gstreamer/docs/manual/highlevel-components.xml
2011-05-16 16:10:49 +02:00

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<chapter id="chapter-components">
<title>Components</title>
<para>
&GStreamer; includes several higher-level components to simplify an
application developer's life. All of the components discussed here (for now) are
targetted at media playback. The idea of each of these components is
to integrate as closely as possible with a &GStreamer; pipeline, but
to hide the complexity of media type detection and several other
rather complex topics that have been discussed in <xref
linkend="part-advanced"/>.
</para>
<para>
We currently recommend people to use either playbin (see <xref
linkend="section-components-playbin"/>) or decodebin (see <xref
linkend="section-components-decodebin"/>), depending on their needs.
Playbin is the recommended solution for everything related to simple
playback of media that should just work. Decodebin is a more flexible
autoplugger that could be used to add more advanced features, such
as playlist support, crossfading of audio tracks and so on. Its
programming interface is more low-level than that of playbin, though.
</para>
<sect1 id="section-components-playbin">
<title>Playbin</title>
<para>
Playbin is an element that can be created using the standard &GStreamer;
API (e.g. <function>gst_element_factory_make ()</function>). The factory
is conveniently called <quote>playbin</quote>. By being a
<classname>GstPipeline</classname> (and thus a
<classname>GstElement</classname>), playbin automatically supports all
of the features of this class, including error handling, tag support,
state handling, getting stream positions, seeking, and so on.
</para>
<para>
Setting up a playbin pipeline is as simple as creating an instance of
the playbin element, setting a file location using the
<quote>uri</quote> property on playbin, and then setting the element
to the <classname>GST_STATE_PLAYING</classname> state (the location has to be a valid
URI, so <quote>&lt;protocol&gt;://&lt;location&gt;</quote>, e.g.
file:///tmp/my.ogg or http://www.example.org/stream.ogg). Internally,
playbin will set up a pipeline to playback the media location.
</para>
<programlisting><!-- example-begin playbin.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end playbin.c a -->
[.. my_bus_callback goes here ..]<!-- example-begin playbin.c b --><!--
static gboolean
my_bus_callback (GstBus *bus,
GstMessage *message,
gpointer data)
{
GMainLoop *loop = data;
switch (GST_MESSAGE_TYPE (message)) {
case GST_MESSAGE_ERROR: {
GError *err;
gchar *debug;
gst_message_parse_error (message, &amp;err, &amp;debug);
g_print ("Error: %s\n", err-&gt;message);
g_error_free (err);
g_free (debug);
g_main_loop_quit (loop);
break;
}
case GST_MESSAGE_EOS:
/* end-of-stream */
g_main_loop_quit (loop);
break;
default:
/* unhandled message */
break;
}
/* remove message from the queue */
return TRUE;
}
--><!-- example-end playbin.c b -->
<!-- example-begin playbin.c c -->
gint
main (gint argc,
gchar *argv[])
{
GMainLoop *loop;
GstElement *play;
GstBus *bus;
/* init GStreamer */
gst_init (&amp;argc, &amp;argv);
loop = g_main_loop_new (NULL, FALSE);
/* make sure we have a URI */
if (argc != 2) {
g_print ("Usage: %s &lt;URI&gt;\n", argv[0]);
return -1;
}
/* set up */
play = gst_element_factory_make ("playbin", "play");
g_object_set (G_OBJECT (play), "uri", argv[1], NULL);
bus = gst_pipeline_get_bus (GST_PIPELINE (play));
gst_bus_add_watch (bus, my_bus_callback, loop);
gst_object_unref (bus);
gst_element_set_state (play, GST_STATE_PLAYING);
/* now run */
g_main_loop_run (loop);
/* also clean up */
gst_element_set_state (play, GST_STATE_NULL);
gst_object_unref (GST_OBJECT (play));
return 0;
}
<!-- example-end playbin.c c --></programlisting>
<para>
Playbin has several features that have been discussed previously:
</para>
<itemizedlist>
<listitem>
<para>
Settable video and audio output (using the <quote>video-sink</quote>
and <quote>audio-sink</quote> properties).
</para>
</listitem>
<listitem>
<para>
Mostly controllable and trackable as a
<classname>GstElement</classname>, including error handling, eos
handling, tag handling, state handling (through the
<classname>GstBus</classname>), media position handling and
seeking.
