Playback tutorial 6: Audio visualization

Goal

GStreamer comes with a set of elements that turn audio into video. They can be used for scientific visualization or to spice up your music player, for example. This tutorial shows:

How to enable audio visualization
How to select the visualization element

Introduction

Enabling audio visualization in playbin is actually very easy. Just set the appropriate playbin flag and, when an audio-only stream is found, it will instantiate the necessary elements to create and display the visualization.

If you want to specify the actual element that you want to use to generate the visualization, you instantiate it yourself and then tell playbin about it through the vis-plugin property.

This tutorial searches the GStreamer registry for all the elements of the Visualization class, tries to select goom (or another one if it is not available) and passes it to playbin.

A fancy music player

Copy this code into a text file named playback-tutorial-6.c.

playback-tutorial-6.c

#include <gst/gst.h>

/* playbin flags */
typedef enum {
  GST_PLAY_FLAG_VIS           = (1 << 3) /* Enable rendering of visualizations when there is no video stream. */
} GstPlayFlags;

/* Return TRUE if this is a Visualization element */
static gboolean filter_vis_features (GstPluginFeature *feature, gpointer data) {
  GstElementFactory *factory;

  if (!GST_IS_ELEMENT_FACTORY (feature))
    return FALSE;
  factory = GST_ELEMENT_FACTORY (feature);
  if (!g_strrstr (gst_element_factory_get_klass (factory), "Visualization"))
    return FALSE;

  return TRUE;
}

int main(int argc, char *argv[]) {
  GstElement *pipeline, *vis_plugin;
  GstBus *bus;
  GstMessage *msg;
  GList *list, *walk;
  GstElementFactory *selected_factory = NULL;
  guint flags;

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

  /* Get a list of all visualization plugins */
  list = gst_registry_feature_filter (gst_registry_get (), filter_vis_features, FALSE, NULL);

  /* Print their names */
  g_print("Available visualization plugins:\n");
  for (walk = list; walk != NULL; walk = g_list_next (walk)) {
    const gchar *name;
    GstElementFactory *factory;

    factory = GST_ELEMENT_FACTORY (walk->data);
    name = gst_element_factory_get_longname (factory);
    g_print("  %s\n", name);

    if (selected_factory == NULL || g_str_has_prefix (name, "GOOM")) {
      selected_factory = factory;
    }
  }

  /* Don't use the factory if it's still empty */
  /* e.g. no visualization plugins found */
  if (!selected_factory) {
    g_print ("No visualization plugins found!\n");
    return -1;
  }

  /* We have now selected a factory for the visualization element */
  g_print ("Selected '%s'\n", gst_element_factory_get_longname (selected_factory));
  vis_plugin = gst_element_factory_create (selected_factory, NULL);
  if (!vis_plugin)
    return -1;

  /* Build the pipeline */
  pipeline = gst_parse_launch ("playbin uri=http://radio.hbr1.com:19800/ambient.ogg", NULL);

  /* Set the visualization flag */
  g_object_get (pipeline, "flags", &flags, NULL);
  flags |= GST_PLAY_FLAG_VIS;
  g_object_set (pipeline, "flags", flags, NULL);

  /* set vis plugin for playbin */
  g_object_set (pipeline, "vis-plugin", vis_plugin, 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_plugin_feature_list_free (list);
  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-6.c -o playback-tutorial-6 `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 plays music streamed from the HBR1 Internet radio station. A window should open displaying somewhat psychedelic color patterns moving with the music. The media is fetched from the Internet, so the window might take a few seconds to appear, depending on your connection speed.

Required libraries: gstreamer-1.0

Walkthrough

First off, we indicate playbin that we want an audio visualization by setting the GST_PLAY_FLAG_VIS flag. If the media already contains video, this flag has no effect.

/* Set the visualization flag */
g_object_get (pipeline, "flags", &flags, NULL);
flags |= GST_PLAY_FLAG_VIS;
g_object_set (pipeline, "flags", flags, NULL);

If no visualization plugin is enforced by the user, playbin will use goom (audio visualization will be disabled if goom is not available). The rest of the tutorial shows how to find out the available visualization elements and enforce one to playbin.

/* Get a list of all visualization plugins */
list = gst_registry_feature_filter (gst_registry_get (), filter_vis_features, FALSE, NULL);

gst_registry_feature_filter() examines all elements currently in the GStreamer registry and selects those for which the filter_vis_features function returns TRUE. This function selects only the Visualization plugins:

/* Return TRUE if this is a Visualization element */
static gboolean filter_vis_features (GstPluginFeature *feature, gpointer data) {
  GstElementFactory *factory;

  if (!GST_IS_ELEMENT_FACTORY (feature))
    return FALSE;
  factory = GST_ELEMENT_FACTORY (feature);
  if (!g_strrstr (gst_element_factory_get_klass (factory), "Visualization"))
    return FALSE;

  return TRUE;
}

A bit of theory regarding the organization of GStreamer elements is in place: Each of the files that GStreamer loads at runtime is known as a Plugin (GstPlugin). A Plugin can contain many Features (GstPluginFeature). There are different kinds of Features, among them, the Element Factories (GstElementFactory) that we have been using to build Elements (GstElement).

This function simply disregards all Features which are not Factories, and then all Factories whose class (obtained with gst_element_factory_get_klass()) does not include “Visualization”. As stated in the documentation for GstElementFactory, a Factory’s class is a “string describing the type of element, as an unordered list separated with slashes (/)”. Examples of classes are “Source/Network”, “Codec/Decoder/Video”, “Codec/Encoder/Audio” or “Visualization”.

/* Print their names */
g_print("Available visualization plugins:\n");
for (walk = list; walk != NULL; walk = g_list_next (walk)) {
  const gchar *name;
  GstElementFactory *factory;

  factory = GST_ELEMENT_FACTORY (walk->data);
  name = gst_element_factory_get_longname (factory);
  g_print("  %s\n", name);

  if (selected_factory == NULL || g_str_has_prefix (name, "GOOM")) {
    selected_factory = factory;
  }
}

Once we have the list of Visualization plugins, we print their names (gst_element_factory_get_longname()) and choose one (in this case, GOOM).

/* We have now selected a factory for the visualization element */
g_print ("Selected '%s'\n", gst_element_factory_get_longname (selected_factory));
vis_plugin = gst_element_factory_create (selected_factory, NULL);
if (!vis_plugin)
  return -1;

The selected factory is used to instantiate an actual GstElement which is then passed to playbin through the vis-plugin property:

/* set vis plugin for playbin */
g_object_set (pipeline, "vis-plugin", vis_plugin, NULL);

And we are done.

Conclusion

This tutorial has shown:

How to enable Audio Visualization in playbin with the GST_PLAY_FLAG_VIS flag
How to enforce one particular visualization element with the vis-plugin playbin property

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

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