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241 lines
8.3 KiB
XML
241 lines
8.3 KiB
XML
<chapter id="cha-bins">
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<title>Bins</title>
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<para>
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A Bin is a container element. You can add elements to a bin. Since a bin is
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an <classname>GstElement</classname> itself, it can also be added to another bin.
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</para>
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<para>
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Bins allow you to combine connected elements into one logical element. You do
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not deal with the individual elements anymore but with just one element, the bin.
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We will see that this is extremely powerful when you are going to construct
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complex pipelines since it allows you to break up the pipeline in smaller chunks.
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</para>
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<para>
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The bin will also manage the elements contained in it. It will figure out how
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the data will flow in the bin and generate an optimal plan for that data flow. Plan
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generation is one of the most complicated procedures in GStreamer.
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</para>
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<figure float="1" id="sec-bin-img">
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<title>Visualisation of a <classname>GstBin</classname> element with some elements in it</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/bin-element.&magic;" format="&magic;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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There are two standard bins available to the GStreamer programmer:
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<itemizedlist>
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<listitem>
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<para>
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A pipeline (<classname>GstPipeline</classname>). Which is a generic container you will
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use most of the time. The toplevel bin has to be a pipeline.
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</para>
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</listitem>
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<listitem>
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<para>
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A thread (<classname>GstThread</classname>). The plan for the
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<classname>GstThread</classname> will be run in a separate thread. You will have to use
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this bin if you have to carefully synchronize audio and video, for example. You will learn
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more about threads in <xref linkend="cha-threads"/>.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<sect1 id="sec-bin-create">
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<title>Creating a bin</title>
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<para>
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Bins register themselves in the GStreamer registry, so they can be created in the normal way:
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</para>
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<programlisting>
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GstElement *bin, *thread, *pipeline;
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/* create a new bin called 'mybin'. this bin will be only for organizational purposes; a normal
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GstBin doesn't affect plan generation */
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bin = gst_element_factory_make ("bin", "mybin");
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/* create a new thread, and give it a unique name */
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thread = gst_element_factory_make ("thread", NULL);
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/* the core bins (GstBin, GstThread, GstPipeline) also have convenience APIs,
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gst_<bintype>_new (). these are equivalent to the gst_element_factory_make () syntax. */
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pipeline = gst_pipeline_new ("pipeline_name");
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</programlisting>
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</sect1>
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<sect1 id="sec-bin-adding">
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<title>Adding elements to a bin</title>
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<para>
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Elements are added to a bin with the following code sample:
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</para>
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<programlisting>
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GstElement *element;
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GstElement *bin;
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bin = gst_bin_new ("mybin");
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element = gst_element_factory_make ("mpg123", "decoder");
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gst_bin_add (GST_BIN (bin), element);
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...
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</programlisting>
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<para>
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Bins and threads can be added to other bins too. This allows you to create nested bins. Note
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that it doesn't make very much sense to add a <classname>GstPipeline</classname> to anything,
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as it's a toplevel bin that needs to be explicitly iterated.
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</para>
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<para>
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To get an element from the bin you can use:
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</para>
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<programlisting>
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GstElement *element;
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element = gst_bin_get_by_name (GST_BIN (bin), "decoder");
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...
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</programlisting>
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<para>
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You can see that the name of the element becomes very handy for retrieving the
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element from an bin by using the element's name. gst_bin_get_by_name () will
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recursively search nested bins.
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</para>
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<para>
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To get a list of elements in a bin, use:
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</para>
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<programlisting>
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GList *elements;
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elements = gst_bin_get_list (GST_BIN (bin));
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while (elements) {
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GstElement *element = GST_ELEMENT (elements->data);
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g_print ("element in bin: %s\n", GST_OBJECT_NAME (GST_OBJECT (element)));
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elements = g_list_next (elements);
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}
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...
