Playback tutorial 7: Custom playbin sinks

Goal

playbin can be further customized by manually selecting its audio and video sinks. This allows applications to rely on playbin to retrieve and decode the media and then manage the final render/display themselves. This tutorial shows:

How to replace the sinks selected by playbin.
How to use a complex pipeline as a sink.

Introduction

Two properties of playbin allow selecting the desired audio and video sinks: audio-sink and video-sink (respectively). The application only needs to instantiate the appropriate GstElement and pass it to playbin through these properties.

This method, though, only allows using a single Element as sink. If a more complex pipeline is required, for example, an equalizer plus an audio sink, it needs to be wrapped in a Bin, so it looks to playbin as if it was a single Element.

A Bin (GstBin) is a container that encapsulates partial pipelines so they can be managed as single elements. As an example, the GstPipeline we have been using in all tutorials is a type of GstBin, which does not interact with external Elements. Elements inside a Bin connect to external elements through Ghost Pads (GstGhostPad), this is, Pads on the surface of the Bin which simply forward data from an external Pad to a given Pad on an internal Element.

Figure 1: A Bin with two Elements and one Ghost Pad.

GstBins are also a type of GstElement, so they can be used wherever an Element is required, in particular, as sinks for playbin (and they are then known as sink-bins).

An equalized player

Copy this code into a text file named playback-tutorial-7.c.

playback-tutorial7.c

#include <gst/gst.h>

int main(int argc, char *argv[]) {
  GstElement *pipeline, *bin, *equalizer, *convert, *sink;
  GstPad *pad, *ghost_pad;
  GstBus *bus;
  GstMessage *msg;

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Build the pipeline */
  pipeline = gst_parse_launch ("playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm", NULL);

  /* Create the elements inside the sink bin */
  equalizer = gst_element_factory_make ("equalizer-3bands", "equalizer");
  convert = gst_element_factory_make ("audioconvert", "convert");
  sink = gst_element_factory_make ("autoaudiosink", "audio_sink");
  if (!equalizer || !convert || !sink) {
    g_printerr ("Not all elements could be created.\n");
    return -1;
  }

  /* Create the sink bin, add the elements and link them */
  bin = gst_bin_new ("audio_sink_bin");
  gst_bin_add_many (GST_BIN (bin), equalizer, convert, sink, NULL);
  gst_element_link_many (equalizer, convert, sink, NULL);
  pad = gst_element_get_static_pad (equalizer, "sink");
  ghost_pad = gst_ghost_pad_new ("sink", pad);
  gst_pad_set_active (ghost_pad, TRUE);
  gst_element_add_pad (bin, ghost_pad);
  gst_object_unref (pad);

  /* Configure the equalizer */
  g_object_set (G_OBJECT (equalizer), "band1", (gdouble)-24.0, NULL);
  g_object_set (G_OBJECT (equalizer), "band2", (gdouble)-24.0, NULL);

  /* Set playbin's audio sink to be our sink bin */
  g_object_set (GST_OBJECT (pipeline), "audio-sink", bin, NULL);

  /* Start playing */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS);

  /* Free resources */
  if (msg != NULL)
    gst_message_unref (msg);
  gst_object_unref (bus);
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  return 0;
}

If you need help to compile this code, refer to the Building the tutorials section for your platform: Mac or Windows or use this specific command on Linux:

gcc playback-tutorial-7.c -o playback-tutorial-7 `pkg-config --cflags --libs gstreamer-1.0`

If you need help to run this code, refer to the Running the tutorials section for your platform: Mac OS X, Windows, for iOS or for android.

This tutorial opens a window and displays a movie, with accompanying audio. The media is fetched from the Internet, so the window might take a few seconds to appear, depending on your connection speed. The higher frequency bands have been attenuated, so the movie sound should have a more powerful bass component.<

Required libraries: gstreamer-1.0

Walkthrough

/* Create the elements inside the sink bin */
equalizer = gst_element_factory_make ("equalizer-3bands", "equalizer");
convert = gst_element_factory_make ("audioconvert", "convert");
sink = gst_element_factory_make ("autoaudiosink", "audio_sink");
if (!equalizer || !convert || !sink) {
  g_printerr ("Not all elements could be created.\n");
  return -1;
}

All the Elements that compose our sink-bin are instantiated. We use an equalizer-3bands and an autoaudiosink, with an audioconvert in between, because we are not sure of the capabilities of the audio sink (since they are hardware-dependant).

/* Create the sink bin, add the elements and link them */
bin = gst_bin_new ("audio_sink_bin");
gst_bin_add_many (GST_BIN (bin), equalizer, convert, sink, NULL);
gst_element_link_many (equalizer, convert, sink, NULL);

This adds the new Elements to the Bin and links them just as we would do if this was a pipeline.

pad = gst_element_get_static_pad (equalizer, "sink");
ghost_pad = gst_ghost_pad_new ("sink", pad);
gst_pad_set_active (ghost_pad, TRUE);
gst_element_add_pad (bin, ghost_pad);
gst_object_unref (pad);

Now we need to create a Ghost Pad so this partial pipeline inside the Bin can be connected to the outside. This Ghost Pad will be connected to a Pad in one of the internal Elements (the sink pad of the equalizer), so we retrieve this Pad with gst_element_get_static_pad(). Remember from that if this was a Request Pad instead of an Always Pad, we would need to use gst_element_request_pad().

The Ghost Pad is created with gst_ghost_pad_new() (pointing to the inner Pad we just acquired), and activated with gst_pad_set_active(). It is then added to the Bin with gst_element_add_pad(), transferring ownership of the Ghost Pad to the bin, so we do not have to worry about releasing it.

Finally, the sink Pad we obtained from the equalizer needs to be release with gst_object_unref().

At this point, we have a functional sink-bin, which we can use as the audio sink in playbin. We just need to instruct playbin to use it:

/* Set playbin's audio sink to be our sink bin */
g_object_set (GST_OBJECT (pipeline), "audio-sink", bin, NULL);

It is as simple as setting the audio-sink property on playbin to the newly created sink.

/* Configure the equalizer */
g_object_set (G_OBJECT (equalizer), "band1", (gdouble)-24.0, NULL);
g_object_set (G_OBJECT (equalizer), "band2", (gdouble)-24.0, NULL);

The only bit remaining is to configure the equalizer. For this example, the two higher frequency bands are set to the maximum attenuation so the bass is boosted. Play a bit with the values to feel the difference (Look at the documentation for the equalizer-3bands element for the allowed range of values).

Exercise

Build a video bin instead of an audio bin, using one of the many interesting video filters GStreamer offers, like solarize, vertigotv or any of the Elements in the effectv plugin. Remember to use the color space conversion element videoconvert if your pipeline fails to link due to incompatible caps.

Conclusion

This tutorial has shown:

How to set your own sinks to playbin using the audio-sink and video-sink properties.
How to wrap a piece of pipeline into a GstBin so it can be used as a sink-bin by playbin.

It has been a pleasure having you here, and see you soon!

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