1 <chapter id="chapter-autoplugging">
2 <title>Autoplugging</title>
4 In <xref linkend="chapter-helloworld"/>, you've learned to build a
5 simple media player for Ogg/Vorbis files. By using alternative elements,
6 you are able to build media players for other media types, such as
7 Ogg/Speex, MP3 or even video formats. However, you would rather want
8 to build an application that can automatically detect the media type
9 of a stream and automatically generate the best possible pipeline
10 by looking at all available elements in a system. This process is called
11 autoplugging, and &GStreamer; contains high-quality autopluggers. If
12 you're looking for an autoplugger, don't read any further and go to
13 <xref linkend="chapter-components"/>. This chapter will explain the
14 <emphasis>concept</emphasis> of autoplugging and typefinding. It will
15 explain what systems &GStreamer; includes to dynamically detect the
16 type of a media stream, and how to generate a pipeline of decoder
17 elements to playback this media. The same principles can also be used
18 for transcoding. Because of the full dynamicity of this concept,
19 &GStreamer; can be automatically extended to support new media types
20 without needing any adaptations to its autopluggers.
23 We will first introduce the concept of MIME types as a dynamic and
24 extendible way of identifying media streams. After that, we will introduce
25 the concept of typefinding to find the type of a media stream. Lastly,
26 we will explain how autoplugging and the &GStreamer; registry can be
27 used to setup a pipeline that will convert media from one mimetype to
28 another, for example for media decoding.
31 <sect1 id="section-mime">
32 <title>MIME-types as a way to identity streams</title>
34 We have previously introduced the concept of capabilities as a way
35 for elements (or, rather, pads) to agree on a media type when
36 streaming data from one element to the next (see <xref
37 linkend="section-caps"/>). We have explained that a capability is
38 a combination of a mimetype and a set of properties. For most
39 container formats (those are the files that you will find on your
40 hard disk; Ogg, for example, is a container format), no properties
41 are needed to describe the stream. Only a MIME-type is needed. A
42 full list of MIME-types and accompanying properties can be found
44 url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-types-definitions.html">the
45 Plugin Writer's Guide</ulink>.
48 An element must associate a MIME-type to its source and sink pads
49 when it is loaded into the system. &GStreamer; knows about the
50 different elements and what type of data they expect and emit through
51 the &GStreamer; registry. This allows for very dynamic and extensible
52 element creation as we will see.
56 In <xref linkend="chapter-helloworld"/>, we've learned to build a
57 music player for Ogg/Vorbis files. Let's look at the MIME-types
58 associated with each pad in this pipeline. <xref
59 linkend="section-mime-img"/> shows what MIME-type belongs to each
62 <!-- FIXME: update for ogg/vorbis rather than mp3 -->
63 <figure float="1" id="section-mime-img">
64 <title>The Hello world pipeline with MIME types</title>
67 <imagedata fileref="images/mime-world.ℑ" format="&IMAGE;"/>
72 Now that we have an idea how &GStreamer; identifies known media
73 streams, we can look at methods &GStreamer; uses to setup pipelines
74 for media handling and for media type detection.
78 <sect1 id="section-typefinding">
79 <title>Media stream type detection</title>
81 Usually, when loading a media stream, the type of the stream is not
82 known. This means that before we can choose a pipeline to decode the
83 stream, we first need to detect the stream type. &GStreamer; uses the
84 concept of typefinding for this. Typefinding is a normal part of a
85 pipeline, it will read data for as long as the type of a stream is
86 unknown. During this period, it will provide data to all plugins
87 that implement a typefinder. when one of the typefinders recognizes
88 the stream, the typefind element will emit a signal and act as a
89 passthrough module from that point on. If no type was found, it will
90 emit an error and further media processing will stop.
93 Once the typefind element has found a type, the application can
94 use this to plug together a pipeline to decode the media stream.
95 This will be discussed in the next section.
98 Plugins in &GStreamer; can, as mentioned before, implement typefinder
99 functionality. A plugin implementing this functionality will submit
100 a mimetype, optionally a set of file extensions commonly used for this
101 media type, and a typefind function. Once this typefind function inside
102 the plugin is called, the plugin will see if the data in this media
103 stream matches a specific pattern that marks the media type identified
104 by that mimetype. If it does, it will notify the typefind element of
105 this fact, telling which mediatype was recognized and how certain we
106 are that this stream is indeed that mediatype. Once this run has been
107 completed for all plugins implementing a typefind functionality, the
108 typefind element will tell the application what kind of media stream
109 it thinks to have recognized.
112 The following code should explain how to use the typefind element.
113 It will print the detected media type, or tell that the media type
114 was not found. The next section will introduce more useful behaviours,
115 such as plugging together a decoding pipeline.
