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138 lines
4.4 KiB
XML
138 lines
4.4 KiB
XML
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<chapter id="text"
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xmlns:xi="http://www.w3.org/2003/XInclude">
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<title>Text</title>
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<epigraph>
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<attribution>Antoine de Saint-Exupery</attribution>
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<para>A rock pile ceases to be a rock pile the moment a single man
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contemplates it, bearing within him the image of a cathedral.</para>
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</epigraph>
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<section id="text-introduction">
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<title>Introduction</title>
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</section>
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<section id="text-shadow">
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<title>Drawing a shadow under the text</title>
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<section>
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<title>Problem</title>
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<para>You want to draw a shadow under the text displayed by a
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ClutterText actor.</para>
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</section>
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<section>
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<title>Solution</title>
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<para>Override the <function>paint</function> signal of
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<type>ClutterText</type> and use the CoglPango API to paint the
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<type>PangoLayout</type> of the actor with the color of the
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shadow at a given offset.</para>
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<screenshot>
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<mediaobject>
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<imageobject>
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<imagedata format="PNG"
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fileref="images/text-shadow.png" />
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</imageobject>
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<alt>
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<para>A ClutterText actor painting a shadow underneath its
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contents</para>
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</alt>
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</mediaobject>
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</screenshot>
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</section>
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<section>
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<title>Discussion</title>
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<para>The <type>ClutterText</type> class provides an actor that
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transforms the <type>PangoLayout</type> object into an element of
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the Clutter scene graph. The underlying layout is painted, though,
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through a subset of the Cogl API, called
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<emphasis>CoglPango</emphasis>.</para>
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<para>It is possible to paint <type>PangoLayout</type> created by a
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<type>ClutterText</type> by invoking
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<function>cogl_pango_render_layout()</function>:</para>
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<informalexample><programlisting>
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<![CDATA[
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void
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cogl_pango_render_layout (PangoLayout *layout,
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int x_offset,
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int y_offset,
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CoglColor *text_color,
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int flags);
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]]>
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</programlisting></informalexample>
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<para>This function will paint the layout at the given offsets using the
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provided color.</para>
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<warning><para>The <function>cogl_pango_render_layout()</function>
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function will only work with <type>PangoLayout</type>s created by
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Clutter.</para></warning>
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<para>Since the shadow of the text is literally the same text but painted
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with a different color and at an offset, we can use the
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<function>paint</function> signal of <type>ClutterText</type> to paint
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the shadow, and then let <type>ClutterText</type> paint its contents on
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top:</para>
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<informalexample><programlisting>
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<![CDATA[
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static void
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_text_paint_cb (ClutterActor *actor)
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{
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ClutterText *text = CLUTTER_TEXT (actor);
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/* Get the PangoLayout that the Text actor is going to paint... */
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PangoLayout *layout;
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layout = clutter_text_get_layout (text);
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/* ... Create the color of the shadow... */
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/* ... Finally, render the Text layout at a given
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* offset using the color of the shadow
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*/
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cogl_pango_render_layout (layout,
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SHADOW_X_OFFSET, SHADOW_Y_OFFSET,
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&color, 0);
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}
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]]>
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</programlisting></informalexample>
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<para>Note that we are using the <type>PangoLayout</type> of the
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<type>ClutterText</type> because the <type>ClutterText</type> actor
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always keeps an updated layout internally. It is, however, possible for
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any <type>ClutterActor</type> to create a <type>PangoLayout</type> using
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<function>clutter_actor_create_pango_layout()</function>, and then paint
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that layout using <function>cogl_pango_render_layout()</function> in
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their implementation of the <function>paint</function> virtual
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function.</para>
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</section>
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<section>
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<title>Full example</title>
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<example id="text-shadow-example">
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<title>Creating a shadow of a text</title>
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<programlisting>
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<xi:include href="examples/text-shadow.c" parse="text">
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<xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback>
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</xi:include>
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</programlisting>
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</example>
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</section>
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</section>
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</chapter>
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