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><A
NAME="section-components-decodebin"
>19.2. Decodebin</A
></H1
><P
> 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 <SPAN
CLASS="QUOTE"
>"new-decoded-pad"</SPAN
>
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 <SPAN
CLASS="QUOTE"
>"unknown-type"</SPAN
> signal. The application is then
responsible for reporting the error to the user.
</P
><PRE
CLASS="programlisting"
> #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_static_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);
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 (&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_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;
}
</PRE
><P
> Decodebin, similar to playbin, supports the following features:
</P
><P
></P
><UL
><LI
><P
> Can decode an unlimited number of contained streams to decoded
output pads.
</P
></LI
><LI
><P
> Is handled as a <CODE
CLASS="classname"
>GstElement</CODE
> in all ways,
including tag or error forwarding and state handling.
</P
></LI
></UL
><P
> Although decodebin is a good autoplugger, there's a whole lot of
things that it does not do and is not intended to do:
</P
><P
></P
><UL
><LI
><P
> Taking care of input streams with a known media type (e.g. a DVD,
an audio-CD or such).
</P
></LI
><LI
><P
> Selection of streams (e.g. which audio track to play in case of
multi-language media streams).
</P
></LI
><LI
><P
> Overlaying subtitles over a decoded video stream.
</P
></LI
></UL
><P
> Decodebin can be easily tested on the commandline, e.g. by using the
command <B
CLASS="command"
>gst-launch-0.10 filesrc location=file.ogg ! decodebin
! audioconvert ! audioresample ! autoaudiosink</B
>.
</P
><P
> New applications should use decodebin2 instead of the old decodebin.
</P
><P
> The uridecodebin element is very similar to decodebin2, only that it
automatically plugs a source plugin based on the protocol of the URI
given.
</P
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