informalexample>
<programlisting>
ClutterEffect *
cb_background_effect_new ()
{
return g_object_new (CB_TYPE_BACKGROUND_EFFECT,
NULL);
}
</programlisting>
</informalexample>
<para>The effect is now ready to be used. The application code
for applying your effect to an actor is the same as for any
other effect:</para>
<informalexample>
<programlisting>
ClutterActor *texture;
ClutterEffect *background_effect;
/* ...initialize texture, load image file etc... */
/* create a gray background effect */
background_effect = cb_background_effect_new ();
/* apply the effect to the actor */
clutter_actor_add_effect (texture, background_effect);
</programlisting>
</informalexample>
<para>Below is an example of applying this effect to a texture loaded
with an image; the image has a transparent background, so the
background is visible through it. The screenshot is from
the <link linkend="effects-basic-example-5">example
application</link>:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/effects-basic-background.png" />
</imageobject>
<alt>
<para>Applying <type>CbBackgroundEffect</type>
to a texture loaded with an image that has a transparent
background</para>
</alt>
</mediaobject>
</screenshot>
</section>
<section>
<title>Discussion</title>
<para>A basic <type>ClutterEffect</type> is particularly useful for
amending the appearance of an actor on the fly: for example,
to highlight an actor in response to a button presses. This
<emphasis>could</emphasis> be done by creating a custom widget
whose appearance could be toggled. But what if you wanted to make
an arbitrary actor's appearance "togglable"? A generic effect
in the style of the border effect in this recipe can be applied
to any actor, and easily toggled by enabling/disabling the
effect.</para>
<para><type>ClutterEffect</type> works best where
you want to overlay or underlay the actor with Cogl paths or
primitives, without changing the actor's geometry. If you want
to do complicated geometry transformations, or other subtle
manipulations of an actor's appearance, it is better to use
a <type>ClutterEffect</type> subclass like
<type>ClutterOffscreenEffect</type>, <type>ClutterDeformEffect</type>,
or <type>ClutterShaderEffect</type>.</para>
<para>In a similar vein, when a <type>ClutterEffect</type> is
applied to an actor, the effect shouldn't paint outside the actor's
allocation. However, if the effect provides a
<function>get_paint_volume()</function> implementation which
returns a volume larger than the actor's allocation, the effect
<emphasis>can</emphasis> paint anywhere within that volume. Though
in most cases, creating a custom paint volume is only going to be
useful for offscreen effects, where you are changing the
actor's geometry.</para>
<section id="effects-basic-discussion-properties">
<title>Effect properties</title>
<para>If your effect has GObject properties, you should
ensure that an actor associated with the effect is queued
for a redraw when those properties change. (You only need to
do this for properties which change the effect's appearance;
but this is likely to include most of an effect's properties.)</para>
<para>In most cases, you're likely define standard GObject
properties for the class; for example,
<link linkend="effects-basic-example-2"><type>CbBorderEffect</type></link>
defines a <varname>width</varname> property like this:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
cb_border_effect_class_init (CbBorderEffectClass *klass)
{
GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
GParamSpec *pspec;
/* ...more class initialization code here... */
pspec = g_param_spec_float ("width",
"Width",
"The width of the border (in pixels)",
1.0, 100.0,
10.0,
G_PARAM_READWRITE);
obj_props[PROP_WIDTH] = pspec;
g_object_class_install_property (gobject_class, PROP_WIDTH, pspec);
/* ...more property definitions...*/
}
]]>
</programlisting>
</informalexample>
<para>It also defines a standard GObject
<function>set_property()</function> function for
<varname>width</varname>:</para>
<informalexample>
<programlisting>
<![CDATA[
static void
cb_border_effect_set_property (GObject *gobject,
guint prop_id,
const GValue *value,
GParamSpec *pspec)
{
CbBorderEffect *effect = CB_BORDER_EFFECT (gobject);
switch (prop_id)
{
/* ...other cases here ... */
case PROP_WIDTH:
cb_border_effect_set_width (effect, g_value_get_float (value));
break;
/* ...default case ... */
}
}
]]>
</programlisting>
</informalexample>
<para>Note that this calls
<function>cb_border_effect_set_width()</function>, which is
also exposed in the public API. This is where the
<varname>width</varname> member variable is actually set in
the private struct; and also where the redraw for the actor
