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https://gitlab.freedesktop.org/gstreamer/gstreamer.git
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617 lines
18 KiB
XML
617 lines
18 KiB
XML
<chapter id="chapter-playback-components">
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<title>Playback Components</title>
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<para>
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&GStreamer; includes several higher-level components to simplify an
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application developer's life. All of the components discussed here (for now) are
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targetted at media playback. The idea of each of these components is
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to integrate as closely as possible with a &GStreamer; pipeline, but
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to hide the complexity of media type detection and several other
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rather complex topics that have been discussed in <xref
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linkend="part-advanced"/>.
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</para>
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<para>
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We currently recommend people to use either playbin (see <xref
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linkend="section-components-playbin"/>) or decodebin (see <xref
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linkend="section-components-decodebin"/>), depending on their needs.
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Playbin is the recommended solution for everything related to simple
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playback of media that should just work. Decodebin is a more flexible
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autoplugger that could be used to add more advanced features, such
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as playlist support, crossfading of audio tracks and so on. Its
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programming interface is more low-level than that of playbin, though.
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</para>
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<sect1 id="section-components-playbin">
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<title>Playbin</title>
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<para>
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Playbin is an element that can be created using the standard &GStreamer;
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API (e.g. <function>gst_element_factory_make ()</function>). The factory
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is conveniently called <quote>playbin</quote>. By being a
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<classname>GstPipeline</classname> (and thus a
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<classname>GstElement</classname>), playbin automatically supports all
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of the features of this class, including error handling, tag support,
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state handling, getting stream positions, seeking, and so on.
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</para>
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<para>
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Setting up a playbin pipeline is as simple as creating an instance of
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the playbin element, setting a file location using the
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<quote>uri</quote> property on playbin, and then setting the element
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to the <classname>GST_STATE_PLAYING</classname> state (the location has to be a valid
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URI, so <quote><protocol>://<location></quote>, e.g.
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file:///tmp/my.ogg or http://www.example.org/stream.ogg). Internally,
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playbin will set up a pipeline to playback the media location.
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</para>
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<programlisting><!-- example-begin playbin.c a -->
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#include <gst/gst.h>
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<!-- example-end playbin.c a -->
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[.. my_bus_callback goes here ..]<!-- example-begin playbin.c b --><!--
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static gboolean
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my_bus_callback (GstBus *bus,
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GstMessage *message,
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gpointer data)
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{
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GMainLoop *loop = data;
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switch (GST_MESSAGE_TYPE (message)) {
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case GST_MESSAGE_ERROR: {
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GError *err;
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gchar *debug;
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gst_message_parse_error (message, &err, &debug);
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g_print ("Error: %s\n", err->message);
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g_error_free (err);
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g_free (debug);
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g_main_loop_quit (loop);
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break;
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}
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case GST_MESSAGE_EOS:
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/* end-of-stream */
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g_main_loop_quit (loop);
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break;
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default:
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/* unhandled message */
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break;
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}
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/* remove message from the queue */
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return TRUE;
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}
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--><!-- example-end playbin.c b -->
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<!-- example-begin playbin.c c -->
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gint
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main (gint argc,
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gchar *argv[])
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{
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GMainLoop *loop;
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GstElement *play;
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GstBus *bus;
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/* init GStreamer */
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gst_init (&argc, &argv);
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loop = g_main_loop_new (NULL, FALSE);
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/* make sure we have a URI */
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if (argc != 2) {
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g_print ("Usage: %s <URI>\n", argv[0]);
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return -1;
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}
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/* set up */
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play = gst_element_factory_make ("playbin", "play");
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g_object_set (G_OBJECT (play), "uri", argv[1], NULL);
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bus = gst_pipeline_get_bus (GST_PIPELINE (play));
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gst_bus_add_watch (bus, my_bus_callback, loop);
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gst_object_unref (bus);
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gst_element_set_state (play, GST_STATE_PLAYING);
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/* now run */
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g_main_loop_run (loop);
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/* also clean up */
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gst_element_set_state (play, GST_STATE_NULL);
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gst_object_unref (GST_OBJECT (play));
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return 0;
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}
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<!-- example-end playbin.c c --></programlisting>
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<para>
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Playbin has several features that have been discussed previously:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Settable video and audio output (using the <quote>video-sink</quote>
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and <quote>audio-sink</quote> properties).
