Pipeline

A Pipeline is a special Bin used as the toplevel container for the filter graph.

The Pipeline will manage the selection and distribution of a global Clock as well as provide a Bus to the application.

Pipeline is used to create a pipeline. when you are done with the pipeline, use unref to free its resources including all added Element objects (if not otherwise referenced).

Elements are added and removed from the pipeline using the Bin methods like add and remove (see Bin).

Before changing the state of the Pipeline (see Element) a Bus should be retrieved with get_bus. This Bus should then be used to receive Message from the elements in the pipeline. Listening to the Bus is necessary for retrieving error messages from the Pipeline and otherwise the Pipeline might stop without any indication, why. Furthermore, the Pipeline posts messages even if nobody listens on the Bus, which will pile up and use up memory.

By default, a Pipeline will automatically flush the pending Bus messages when going to the NULL state to ensure that no circular references exist when no messages are read from the Bus. This behaviour can be changed with set_auto_flush_bus.

When the Pipeline performs the PAUSED to PLAYING state change it will select a clock for the elements. The clock selection algorithm will by default select a clock provided by an element that is most upstream (closest to the source). For live pipelines (ones that return NO_PREROLL from the set_state call) this will select the clock provided by the live source. For normal pipelines this will select a clock provided by the sinks (most likely the audio sink). If no element provides a clock, a default SystemClock is used.

The clock selection can be controlled with the use_clock method, which will enforce a given clock on the pipeline. With auto_clock the default clock selection algorithm can be restored.

A Pipeline maintains a running time for the elements. The running time is defined as the difference between the current clock time and the base time. When the pipeline goes to READY or a flushing seek is performed on it, the running time is reset to 0. When the pipeline is set from PLAYING to PAUSED, the current clock time is sampled and used to configure the base time for the elements when the pipeline is set to PLAYING again. The effect is that the running time (as the difference between the clock time and the base time) will count how much time was spent in the PLAYING state. This default behaviour can be changed with the set_start_time method.

Example: Hello World:

// See http://docs.gstreamer.com/x/NwAF
// for a detailed description

public static int main (string[] args) {
	// Initialize GStreamer:
	Gst.init (ref args);


	// Build the pipeline:
	Gst.Element pipeline;
	try {
		pipeline = Gst.parse_launch ("playbin uri=http://docs.gstreamer.com/media/sintel_trailer-480p.webm");
	} catch (Error e) {
		stderr.printf ("Error: %s\n", e.message);
		return 0;
	}

	// Start playing:
	pipeline.set_state (Gst.State.PLAYING);

	// Wait until error or EOS:
	Gst.Bus bus = pipeline.get_bus ();
	bus.timed_pop_filtered (Gst.CLOCK_TIME_NONE, Gst.MessageType.ERROR | Gst.MessageType.EOS);

	// Free resources:
	pipeline.set_state (Gst.State.NULL);

	return 0;
}

valac --pkg gstreamer-1.0 hello-world.vala