117 <programlisting><!-- example-begin typefind.c a -->
118 #include <gst/gst.h>
119 <!-- example-end typefind.c a -->
120 [.. my_bus_callback goes here ..]<!-- example-begin typefind.c b --><!--
122 my_bus_callback (GstBus *bus,
126 GMainLoop *loop = data;
128 switch (GST_MESSAGE_TYPE (message)) {
129 case GST_MESSAGE_ERROR: {
133 gst_message_parse_error (message, &err, &debug);
134 g_print ("Error: %s\n", err->message);
138 g_main_loop_quit (loop);
141 case GST_MESSAGE_EOS:
143 g_main_loop_quit (loop);
149 /* remove from queue */
152 --><!-- example-end typefind.c b -->
153 <!-- example-begin typefind.c c -->
155 idle_exit_loop (gpointer data)
157 g_main_loop_quit ((GMainLoop *) data);
164 cb_typefound (GstElement *typefind,
169 GMainLoop *loop = data;
172 type = gst_caps_to_string (caps);
173 g_print ("Media type %s found, probability %d%%\n", type, probability);
176 /* since we connect to a signal in the pipeline thread context, we need
177 * to set an idle handler to exit the main loop in the mainloop context.
178 * Normally, your app should not need to worry about such things. */
179 g_idle_add (idle_exit_loop, loop);
187 GstElement *pipeline, *filesrc, *typefind;
191 gst_init (&argc, &argv);
192 loop = g_main_loop_new (NULL, FALSE);
196 g_print ("Usage: %s <filename>\n", argv[0]);
200 /* create a new pipeline to hold the elements */
201 pipeline = gst_pipeline_new ("pipe");
203 bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
204 gst_bus_add_watch (bus, my_bus_callback, NULL);
205 gst_object_unref (bus);
207 /* create file source and typefind element */
208 filesrc = gst_element_factory_make ("filesrc", "source");
209 g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
210 typefind = gst_element_factory_make ("typefind", "typefinder");
211 g_signal_connect (typefind, "have-type", G_CALLBACK (cb_typefound), loop);
214 gst_bin_add_many (GST_BIN (pipeline), filesrc, typefind, NULL);
215 gst_element_link (filesrc, typefind);
216 gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
217 g_main_loop_run (loop);
220 gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);
221 gst_object_unref (GST_OBJECT (pipeline));
225 <!-- example-end typefind.c c --></programlisting>
227 Once a media type has been detected, you can plug an element (e.g. a
228 demuxer or decoder) to the source pad of the typefind element, and
229 decoding of the media stream will start right after.
233 <sect1 id="section-dynamic">
234 <title>Plugging together dynamic pipelines</title>
236 The code in this section is broken, outdated and overly complicated.
237 Also, you should use decodebin, playbin or uridecodebin to get
238 decoders plugged automatically.
241 In this chapter we will see how you can create a dynamic pipeline. A
242 dynamic pipeline is a pipeline that is updated or created while data
243 is flowing through it. We will create a partial pipeline first and add
244 more elements while the pipeline is playing. The basis of this player
245 will be the application that we wrote in the previous section (<xref
246 linkend="section-typefinding"/>) to identify unknown media streams.
248 <!-- example-begin dynamic.c a --><!--
249 #include <gst/gst.h>
251 GstElement *pipeline;
252 --><!-- example-end dynamic.c a -->
254 Once the type of the media has been found, we will find elements in
255 the registry that can decode this streamtype. For this, we will get
256 all element factories (which we've seen before in <xref
257 linkend="section-elements-create"/>) and find the ones with the
258 given MIME-type and capabilities on their sinkpad. Note that we will
259 only use parsers, demuxers and decoders. We will not use factories for
260 any other element types, or we might get into a loop of encoders and
261 decoders. For this, we will want to build a list of <quote>allowed</quote>
262 factories right after initializing &GStreamer;.
264 <programlisting><!-- example-begin dynamic.c b -->
265 static GList *factories;
268 * This function is called by the registry loader. Its return value
269 * (TRUE or FALSE) decides whether the given feature will be included
270 * in the list that we're generating further down.
274 cb_feature_filter (GstPluginFeature *feature,
280 /* we only care about element factories */
281 if (!GST_IS_ELEMENT_FACTORY (feature))
284 /* only parsers, demuxers and decoders */
285 klass = gst_element_factory_get_klass (GST_ELEMENT_FACTORY (feature));
286 if (g_strrstr (klass, "Demux") == NULL &&
287 g_strrstr (klass, "Decoder") == NULL &&
288 g_strrstr (klass, "Parse") == NULL)
291 /* only select elements with autoplugging rank */
292 rank = gst_plugin_feature_get_rank (feature);
293 if (rank < GST_RANK_MARGINAL)
300 * This function is called to sort features by rank.