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</para>
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</listitem>
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<listitem>
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<para>
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Mostly controllable and trackable as a
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<classname>GstElement</classname>, including error handling, eos
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handling, tag handling, state handling (through the
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<classname>GstBus</classname>), media position handling and
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seeking.
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</para>
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</listitem>
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<listitem>
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<para>
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Buffers network-sources, with buffer fullness notifications being
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passed through the <classname>GstBus</classname>.
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</para>
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</listitem>
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<listitem>
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<para>
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Supports visualizations for audio-only media.
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</para>
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</listitem>
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<listitem>
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<para>
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Supports subtitles, both in the media as well as from separate
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files. For separate subtitle files, use the <quote>suburi</quote>
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property.
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</para>
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</listitem>
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<listitem>
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<para>
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Supports stream selection and disabling. If your media has
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multiple audio or subtitle tracks, you can dynamically choose
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which one to play back, or decide to turn it off altogether
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(which is especially useful to turn off subtitles). For each
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of those, use the <quote>current-text</quote> and other related
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properties.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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For convenience, it is possible to test <quote>playbin</quote> on
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the commandline, using the command <quote>gst-launch-1.0 playbin
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uri=file:///path/to/file</quote>.
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</para>
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</sect1>
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<sect1 id="section-components-decodebin">
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<title>Decodebin</title>
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<para>
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Decodebin is the actual autoplugger backend of playbin, which was
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discussed in the previous section. Decodebin will, in short, accept
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input from a source that is linked to its sinkpad and will try to
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detect the media type contained in the stream, and set up decoder
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routines for each of those. It will automatically select decoders.
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For each decoded stream, it will emit the <quote>pad-added</quote>
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signal, to let the client know about the newly found decoded stream.
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For unknown streams (which might be the whole stream), it will emit
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the <quote>unknown-type</quote> signal. The application is then
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responsible for reporting the error to the user.
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</para>
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<programlisting><!-- example-begin decodebin.c a -->
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<![CDATA[
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#include <gst/gst.h>
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]]>
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<!-- example-end decodebin.c a -->
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[.. my_bus_callback goes here ..]<!-- example-begin decodebin.c b -->
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<!--
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static gboolean
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my_bus_callback (GstBus *bus,
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GstMessage *message,
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gpointer data)
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{
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GMainLoop *loop = data;
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switch (GST_MESSAGE_TYPE (message)) {
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case GST_MESSAGE_ERROR: {
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GError *err;
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gchar *debug;
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gst_message_parse_error (message, &err, &debug);
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g_print ("Error: %s\n", err->message);
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g_error_free (err);
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g_free (debug);
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g_main_loop_quit (loop);
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break;
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}
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case GST_MESSAGE_EOS:
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/* end-of-stream */
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g_main_loop_quit (loop);
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break;
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default:
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/* unhandled message */
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break;
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}
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/* remove message from the queue */
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return TRUE;
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}
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--><!-- example-end decodebin.c b -->
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<!-- example-begin decodebin.c c -->
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<![CDATA[
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GstElement *pipeline, *audio;
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static void
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cb_newpad (GstElement *decodebin,
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GstPad *pad,
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gpointer data)
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{
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GstCaps *caps;
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GstStructure *str;
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GstPad *audiopad;
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/* only link once */
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audiopad = gst_element_get_static_pad (audio, "sink");
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if (GST_PAD_IS_LINKED (audiopad)) {
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g_object_unref (audiopad);
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return;
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}
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/* check media type */
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caps = gst_pad_query_caps (pad, NULL);
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str = gst_caps_get_structure (caps, 0);
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if (!g_strrstr (gst_structure_get_name (str), "audio")) {
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gst_caps_unref (caps);
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gst_object_unref (audiopad);
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return;
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}
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gst_caps_unref (caps);
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/* link'n'play */
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gst_pad_link (pad, audiopad);
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g_object_unref (audiopad);
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}
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gint
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main (gint argc,
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gchar *argv[])
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{
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GMainLoop *loop;
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GstElement *src, *dec, *conv, *sink;
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GstPad *audiopad;
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GstBus *bus;
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/* init GStreamer */
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gst_init (&argc, &argv);
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loop = g_main_loop_new (NULL, FALSE);
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/* make sure we have input */
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if (argc != 2) {
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g_print ("Usage: %s <filename>\n", argv[0]);
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return -1;
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}
