19.2. Decodebin

Decodebin is the actual autoplugger backend of playbin, which was discussed in the previous section. Decodebin will, in short, accept input from a source that is linked to its sinkpad and will try to detect the media type contained in the stream, and set up decoder routines for each of those. It will automatically select decoders. For each decoded stream, it will emit the "new-decoded-pad" signal, to let the client know about the newly found decoded stream. For unknown streams (which might be the whole stream), it will emit the "unknown-type" signal. The application is then responsible for reporting the error to the user.


#include <gst/gst.h>

[.. my_bus_callback goes here ..]

GstElement *pipeline, *audio;

static void
cb_newpad (GstElement *decodebin,
	   GstPad     *pad,
	   gboolean    last,
	   gpointer    data)
{
  GstCaps *caps;
  GstStructure *str;
  GstPad *audiopad;

  /* only link once */
  audiopad = gst_element_get_pad (audio, "sink");
  if (GST_PAD_IS_LINKED (audiopad)) {
    g_object_unref (audiopad);
    return;
  }

  /* check media type */
  caps = gst_pad_get_caps (pad);
  str = gst_caps_get_structure (caps, 0);
  if (!g_strrstr (gst_structure_get_name (str), "audio")) {
    gst_caps_unref (caps);
    gst_object_unref (audiopad);
    return;
  }
  gst_caps_unref (caps);

  /* link'n'play */
  gst_pad_link (pad, audiopad);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *src, *dec, *conv, *sink;
  GstPad *audiopad;
  GstBus *bus;

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

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

  /* setup */
  pipeline = gst_pipeline_new ("pipeline");

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

  src = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (src), "location", argv[1], NULL);
  dec = gst_element_factory_make ("decodebin", "decoder");
  g_signal_connect (dec, "new-decoded-pad", G_CALLBACK (cb_newpad), NULL);
  gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
  gst_element_link (src, dec);

  /* create audio output */
  audio = gst_bin_new ("audiobin");
  conv = gst_element_factory_make ("audioconvert", "aconv");
  audiopad = gst_element_get_pad (conv, "sink");
  sink = gst_element_factory_make ("alsasink", "sink");
  gst_bin_add_many (GST_BIN (audio), conv, sink, NULL);
  gst_element_link (conv, sink);
  gst_element_add_pad (audio,
      gst_ghost_pad_new ("sink", audiopad));
  gst_object_unref (audiopad);
  gst_bin_add (GST_BIN (pipeline), audio);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* cleanup */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}
    

Decodebin, similar to playbin, supports the following features:

Although decodebin is a good autoplugger, there's a whole lot of things that it does not do and is not intended to do:

Decodebin can be easily tested on the commandline, e.g. by using the command gst-launch-0.8 filesrc location=file.ogg ! decodebin ! audioconvert ! alsasink.