304 cb_compare_ranks (GstPluginFeature *f1,
305 GstPluginFeature *f2)
307 return gst_plugin_feature_get_rank (f2) - gst_plugin_feature_get_rank (f1);
311 init_factories (void)
313 /* first filter out the interesting element factories */
314 factories = gst_registry_feature_filter (
315 gst_registry_get_default (),
316 (GstPluginFeatureFilter) cb_feature_filter, FALSE, NULL);
318 /* sort them according to their ranks */
319 factories = g_list_sort (factories, (GCompareFunc) cb_compare_ranks);
321 <!-- example-end dynamic.c b --></programlisting>
323 From this list of element factories, we will select the one that most
324 likely will help us decoding a media stream to a given output type.
325 For each newly created element, we will again try to autoplug new
326 elements to its source pad(s). Also, if the element has dynamic pads
327 (which we've seen before in <xref linkend="section-pads-dynamic"/>),
328 we will listen for newly created source pads and handle those, too.
329 The following code replaces the <function>cb_type_found</function>
330 from the previous section with a function to initiate autoplugging,
331 which will continue with the above approach.
333 <programlisting><!-- example-begin dynamic.c c -->
334 static void try_to_plug (GstPad *pad, const GstCaps *caps);
336 static GstElement *audiosink;
339 cb_newpad (GstElement *element,
345 caps = gst_pad_get_caps (pad);
346 try_to_plug (pad, caps);
347 gst_caps_unref (caps);
351 close_link (GstPad *srcpad,
352 GstElement *sinkelement,
353 const gchar *padname,
354 const GList *templlist)
357 gboolean has_dynamic_pads = FALSE;
359 g_print ("Plugging pad %s:%s to newly created %s:%s\n",
360 gst_object_get_name (GST_OBJECT (gst_pad_get_parent (srcpad))),
361 gst_pad_get_name (srcpad),
362 gst_object_get_name (GST_OBJECT (sinkelement)), padname);
364 /* add the element to the pipeline and set correct state */
365 if (sinkelement != audiosink) {
366 gst_bin_add (GST_BIN (pipeline), sinkelement);
367 gst_element_set_state (sinkelement, GST_STATE_READY);
369 pad = gst_element_get_pad (sinkelement, padname);
370 gst_pad_link (srcpad, pad);
371 if (sinkelement != audiosink) {
372 gst_element_set_state (sinkelement, GST_STATE_PAUSED);
374 gst_object_unref (GST_OBJECT (pad));
376 /* if we have static source pads, link those. If we have dynamic
377 * source pads, listen for pad-added signals on the element */
378 for ( ; templlist != NULL; templlist = templlist->next) {
379 GstStaticPadTemplate *templ = templlist->data;
381 /* only sourcepads, no request pads */
382 if (templ->direction != GST_PAD_SRC ||
383 templ->presence == GST_PAD_REQUEST) {
387 switch (templ->presence) {
388 case GST_PAD_ALWAYS: {
389 GstPad *pad = gst_element_get_pad (sinkelement, templ->name_template);
390 GstCaps *caps = gst_pad_get_caps (pad);
393 try_to_plug (pad, caps);
394 gst_object_unref (GST_OBJECT (pad));
395 gst_caps_unref (caps);
398 case GST_PAD_SOMETIMES:
399 has_dynamic_pads = TRUE;
406 /* listen for newly created pads if this element supports that */
407 if (has_dynamic_pads) {
408 g_signal_connect (sinkelement, "pad-added", G_CALLBACK (cb_newpad), NULL);
413 try_to_plug (GstPad *pad,
416 GstObject *parent = GST_OBJECT (GST_OBJECT_PARENT (pad));
419 GstCaps *res, *audiocaps;
421 /* don't plug if we're already plugged - FIXME: memleak for pad */
422 if (GST_PAD_IS_LINKED (gst_element_get_pad (audiosink, "sink"))) {
423 g_print ("Omitting link for pad %s:%s because we're already linked\n",
424 GST_OBJECT_NAME (parent), GST_OBJECT_NAME (pad));
428 /* as said above, we only try to plug audio... Omit video */
429 mime = gst_structure_get_name (gst_caps_get_structure (caps, 0));
430 if (g_strrstr (mime, "video")) {
431 g_print ("Omitting link for pad %s:%s because mimetype %s is non-audio\n",
432 GST_OBJECT_NAME (parent), GST_OBJECT_NAME (pad), mime);
436 /* can it link to the audiopad? */
437 audiocaps = gst_pad_get_caps (gst_element_get_pad (audiosink, "sink"));
438 res = gst_caps_intersect (caps, audiocaps);
439 if (res && !gst_caps_is_empty (res)) {
440 g_print ("Found pad to link to audiosink - plugging is now done\n");
441 close_link (pad, audiosink, "sink", NULL);
442 gst_caps_unref (audiocaps);
443 gst_caps_unref (res);