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/* setup */
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pipeline = gst_pipeline_new ("pipeline");
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bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
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gst_bus_add_watch (bus, my_bus_callback, loop);
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gst_object_unref (bus);
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src = gst_element_factory_make ("filesrc", "source");
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g_object_set (G_OBJECT (src), "location", argv[1], NULL);
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dec = gst_element_factory_make ("decodebin", "decoder");
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g_signal_connect (dec, "pad-added", G_CALLBACK (cb_newpad), NULL);
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gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
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gst_element_link (src, dec);
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/* create audio output */
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audio = gst_bin_new ("audiobin");
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conv = gst_element_factory_make ("audioconvert", "aconv");
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audiopad = gst_element_get_static_pad (conv, "sink");
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sink = gst_element_factory_make ("alsasink", "sink");
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gst_bin_add_many (GST_BIN (audio), conv, sink, NULL);
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gst_element_link (conv, sink);
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gst_element_add_pad (audio,
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gst_ghost_pad_new ("sink", audiopad));
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gst_object_unref (audiopad);
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gst_bin_add (GST_BIN (pipeline), audio);
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/* run */
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gst_element_set_state (pipeline, GST_STATE_PLAYING);
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g_main_loop_run (loop);
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/* cleanup */
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gst_element_set_state (pipeline, GST_STATE_NULL);
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gst_object_unref (GST_OBJECT (pipeline));
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return 0;
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}
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]]>
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<!-- example-end decodebin.c c --></programlisting>
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<para>
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Decodebin, similar to playbin, supports the following features:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Can decode an unlimited number of contained streams to decoded
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output pads.
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</para>
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</listitem>
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<listitem>
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<para>
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Is handled as a <classname>GstElement</classname> in all ways,
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including tag or error forwarding and state handling.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Although decodebin is a good autoplugger, there's a whole lot of
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things that it does not do and is not intended to do:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Taking care of input streams with a known media type (e.g. a DVD,
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an audio-CD or such).
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</para>
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</listitem>
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<listitem>
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<para>
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Selection of streams (e.g. which audio track to play in case of
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multi-language media streams).
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</para>
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</listitem>
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<listitem>
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<para>
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Overlaying subtitles over a decoded video stream.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Decodebin can be easily tested on the commandline, e.g. by using the
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command <command>gst-launch-1.0 filesrc location=file.ogg ! decodebin
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! audioconvert ! audioresample ! autoaudiosink</command>.
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</para>
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</sect1>
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<sect1 id="section-components-uridecodebin">
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<title>URIDecodebin</title>
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<para>
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The uridecodebin element is very similar to decodebin, only that it
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automatically plugs a source plugin based on the protocol of the URI
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given.
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</para>
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<para>
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Uridecodebin will also automatically insert buffering elements when
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the uri is a slow network source. The buffering element will post
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BUFFERING messages that the application needs to handle as explained
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in <xref linkend="chapter-buffering"/>.
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The following properties can be used to configure the buffering method:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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The buffer-size property allows you to configure a maximum size in
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bytes for the buffer element.
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</para>
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</listitem>
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<listitem>
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<para>
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The buffer-duration property allows you to configure a maximum size
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in time for the buffer element. The time will be estimated based on
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the bitrate of the network.
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</para>
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</listitem>
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<listitem>
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<para>
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With the download property you can enable the download buffering
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method as described in <xref linkend="section-buffering-download"/>.
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Setting this option to TRUE will only enable download buffering
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for selected formats such as quicktime, flash video, avi and
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webm.
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</para>
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</listitem>
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<listitem>
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<para>
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You can also enable buffering on the parsed/demuxed data with the
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use-buffering property. This is interesting to enable buffering
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on slower random access media such as a network file server.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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URIDecodebin can be easily tested on the commandline, e.g. by using the
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command <command>gst-launch-1.0 uridecodebin uri=file:///file.ogg !
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! audioconvert ! audioresample ! autoaudiosink</command>.
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</para>
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</sect1>
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<sect1 id="section-components-playsink">
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<title>Playsink</title>
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<para>
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The playsink element is a powerful sink element. It has request pads
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for raw decoded audio, video and text and it will configure itself to
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play the media streams. It has the following features:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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It exposes GstStreamVolume, GstVideoOverlay, GstNavigation and
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GstColorBalance interfaces and automatically plugs software
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elements to implement the interfaces when needed.
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</para>
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</listitem>
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<listitem>
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<para>
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It will automatically plug conversion elements.
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</para>
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</listitem>
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<listitem>
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<para>
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Can optionally render visualizations when there is no video input.
