Bins
A Bin is a container element. You can add elements to a bin. Since a bin is
an GstElement itself, it can also be added to another bin.
Bins allow you to combine connected elements into one logical element. You do
not deal with the individual elements anymore but with just one element, the bin.
We will see that this is extremely powerfull when you are going to construct
complex pipelines since it allows you to break up the pipeline in smaller chunks.
The bin will also manage the elements contained in it. It will figure out how
the data will flow in the bin and generate an optimal plan for that data flow. Plan
generation is one of the most complicated procedures in GStreamer.
Visualisation of a <classname>GstBin</classname> element with some elements in it
There are two standard bins available to the GStreamer programmer:
A pipeline (GstPipeline). Which is a generic container you will
use most of the time.
A thread (GstThread). All the elements in the thread bin will
run in a separate thread. You will haver to use this bin if you carfully have to
synchronize audio and video for example. You will learn more about threads in..
The application programmer can create custom bins packed with elements to perform a
specific task. This allow you to write an MPEG audio decoder with just the follwing lines
of code:
// create the mp3player element
GstElement *mp3player = gst_elementfactory_make("mp3player","mp3player");
// set the source mp3 audio file
gtk_object_set(GTK_OBJECT(mp3player), "location", "helloworld.mp3", NULL);
// tell the mp3player to prepare itself
gst_element_set_state(GST_ELEMENT(mp3player),GST_STATE_READY);
// start playback
gst_element_set_state(GST_ELEMENT(mp3player),GST_STATE_PLAYING);
...
// pause playback
gst_element_set_state(GST_ELEMENT(mp3player),GST_STATE_PAUSED);
...
// stop
gst_element_set_state(GST_ELEMENT(mp3player),GST_STATE_NULL);
Custom bins can be created with a plugin or an XML description.
Ghostpads
You can see from figure ... how a bin has no pads of its own. This is where Ghostpads
come into play.
A ghostpad is a pad from some element in the bin that has been promoted to the bin.
This way, the bin also has a pad. The bin becomes just another element with a pad and
you can then use the bin just like any other element.
Visualisation of a <classname>GstBin</classname> element with a ghostpad
Above is a representation of a ghostpad. the sinkpad of element one is now also a pad
of the bin.