446 gst_caps_unref (audiocaps);
447 gst_caps_unref (res);
449 /* try to plug from our list */
450 for (item = factories; item != NULL; item = item->next) {
451 GstElementFactory *factory = GST_ELEMENT_FACTORY (item->data);
454 for (pads = gst_element_factory_get_static_pad_templates (factory);
455 pads != NULL; pads = pads->next) {
456 GstStaticPadTemplate *templ = pads->data;
458 /* find the sink template - need an always pad*/
459 if (templ->direction != GST_PAD_SINK ||
460 templ->presence != GST_PAD_ALWAYS) {
465 res = gst_caps_intersect (caps,
466 gst_static_caps_get (&templ->static_caps));
467 if (res && !gst_caps_is_empty (res)) {
469 gchar *name_template = g_strdup (templ->name_template);
471 /* close link and return */
472 gst_caps_unref (res);
473 element = gst_element_factory_create (factory, NULL);
474 close_link (pad, element, name_template,
475 gst_element_factory_get_static_pad_templates (factory));
476 g_free (name_template);
479 gst_caps_unref (res);
481 /* we only check one sink template per factory, so move on to the
482 * next factory now */
487 /* if we get here, no item was found */
488 g_print ("No compatible pad found to decode %s on %s:%s\n",
489 mime, GST_OBJECT_NAME (parent), GST_OBJECT_NAME (pad));
493 cb_typefound (GstElement *typefind,
501 s = gst_caps_to_string (caps);
502 g_print ("Detected media type %s\n", s);
505 /* actually plug now */
506 pad = gst_element_get_pad (typefind, "src");
507 try_to_plug (pad, caps);
508 gst_object_unref (GST_OBJECT (pad));
510 <!-- example-end dynamic.c c --></programlisting>
512 By doing all this, we will be able to make a simple autoplugger that
513 can automatically setup a pipeline for any media type. In the example
514 below, we will do this for audio only. However, we can also do this
515 for video to create a player that plays both audio and video.
517 <!-- example-begin dynamic.c d --><!--
519 my_bus_callback (GstBus *bus,
523 GMainLoop *loop = data;
525 switch (GST_MESSAGE_TYPE (message)) {
526 case GST_MESSAGE_ERROR: {
530 gst_message_parse_error (message, &err, &debug);
531 g_print ("Error: %s\n", err->message);
535 g_main_loop_quit (loop);
538 case GST_MESSAGE_EOS:
540 g_main_loop_quit (loop);
546 /* remove from queue */
555 GstElement *typefind, *realsink;
560 /* init GStreamer and ourselves */
561 gst_init (&argc, &argv);
562 loop = g_main_loop_new (NULL, FALSE);
567 g_print ("Usage: %s <filename>\n", argv[0]);
572 p = g_strdup_printf ("filesrc location=\"%s\" ! typefind name=tf", argv[1]);
573 pipeline = gst_parse_launch (p, &err);
577 g_error ("Could not construct pipeline: %s", err->message);
582 bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
583 gst_bus_add_watch (bus, my_bus_callback, NULL);
584 gst_object_unref (bus);
586 typefind = gst_bin_get_by_name (GST_BIN (pipeline), "tf");
587 g_signal_connect (typefind, "have-type", G_CALLBACK (cb_typefound), NULL);
588 gst_object_unref (GST_OBJECT (typefind));
589 audiosink = gst_element_factory_make ("audioconvert", "aconv");
590 realsink = gst_element_factory_make ("alsasink", "audiosink");
591 gst_bin_add_many (GST_BIN (pipeline), audiosink, realsink, NULL);
592 gst_element_link (audiosink, realsink);
593 gst_element_set_state (pipeline, GST_STATE_PLAYING);
596 g_main_loop_run (loop);
599 gst_element_set_state (pipeline, GST_STATE_NULL);
600 gst_object_unref (GST_OBJECT (pipeline));
604 --><!-- example-end dynamic.c d -->
606 The example above is a good first try for an autoplugger. Next steps
607 would be to listen for <quote>pad-removed</quote> signals, so we
608 can dynamically change the plugged pipeline if the stream changes
609 (this happens for DVB or Ogg radio). Also, you might want special-case
610 code for input with known content (such as a DVD or an audio-CD),
611 and much, much more. Moreover, you'll want many checks to prevent
612 infinite loops during autoplugging, maybe you'll want to implement
613 shortest-path-finding to make sure the most optimal pipeline is chosen,
614 and so on. Basically, the features that you implement in an autoplugger
615 depend on what you want to use it for. For full-blown implementations,
616 see the <quote>playbin</quote> and <quote>decodebin</quote> elements.