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</para>
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</listitem>
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<listitem>
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<para>
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Configurable sink elements.
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</para>
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</listitem>
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<listitem>
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<para>
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Configurable audio/video sync offset to fine-tune synchronization
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in badly muxed files.
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</para>
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</listitem>
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<listitem>
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<para>
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Support for taking a snapshot of the last video frame.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Below is an example of how you can use playsink. We use a uridecodebin
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element to decode into raw audio and video streams which we then link
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to the playsink request pads. We only link the first audio and video
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pads, you could use an input-selector to link all pads.
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</para>
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<programlisting>
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<!-- example-begin playsink.c a -->
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<![CDATA[
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#include <gst/gst.h>
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]]>
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<!-- example-end playsink.c a -->
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[.. my_bus_callback goes here ..]
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<!-- example-begin playsink.c b -->
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<!--
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static gboolean
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my_bus_callback (GstBus *bus,
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GstMessage *message,
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gpointer data)
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{
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GMainLoop *loop = data;
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switch (GST_MESSAGE_TYPE (message)) {
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case GST_MESSAGE_ERROR: {
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GError *err;
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gchar *debug;
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gst_message_parse_error (message, &err, &debug);
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g_print ("Error: %s\n", err->message);
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g_error_free (err);
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g_free (debug);
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g_main_loop_quit (loop);
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break;
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}
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case GST_MESSAGE_EOS:
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/* end-of-stream */
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g_main_loop_quit (loop);
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break;
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default:
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/* unhandled message */
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break;
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}
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/* remove message from the queue */
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return TRUE;
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}
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-->
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<!-- example-end playsink.c b -->
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<!-- example-begin playsink.c c -->
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<![CDATA[
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GstElement *pipeline, *sink;
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static void
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cb_pad_added (GstElement *dec,
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GstPad *pad,
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gpointer data)
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|
{
|
|
GstCaps *caps;
|
|
GstStructure *str;
|
|
const gchar *name;
|
|
GstPadTemplate *templ;
|
|
GstElementClass *klass;
|
|
|
|
/* check media type */
|
|
caps = gst_pad_query_caps (pad, NULL);
|
|
str = gst_caps_get_structure (caps, 0);
|
|
name = gst_structure_get_name (str);
|
|
|
|
klass = GST_ELEMENT_GET_CLASS (sink);
|
|
|
|
if (g_str_has_prefix (name, "audio")) {
|
|
templ = gst_element_class_get_pad_template (klass, "audio_sink");
|
|
} else if (g_str_has_prefix (name, "video")) {
|
|
templ = gst_element_class_get_pad_template (klass, "video_sink");
|
|
} else if (g_str_has_prefix (name, "text")) {
|
|
templ = gst_element_class_get_pad_template (klass, "text_sink");
|
|
} else {
|
|
templ = NULL;
|
|
}
|
|
|
|
if (templ) {
|
|
GstPad *sinkpad;
|
|
|
|
sinkpad = gst_element_request_pad (sink, templ, NULL, NULL);
|
|
|
|
if (!gst_pad_is_linked (sinkpad))
|
|
gst_pad_link (pad, sinkpad);
|
|
|
|
gst_object_unref (sinkpad);
|
|
}
|
|
}
|
|
|
|
gint
|
|
main (gint argc,
|
|
gchar *argv[])
|
|
{
|
|
GMainLoop *loop;
|
|
GstElement *dec;
|
|
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 <uri>\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);
|
|
|
|
dec = gst_element_factory_make ("uridecodebin", "source");
|
|
g_object_set (G_OBJECT (dec), "uri", argv[1], NULL);
|
|
g_signal_connect (dec, "pad-added", G_CALLBACK (cb_pad_added), NULL);
|
|
|
|
/* create audio output */
|
|
sink = gst_element_factory_make ("playsink", "sink");
|
|
gst_util_set_object_arg (G_OBJECT (sink), "flags",
|
|
"soft-colorbalance+soft-volume+vis+text+audio+video");
|
|
gst_bin_add_many (GST_BIN (pipeline), dec, sink, NULL);
|
|
|
|
/* 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;
|
|
}
|
|
]]>
|
|
<!-- example-end playsink.c c -->
|
|
</programlisting>
|
|
<para>
|
|
This example will show audio and video depending on what you
|
|
give it. Try this example on an audio file and you will see that
|
|
it shows visualizations. You can change the visualization at runtime by
|
|
changing the vis-plugin property.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|