Autoplugging

In Your first application, you've learned to build a simple media player for Ogg/Vorbis files. By using alternative elements, you are able to build media players for other media types, such as Ogg/Speex, MP3 or even video formats. However, you would rather want to build an application that can automatically detect the media type of a stream and automatically generate the best possible pipeline by looking at all available elements in a system. This process is called autoplugging, and GStreamer contains high-quality autopluggers. If you're looking for an autoplugger, don't read any further and go to Playback Components. This chapter will explain the concept of autoplugging and typefinding. It will explain what systems GStreamer includes to dynamically detect the type of a media stream, and how to generate a pipeline of decoder elements to playback this media. The same principles can also be used for transcoding. Because of the full dynamicity of this concept, GStreamer can be automatically extended to support new media types without needing any adaptations to its autopluggers.

We will first introduce the concept of Media types as a dynamic and extendible way of identifying media streams. After that, we will introduce the concept of typefinding to find the type of a media stream. Lastly, we will explain how autoplugging and the GStreamer registry can be used to setup a pipeline that will convert media from one mediatype to another, for example for media decoding.

Media types as a way to identify streams

We have previously introduced the concept of capabilities as a way for elements (or, rather, pads) to agree on a media type when streaming data from one element to the next (see Capabilities of a pad)). We have explained that a capability is a combination of a media type and a set of properties. For most container formats (those are the files that you will find on your hard disk; Ogg, for example, is a container format), no properties are needed to describe the stream. Only a media type is needed. A full list of media types and accompanying properties can be found in the Plugin Writer's Guide.

An element must associate a media type to its source and sink pads when it is loaded into the system. GStreamer knows about the different elements and what type of data they expect and emit through the GStreamer registry. This allows for very dynamic and extensible element creation as we will see.

In Your first application, we've learned to build a music player for Ogg/Vorbis files. Let's look at the media types associated with each pad in this pipeline. The Hello world pipeline with media types shows what media type belongs to each pad in this pipeline.

The Hello world pipeline with media types

Now that we have an idea how GStreamer identifies known media streams, we can look at methods GStreamer uses to setup pipelines for media handling and for media type detection.

Media stream type detection

Usually, when loading a media stream, the type of the stream is not known. This means that before we can choose a pipeline to decode the stream, we first need to detect the stream type. GStreamer uses the concept of typefinding for this. Typefinding is a normal part of a pipeline, it will read data for as long as the type of a stream is unknown. During this period, it will provide data to all plugins that implement a typefinder. When one of the typefinders recognizes the stream, the typefind element will emit a signal and act as a passthrough module from that point on. If no type was found, it will emit an error and further media processing will stop.

Once the typefind element has found a type, the application can use this to plug together a pipeline to decode the media stream. This will be discussed in the next section.

Plugins in GStreamer can, as mentioned before, implement typefinder functionality. A plugin implementing this functionality will submit a media type, optionally a set of file extensions commonly used for this media type, and a typefind function. Once this typefind function inside the plugin is called, the plugin will see if the data in this media stream matches a specific pattern that marks the media type identified by that media type. If it does, it will notify the typefind element of this fact, telling which mediatype was recognized and how certain we are that this stream is indeed that mediatype. Once this run has been completed for all plugins implementing a typefind functionality, the typefind element will tell the application what kind of media stream it thinks to have recognized.

The following code should explain how to use the typefind element. It will print the detected media type, or tell that the media type was not found. The next section will introduce more useful behaviours, such as plugging together a decoding pipeline.

#include <gst/gst.h>

[.. my_bus_callback goes here ..]

static gboolean
idle_exit_loop (gpointer data)
{
  g_main_loop_quit ((GMainLoop *) data);

  /* once */
  return FALSE;
}

static void
cb_typefound (GstElement *typefind,
          guint       probability,
          GstCaps    *caps,
          gpointer    data)
{
  GMainLoop *loop = data;
  gchar *type;

  type = gst_caps_to_string (caps);
  g_print ("Media type %s found, probability %d%%\n", type, probability);
  g_free (type);

  /* since we connect to a signal in the pipeline thread context, we need
   * to set an idle handler to exit the main loop in the mainloop context.
   * Normally, your app should not need to worry about such things. */
  g_idle_add (idle_exit_loop, loop);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline, *filesrc, *typefind, *fakesink;
  GstBus *bus;

  /* init GStreamer */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* check args */
  if (argc != 2) {
    g_print ("Usage: %s <filename>\n", argv[0]);
    return -1;
  }

  /* create a new pipeline to hold the elements */
  pipeline = gst_pipeline_new ("pipe");

  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_add_watch (bus, my_bus_callback, NULL);
  gst_object_unref (bus);

  /* create file source and typefind element */
  filesrc = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
  typefind = gst_element_factory_make ("typefind", "typefinder");
  g_signal_connect (typefind, "have-type", G_CALLBACK (cb_typefound), loop);
  fakesink = gst_element_factory_make ("fakesink", "sink");

  /* setup */
  gst_bin_add_many (GST_BIN (pipeline), filesrc, typefind, fakesink, NULL);
  gst_element_link_many (filesrc, typefind, fakesink, NULL);
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* unset */
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}

Once a media type has been detected, you can plug an element (e.g. a demuxer or decoder) to the source pad of the typefind element, and decoding of the media stream will start right after.

Dynamically autoplugging a pipeline

See Playback Components for using the high level object that you can use to dynamically construct pipelines.

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