associated with the effect should be queued:</para>
<informalexample>
<programlisting>
<![CDATA[
/* queues a redraw of the actor associated with the effect, if there is one */
static void
cb_border_effect_update (CbBorderEffect *self)
{
ClutterActor *actor = clutter_actor_meta_get_actor (CLUTTER_ACTOR_META (self));
/* this guard is necessary as an effect's properties can be manipulated
* before it has an actor associated with it
*/
if (actor != NULL)
clutter_actor_queue_redraw (actor);
}
/* public setter for the width property, which calls the update function */
void
cb_border_effect_set_width (CbBorderEffect *self,
gfloat width)
{
CbBorderEffectPrivate *priv;
g_return_if_fail (CB_IS_BORDER_EFFECT (self));
priv = CB_BORDER_EFFECT_GET_PRIVATE (self);
priv->width = width;
/* the property has been updated, so queue a redraw of the actor (if set) */
cb_border_effect_update (self);
}
]]>
</programlisting>
</informalexample>
<para>Any other property setters which affect the associated
actor's appearance (i.e. color in the case of
<type>CbBorderEffect</type>) should also call the update
function after setting the property.</para>
<tip>
<para>If your effect exposes GObject properties in this way,
it can also be animated with the Clutter animation API as usual.
For example, you could animate the border effect in this recipe
so that the border gradually becomes thinner or thicker.</para>
</tip>
</section>
</section>
<section id="effects-basic-example">
<title>Full example</title>
<para>The example application applies two effects to a
group of <type>ClutterTextures</type>:</para>
<itemizedlist>
<listitem>
<para>A <type>CbBackgroundEffect</type> which draws a gray
background under each actor. The effect is implemented in
<link linkend="effects-basic-example-1">a header
file</link> and <link linkend="effects-basic-example-2">a C
code file</link>.</para>
</listitem>
<listitem>
<para>A <type>CbBorderEffect</type> which draws a
red border on top of an actor; this is toggled by clicking
on the actor. The effect is implemented in
<link linkend="effects-basic-example-3">a header
file</link> and <link linkend="effects-basic-example-4">a C
code file</link>.</para>
</listitem>
</itemizedlist>
<para>The <link linkend="effects-basic-example-5">application</link>
creates textures from the file paths specified
on the command line then applies both of these effects to
each texture. In the case of the <type>CbBorderEffect</type>,
a 5 pixel red border is applied; this is also disabled by default,
and enabled when a texture is clicked.</para>
<para>Here is an example of the output when the application is loaded
with four images:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/effects-basic.png" />
</imageobject>
<alt>
<para>Applying <type>CbBackgroundEffect</type>
and a togglable <type>CbBorderEffect</type>
to a several textures</para>
</alt>
</mediaobject>
</screenshot>
<section id="effects-basic-example-cbbackgroundeffect">
<title><type>CbBackgroundEffect</type></title>
<example id="effects-basic-example-1">
<title><filename>cb-background-effect.h</filename> (header file)</title>
<programlisting>
<xi:include href="examples/cb-background-effect.h" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="effects-basic-example-2">
<title><filename>cb-background-effect.c</filename> (code file)</title>
<programlisting>
<xi:include href="examples/cb-background-effect.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
<section id="effects-basic-example-cbbordereffect">
<title><type>CbBorderEffect</type></title>
<para>This is a more sophisticated effect with configurable
border color and width.</para>
<example id="effects-basic-example-3">
<title><filename>cb-border-effect.h</filename> (header file)</title>
<programlisting>
<xi:include href="examples/cb-border-effect.h" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="effects-basic-example-4">
<title><filename>cb-border-effect.c</filename> (code file)</title>
<programlisting>
<xi:include href="examples/cb-border-effect.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
<section id="effects-basic-example-application">
<title>Application</title>
<example id="effects-basic-example-5">
<title>Application which applies <type>CbBorderEffect</type>
and <type>CbBackgroundEffect</type> to a group of
<type>ClutterTextures</type>.</title>
<programlisting>
<xi:include href="examples/effects-basic.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
</section>
</section>
<section id="effects-custom-deform">
<title>Creating and animating a custom <type>ClutterDeformEffect</type></title>

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