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92 lines
3.5 KiB
Text
92 lines
3.5 KiB
Text
<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 powerfull 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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<graphic fileref="images/bin-element" format="png"></graphic>
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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.
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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>). All the elements in the thread bin will
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run in a separate thread. You will haver to use this bin if you carfully have to
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synchronize audio and video for example. You will learn more about threads in.. <!-- FIXME -->
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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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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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<programlisting>
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// create the mp3player element
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GstElement *mp3player = gst_elementfactory_make("mp3player","mp3player");
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// set the source mp3 audio file
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gtk_object_set(GTK_OBJECT(mp3player), "location", "helloworld.mp3", NULL);
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// tell the mp3player to prepare itself
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gst_element_set_state(GST_ELEMENT(mp3player),GST_STATE_READY);
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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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Custom bins can be created with a plugin or an XML description.
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</para>
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<sect1 id="sec-bin-ghostpads">
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<title>Ghostpads</title>
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<para>
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You can see from figure ... how a bin has no pads of its own. This is where Ghostpads
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come into play.
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</para>
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<para>
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A ghostpad 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.
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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 ghostpad</title>
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<graphic fileref="images/bin-element-ghost" format="png"></graphic>
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</figure>
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<para>
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Above is a representation of a ghostpad. the sinkpad of element one is now also a pad
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of the bin.
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</para>
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</sect1>
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</chapter>
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