Your first application

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.

 

现在结合Hello world的示例来讲解:

#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;
/* Initialisation */
gst_init (&argc, &argv);
loop = g_main_loop_new (NULL, FALSE);
/* Check input arguments */
if (argc != 2) {
g_printerr ("Usage: %s \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));
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));
return 0;
}

 

元素是最小的功能单位（注意，bin或者pipeline也是元素，它们可以提供复合的功能），它们被包含在不同的插件中（一个插件可能含有一个或者多个元素），而这些插件被分类为：

• gst-plugins-base: an essential exemplary set of elements
• gst-plugins-good: a set of good-quality plug-ins under LGPL
• gst-plugins-ugly: a set of good-quality plug-ins that might pose distribution problems
• gst-plugins-bad: a set of plug-ins that need more quality
• gst-python: the python bindings
• a few others packages

而按照功能的分类:

• protocols handling
• sources: for audio and video (involves protocol plugins)
• formats: parsers, formaters, muxers, demuxers, metadata, subtitles
• codecs: coders and decoders
• filters: converters, mixers, effects, ...
• sinks: for audio and video (involves protocol plugins)

 

再次看这种示意图：

 

 

 现在把示例改为播放MP3文件:

问题在于文件中的tags信息，如果没有被处理而直接传给decoder的话，就会发生error而停止。虽然知道有id3demux等提取tags信息的，但是提取后的信息要传个fakesink(这样会在控制台打印出来），而不是audiosink,把它们从流中取走，而把净数据传个decoder的机制一定存在。