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</programlisting>
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<para>
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To remove an element from a bin use:
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</para>
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<programlisting>
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GstElement *element;
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gst_bin_remove (GST_BIN (bin), element);
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...
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</programlisting>
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<para>
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To add many elements to a bin at the same time, try the gst_bin_add_many () API. Remember to
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pass NULL as the last argument.
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</para>
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<programlisting>
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GstElement *filesrc, *decoder, *audiosink;
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GstBin *bin;
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/* instantiate the elements and the bins... */
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gst_bin_add_many (bin, filesrc, decoder, audiosink, NULL);
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</programlisting>
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</sect1>
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<sect1 id="sec-bin-custom">
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<title>Custom bins</title>
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<para>
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The application programmer can create custom bins packed with elements to perform a
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specific task. This allow you to write an MPEG audio decoder with just the follwing lines
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of code:
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</para>
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<programlisting>
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/* create the mp3player element */
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GstElement *mp3player = gst_element_factory_make ("mp3player", "mp3player");
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/* set the source mp3 audio file */
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g_object_set (G_OBJECT (mp3player), "location", "helloworld.mp3", NULL);
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/* start playback */
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gst_element_set_state (GST_ELEMENT (mp3player), GST_STATE_PLAYING);
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...
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/* pause playback */
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gst_element_set_state (GST_ELEMENT (mp3player), GST_STATE_PAUSED);
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...
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/* stop */
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gst_element_set_state (GST_ELEMENT (mp3player), GST_STATE_NULL);
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</programlisting>
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<para>
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Note that the above code assumes that the mp3player bin derives itself from a
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<classname>GstThread</classname>, which begins to play as soon as its state is set to PLAYING.
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Other bin types may need explicit iteration. For more information, see <xref
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linkend="cha-threads"/>.
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Custom bins can be created with a plugin or an XML description. You will find more
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information about creating custom bin in the Plugin Writers Guide (FIXME ref).
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</para>
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</sect1>
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<sect1 id="sec-bin-ghostpads">
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<title>Ghost pads</title>
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<para>
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You can see from figure <xref linkend="sec-bin-noghost-img"/> how a bin has no pads of its own.
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This is where "ghost pads" come into play.
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</para>
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<figure float="1" id="sec-bin-noghost-img">
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<title>Visualisation of a <classname>GstBin</classname> element without ghost pads</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/bin-element-noghost.&magic;" format="&magic;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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A ghost pad is a pad from some element in the bin that has been promoted to the bin.
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This way, the bin also has a pad. The bin becomes just another element with a pad and
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you can then use the bin just like any other element. This is a very important feature
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for creating custom bins.
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</para>
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<figure float="1" id="sec-bin-ghost-img">
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<title>Visualisation of a <classname>GstBin</classname> element with a ghost pad</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/bin-element-ghost.&magic;" format="&magic;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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Above is a representation of a ghost pad. The sink pad of element one is now also a pad
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of the bin.
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</para>
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<para>
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Ghost pads can actually be added to all <classname>GstElement</classname>s and not just
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<classname>GstBin</classname>s. Use the following code example to add a ghost pad to a bin:
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</para>
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<programlisting>
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GstElement *bin;
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GstElement *element;
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element = gst_element_factory_create ("mad", "decoder");
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bin = gst_bin_new ("mybin");
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gst_bin_add (GST_BIN (bin), element);
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gst_element_add_ghost_pad (bin, gst_element_get_pad (element, "sink"), "sink");
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</programlisting>
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<para>
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In the above example, the bin now also has a pad: the pad called 'sink' of the
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given element. We can now, for example, connect the srcpad of a filesrc to the
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bin with:
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</para>
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<programlisting>
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GstElement *filesrc;
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filesrc = gst_element_factory_create ("filesrc", "disk_reader");
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gst_element_connect_pads (filesrc, "src", bin, "sink");
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...
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</programlisting>
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</sect1>
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</chapter>
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