</para>
</listitem>
<listitem>
<para>
Buffers network-sources, with buffer fullness notifications being
passed through the <classname>GstBus</classname>.
</para>
</listitem>
<listitem>
<para>
Supports visualizations for audio-only media.
</para>
</listitem>
<listitem>
<para>
Supports subtitles, both in the media as well as from separate
files. For separate subtitle files, use the <quote>suburi</quote>
property.
</para>
</listitem>
<listitem>
<para>
Supports stream selection and disabling. If your media has
multiple audio or subtitle tracks, you can dynamically choose
which one to play back, or decide to turn it off alltogther
(which is especially useful to turn off subtitles). For each
of those, use the <quote>current-text</quote> and other related
properties.
</para>
</listitem>
</itemizedlist>
<para>
For convenience, it is possible to test <quote>playbin</quote> on
the commandline, using the command <quote>gst-launch-0.10 playbin
uri=file:///path/to/file</quote>.
</para>
<para>
New applications should use playbin2 instead of the old playbin.
</para>
</sect1>
<sect1 id="section-components-decodebin">
<title>Decodebin</title>
<para>
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 <quote>new-decoded-pad</quote>
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 <quote>unknown-type</quote> signal. The application is then
responsible for reporting the error to the user.
</para>
<programlisting><!-- example-begin decodebin.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end decodebin.c a -->
[.. my_bus_callback goes here ..]<!-- example-begin decodebin.c b --><!--
static gboolean
my_bus_callback (GstBus *bus,
GstMessage *message,
gpointer data)
{
GMainLoop *loop = data;
switch (GST_MESSAGE_TYPE (message)) {
case GST_MESSAGE_ERROR: {
GError *err;
gchar *debug;
gst_message_parse_error (message, &amp;err, &amp;debug);
g_print ("Error: %s\n", err-&gt;message);
g_error_free (err);
g_free (debug);
g_main_loop_quit (loop);
break;
}
case GST_MESSAGE_EOS:
/* end-of-stream */
g_main_loop_quit (loop);
break;
default:
/* unhandled message */
break;
}
/* remove message from the queue */
return TRUE;
}
--><!-- example-end decodebin.c b -->
<!-- example-begin decodebin.c c -->
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_static_pad (audio, "sink");
if (GST_PAD_IS_LINKED (audiopad)) {
g_object_unref (audiopad);
return;
}
/* check media type */
caps = gst_pad_get_caps (pad, NULL);
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);
g_object_unref (audiopad);
}
gint
main (gint argc,
gchar *argv[])
{
GMainLoop *loop;
GstElement *src, *dec, *conv, *sink;
GstPad *audiopad;
GstBus *bus;
/* init GStreamer */
gst_init (&amp;argc, &amp;argv);
loop = g_main_loop_new (NULL, FALSE);
/* make sure we have input */
if (argc != 2) {
g_print ("Usage: %s &lt;filename&gt;\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_static_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;
}
<!-- example-end decodebin.c c --></programlisting>
<para>
Decodebin, similar to playbin, supports the following features:
</para>
<itemizedlist>
<listitem>
<para>
Can decode an unlimited number of contained streams to decoded
output pads.
</para>
</listitem>
<listitem>
<para>
Is handled as a <classname>GstElement</classname> in all ways,
including tag or error forwarding and state handling.
</para>
</listitem>
</itemizedlist>
<para>
Although decodebin is a good autoplugger, there's a whole lot of
things that it does not do and is not intended to do:
</para>
<itemizedlist>
<listitem>
<para>
Taking care of input streams with a known media type (e.g. a DVD,
an audio-CD or such).
</para>
</listitem>
<listitem>
<para>
Selection of streams (e.g. which audio track to play in case of
multi-language media streams).
</para>
</listitem>
<listitem>
<para>
Overlaying subtitles over a decoded video stream.
</para>
</listitem>
</itemizedlist>
<para>
Decodebin can be easily tested on the commandline, e.g. by using the
command <command>gst-launch-0.10 filesrc location=file.ogg ! decodebin
! audioconvert ! audioresample ! autoaudiosink</command>.
</para>
<para>
New applications should use decodebin2 instead of the old decodebin.
</para>
<para>
The uridecodebin element is very similar to decodebin2, only that it
automatically plugs a source plugin based on the protocol of the URI
given.
</para>
</sect1>
</chapter>