Table of Contents
This chapter will summarize everything you've learned in the previous chapters. It describes all aspects of a simple GStreamer application, including initializing libraries, creating elements, packing elements together in a pipeline and playing this pipeline. By doing all this, you will be able to build a simple Ogg/Vorbis audio player.
We're going to create a simple first application, a simple Ogg/Vorbis command-line audio player. For this, we will use only standard GStreamer components. The player will read a file specified on the command-line. Let's get started!
We've learned, in Chapter 4, Initializing GStreamer, that the first thing
to do in your application is to initialize GStreamer by calling
gst_init ()
. Also, make sure that the application
includes gst/gst.h
so all function names and
objects are properly defined. Use #include
<gst/gst.h>
to do that.
Next, you'll want to create the different elements using
gst_element_factory_make ()
. For an Ogg/Vorbis
audio player, we'll need a source element that reads files from a
disk. GStreamer includes this element under the name
“filesrc”. Next, we'll need something to parse the
file and decode it into raw audio. GStreamer has two elements
for this: the first parses Ogg streams into elementary streams (video,
audio) and is called “oggdemux”. The second is a Vorbis
audio decoder, it's conveniently called “vorbisdec”.
Since “oggdemux” creates dynamic pads for each elementary
stream, you'll need to set a “pad-added” event handler
on the “oggdemux” element, like you've learned in
the section called “Dynamic (or sometimes) pads”, to link the Ogg demuxer and
the Vorbis decoder elements together. At last, we'll also need an
audio output element, we will use “autoaudiosink”, which
automatically detects your audio device.
The last thing left to do is to add all elements into a container
element, a GstPipeline
, and wait until
we've played the whole song. We've previously
learned how to add elements to a container bin in Chapter 6, Bins, and we've learned about element states
in the section called “Element States”. We will also attach
a message handler to the pipeline bus so we can retrieve errors
and detect the end-of-stream.
Let's now add all the code together to get our very first audio player:
#include <gst/gst.h> #include <glib.h> static gboolean bus_call (GstBus *bus, GstMessage *msg, gpointer data) { GMainLoop *loop = (GMainLoop *) data; switch (GST_MESSAGE_TYPE (msg)) { case GST_MESSAGE_EOS: g_print ("End of stream\n"); g_main_loop_quit (loop); break; case GST_MESSAGE_ERROR: { gchar *debug; GError *error; gst_message_parse_error (msg, &error, &debug); g_free (debug); g_printerr ("Error: %s\n", error->message); g_error_free (error); g_main_loop_quit (loop); break; } default: break; } return TRUE; } static void on_pad_added (GstElement *element, GstPad *pad, gpointer data) { GstPad *sinkpad; GstElement *decoder = (GstElement *) data; /* We can now link this pad with the vorbis-decoder sink pad */ g_print ("Dynamic pad created, linking demuxer/decoder\n"); sinkpad = gst_element_get_static_pad (decoder, "sink"); gst_pad_link (pad, sinkpad); gst_object_unref (sinkpad); } int main (int argc, char *argv[]) { GMainLoop *loop; GstElement *pipeline, *source, *demuxer, *decoder, *conv, *sink; GstBus *bus; guint bus_watch_id; /* Initialisation */ gst_init (&argc, &argv); loop = g_main_loop_new (NULL, FALSE); /* Check input arguments */ if (argc != 2) { g_printerr ("Usage: %s <Ogg/Vorbis filename>\n", argv[0]); return -1; } /* Create gstreamer elements */ pipeline = gst_pipeline_new ("audio-player"); source = gst_element_factory_make ("filesrc", "file-source"); demuxer = gst_element_factory_make ("oggdemux", "ogg-demuxer"); decoder = gst_element_factory_make ("vorbisdec", "vorbis-decoder"); conv = gst_element_factory_make ("audioconvert", "converter"); sink = gst_element_factory_make ("autoaudiosink", "audio-output"); if (!pipeline || !source || !demuxer || !decoder || !conv || !sink) { g_printerr ("One element could not be created. Exiting.\n"); return -1; } /* Set up the pipeline */ /* we set the input filename to the source element */ g_object_set (G_OBJECT (source), "location", argv[1], NULL); /* we add a message handler */ bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline)); bus_watch_id = gst_bus_add_watch (bus, bus_call, loop); gst_object_unref (bus); /* we add all elements into the pipeline */ /* file-source | ogg-demuxer | vorbis-decoder | converter | alsa-output */ gst_bin_add_many (GST_BIN (pipeline), source, demuxer, decoder, conv, sink, NULL); /* we link the elements together */ /* file-source -> ogg-demuxer ~> vorbis-decoder -> converter -> alsa-output */ gst_element_link (source, demuxer); gst_element_link_many (decoder, conv, sink, NULL); g_signal_connect (demuxer, "pad-added", G_CALLBACK (on_pad_added), decoder); /* note that the demuxer will be linked to the decoder dynamically. The reason is that Ogg may contain various streams (for example audio and video). The source pad(s) will be created at run time, by the demuxer when it detects the amount and nature of streams. Therefore we connect a callback function which will be executed when the "pad-added" is emitted.*/ /* Set the pipeline to "playing" state*/ g_print ("Now playing: %s\n", argv[1]); gst_element_set_state (pipeline, GST_STATE_PLAYING); /* Iterate */ g_print ("Running...\n"); g_main_loop_run (loop); /* Out of the main loop, clean up nicely */ g_print ("Returned, stopping playback\n"); gst_element_set_state (pipeline, GST_STATE_NULL); g_print ("Deleting pipeline\n"); gst_object_unref (GST_OBJECT (pipeline)); g_source_remove (bus_watch_id); g_main_loop_unref (loop); return 0; }
We now have created a complete pipeline. We can visualise the pipeline as follows: