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237 lines
8.9 KiB
XML
237 lines
8.9 KiB
XML
<chapter id="chapter-advanced-interfaces">
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<title>Interfaces</title>
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<para>
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Previously, in the chapter <xref linkend="chapter-building-args"/>, we have
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introduced the concept of GObject properties of controlling an element's
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behaviour. This is very powerful, but it has two big disadvantages:
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first of all, it is too generic, and second, it isn't dynamic.
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</para>
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<para>
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The first disadvantage is related to the customizability of the end-user
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interface that will be built to control the element. Some properties are
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more important than others. Some integer properties are better shown in a
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spin-button widget, whereas others would be better represented by a slider
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widget. Such things are not possible because the UI has no actual meaning
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in the application. A UI widget that represents a bitrate property is the
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same as a UI widget that represents the size of a video, as long as both
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are of the same <classname>GParamSpec</classname> type. Another problem,
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is that things like parameter grouping, function grouping, or parameter
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coupling are not
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really possible.
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</para>
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<para>
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The second problem with parameters are that they are not dynamic. In
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many cases, the allowed values for a property are not fixed, but depend
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on things that can only be detected at runtime. The names of inputs for
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a TV card in a video4linux source element, for example, can only be
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retrieved from the kernel driver when we've opened the device; this only
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happens when the element goes into the READY state. This means that we
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cannot create an enum property type to show this to the user.
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</para>
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<para>
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The solution to those problems is to create very specialized types of
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controls for certain often-used controls. We use the concept of interfaces
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to achieve this. The basis of this all is the glib
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<classname>GTypeInterface</classname> type. For each case where we think
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it's useful, we've created interfaces which can be implemented by elements
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at their own will.
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</para>
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<para>
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One important note: interfaces do <emphasis>not</emphasis> replace
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properties. Rather, interfaces should be built <emphasis>next to</emphasis>
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properties. There are two important reasons for this. First of all,
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properties can be more easily introspected. Second, properties can be
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specified on the commandline (<filename>gst-launch</filename>).
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</para>
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<sect1 id="section-iface-general" xreflabel="How to Implement Interfaces">
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<title>How to Implement Interfaces</title>
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<para>
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Implementing interfaces is initiated in the <function>_get_type ()</function>
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of your element. You can register one or more interfaces after having
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registered the type itself. Some interfaces have dependencies on other
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interfaces or can only be registered by certain types of elements. You
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will be notified of doing that wrongly when using the element: it will
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quit with failed assertions, which will explain what went wrong.
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If it does, you need to register support for <emphasis>that</emphasis>
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interface before registering support for the interface that you're
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wanting to support. The example below explains how to add support for a
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simple interface with no further dependencies.
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</para>
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<programlisting>
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static void gst_my_filter_some_interface_init (GstSomeInterface *iface);
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GType
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gst_my_filter_get_type (void)
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{
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static GType my_filter_type = 0;
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if (!my_filter_type) {
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static const GTypeInfo my_filter_info = {
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sizeof (GstMyFilterClass),
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NULL,
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NULL,
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(GClassInitFunc) gst_my_filter_class_init,
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NULL,
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NULL,
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sizeof (GstMyFilter),
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0,
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(GInstanceInitFunc) gst_my_filter_init
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};
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static const GInterfaceInfo some_interface_info = {
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(GInterfaceInitFunc) gst_my_filter_some_interface_init,
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NULL,
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NULL
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};
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my_filter_type =
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g_type_register_static (GST_TYPE_ELEMENT,
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"GstMyFilter",
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&my_filter_info, 0);
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g_type_add_interface_static (my_filter_type,
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GST_TYPE_SOME_INTERFACE,
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&some_interface_info);
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}
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return my_filter_type;
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}
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static void
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gst_my_filter_some_interface_init (GstSomeInterface *iface)
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{
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/* here, you would set virtual function pointers in the interface */
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}
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</programlisting>
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<para>
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Or more conveniently:
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</para>
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<programlisting>
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static void gst_my_filter_some_interface_init (GstSomeInterface *iface);
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G_DEFINE_TYPE_WITH_CODE (GstMyFilter, gst_my_filter,GST_TYPE_ELEMENT,
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G_IMPLEMENT_INTERFACE (GST_TYPE_SOME_INTERFACE,
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gst_my_filter_some_interface_init));
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</programlisting>
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</sect1>
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<sect1 id="section-iface-uri" xreflabel="URI interface">
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<title>URI interface</title>
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<para>
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WRITEME
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</para>
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</sect1>
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<sect1 id="section-iface-colorbalance" xreflabel="Color Balance Interface">
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<title>Color Balance Interface</title>
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<para>
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WRITEME
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</para>
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</sect1>
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<sect1 id="section-iface-xoverlay" xreflabel="Video Overlay Interface">
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<title>Video Overlay Interface</title>
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<para>
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The #GstVideoOverlay interface is used for 2 main purposes :
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<itemizedlist>
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<listitem>
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<para>
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To get a grab on the Window where the video sink element is going to render.
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This is achieved by either being informed about the Window identifier that
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the video sink element generated, or by forcing the video sink element to use
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a specific Window identifier for rendering.
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</para>
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</listitem>
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<listitem>
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<para>
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To force a redrawing of the latest video frame the video sink element
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displayed on the Window. Indeed if the #GstPipeline is in #GST_STATE_PAUSED
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state, moving the Window around will damage its content. Application
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developers will want to handle the Expose events themselves and force the
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video sink element to refresh the Window's content.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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A plugin drawing video output in a video window will need to have that
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window at one stage or another. Passive mode simply means that no window
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has been given to the plugin before that stage, so the plugin created the
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window by itself. In that case the plugin is responsible of destroying
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that window when it's not needed any more and it has to tell the
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applications that a window has been created so that the application can
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use it. This is done using the <classname>have-window-handle</classname>
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message that can be posted from the plugin with the
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<function>gst_video_overlay_got_window_handle</function> method.
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</para>
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<para>
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As you probably guessed already active mode just means sending a video
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window to the plugin so that video output goes there. This is done using
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the <function>gst_video_overlay_set_window_handle</function> method.
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</para>
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<para>
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It is possible to switch from one mode to another at any moment, so the
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plugin implementing this interface has to handle all cases. There are only
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2 methods that plugins writers have to implement and they most probably
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look like that :
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</para>
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<programlisting><![CDATA[
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static void
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gst_my_filter_set_window_handle (GstVideoOverlay *overlay, guintptr handle)
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{
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GstMyFilter *my_filter = GST_MY_FILTER (overlay);
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if (my_filter->window)
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gst_my_filter_destroy_window (my_filter->window);
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my_filter->window = handle;
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}
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static void
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gst_my_filter_xoverlay_init (GstVideoOverlayClass *iface)
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{
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iface->set_window_handle = gst_my_filter_set_window_handle;
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}
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]]></programlisting>
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<para>
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You will also need to use the interface methods to post messages when
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needed such as when receiving a CAPS event where you will know the video
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geometry and maybe create the window.
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</para>
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<programlisting><![CDATA[
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static MyFilterWindow *
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gst_my_filter_window_create (GstMyFilter *my_filter, gint width, gint height)
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{
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MyFilterWindow *window = g_new (MyFilterWindow, 1);
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...
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gst_video_overlay_got_window_handle (GST_VIDEO_OVERLAY (my_filter), window->win);
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}
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/* called from the event handler for CAPS events */
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static gboolean
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gst_my_filter_sink_set_caps (GstMyFilter *my_filter, GstCaps *caps)
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{
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gint width, height;
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gboolean ret;
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...
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ret = gst_structure_get_int (structure, "width", &width);
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ret &= gst_structure_get_int (structure, "height", &height);
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if (!ret) return FALSE;
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gst_video_overlay_prepare_window_handle (GST_VIDEO_OVERLAY (my_filter));
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if (!my_filter->window)
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my_filter->window = gst_my_filter_create_window (my_filter, width, height);
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...
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}
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]]></programlisting>
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</sect1>
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<sect1 id="section-iface-navigation" xreflabel="Navigation Interface">
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<title>Navigation Interface</title>
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<para>
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WRITEME
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</para>
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</sect1>
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</chapter>
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