1 <chapter id="chapter-metadata">
2 <title>Metadata</title>
5 &GStreamer; makes a clear distinction between two types of metadata, and
6 has support for both types. The first is stream tags, which describe the
7 content of a stream in a non-technical way. Examples include the author
8 of a song, the title of that very same song or the album it is a part of.
9 The other type of metadata is stream-info, which is a somewhat technical
10 description of the properties of a stream. This can include video size,
11 audio samplerate, codecs used and so on. Tags are handled using the
12 &GStreamer; tagging system. Stream-info can be retrieved from a
13 <classname>GstPad</classname>.
16 <sect1 id="section-tags-read">
17 <title>Metadata reading</title>
20 Stream information can most easily be read by reading it from a
21 <classname>GstPad</classname>. This has already been discussed before
22 in <xref linkend="section-caps-metadata"/>. Therefore, we will skip
23 it here. Note that this requires access to all pads of which you
24 want stream information.
28 Tag reading is done through a bus in &GStreamer;, which has been
29 discussed previously in <xref linkend="chapter-bus"/>. You can
30 listen for <classname>GST_MESSAGE_TAG</classname> messages and handle
34 Note, however, that the <classname>GST_MESSAGE_TAG</classname>
35 message may be fired multiple times in the pipeline. It is the
36 application's responsibility to put all those tags together and
37 display them to the user in a nice, coherent way. Usually, using
38 <function>gst_tag_list_merge ()</function> is a good enough way
39 of doing this; make sure to empty the cache when loading a new song,
40 or after every few minutes when listening to internet radio. Also,
41 make sure you use <classname>GST_TAG_MERGE_PREPEND</classname> as
42 merging mode, so that a new title (which came in later) has a
43 preference over the old one for display.
46 The following example will extract tags from a file and print them:
50 * gcc -o tags tags.c `pkg-config --cflags --libs gstreamer-0.10` */
51 #include <gst/gst.h>
54 print_one_tag (const GstTagList * list, const gchar * tag, gpointer user_data)
58 num = gst_tag_list_get_tag_size (list, tag);
59 for (i = 0; i < num; ++i) {
62 /* Note: when looking for specific tags, use the g_tag_list_get_xyz() API,
63 * we only use the GValue approach here because it is more generic */
64 val = gst_tag_list_get_value_index (list, tag, i);
65 if (G_VALUE_HOLDS_STRING (val)) {
66 g_print ("\t%20s : %s\n", tag, g_value_get_string (val));
67 } else if (G_VALUE_HOLDS_UINT (val)) {
68 g_print ("\t%20s : %u\n", tag, g_value_get_uint (val));
69 } else if (G_VALUE_HOLDS_DOUBLE (val)) {
70 g_print ("\t%20s : %g\n", tag, g_value_get_double (val));
71 } else if (G_VALUE_HOLDS_BOOLEAN (val)) {
72 g_print ("\t%20s : %s\n", tag,
73 (g_value_get_boolean (val)) ? "true" : "false");
74 } else if (GST_VALUE_HOLDS_BUFFER (val)) {
75 g_print ("\t%20s : buffer of size %u\n", tag,
76 GST_BUFFER_SIZE (gst_value_get_buffer (val)));
77 } else if (GST_VALUE_HOLDS_DATE (val)) {
78 g_print ("\t%20s : date (year=%u,...)\n", tag,
79 g_date_get_year (gst_value_get_date (val)));
81 g_print ("\t%20s : tag of type '%s'\n", tag, G_VALUE_TYPE_NAME (val));
87 on_new_pad (GstElement * dec, GstPad * pad, GstElement * fakesink)
91 sinkpad = gst_element_get_static_pad (fakesink, "sink");
92 if (!gst_pad_is_linked (sinkpad)) {
93 if (gst_pad_link (pad, sinkpad) != GST_PAD_LINK_OK)
94 g_error ("Failed to link pads!");
96 gst_object_unref (sinkpad);
100 main (int argc, char ** argv)
102 GstElement *pipe, *dec, *sink;
105 gst_init (&argc, &argv);
107 if (argc < 2 || !gst_uri_is_valid (argv[1]))
108 g_error ("Usage: %s file:///path/to/file", argv[0]);
110 pipe = gst_pipeline_new ("pipeline");
112 dec = gst_element_factory_make ("uridecodebin", NULL);
113 g_object_set (dec, "uri", argv[1], NULL);
114 gst_bin_add (GST_BIN (pipe), dec);
116 sink = gst_element_factory_make ("fakesink", NULL);
117 gst_bin_add (GST_BIN (pipe), sink);
119 g_signal_connect (dec, "pad-added", G_CALLBACK (on_new_pad), sink);
121 gst_element_set_state (pipe, GST_STATE_PAUSED);
124 GstTagList *tags = NULL;
126 msg = gst_bus_timed_pop_filtered (GST_ELEMENT_BUS (pipe),
128 GST_MESSAGE_ASYNC_DONE | GST_MESSAGE_TAG | GST_MESSAGE_ERROR);
130 if (GST_MESSAGE_TYPE (msg) != GST_MESSAGE_TAG) /* error or async_done */
133 gst_message_parse_tag (msg, &tags);
135 g_print ("Got tags from element %s:\n", GST_OBJECT_NAME (msg->src));
136 gst_tag_list_foreach (tags, print_one_tag, NULL);
138 gst_tag_list_free (tags);
140 gst_message_unref (msg);
143 if (GST_MESSAGE_TYPE (msg) == GST_MESSAGE_ERROR)
144 g_error ("Got error");
146 gst_message_unref (msg);
147 gst_element_set_state (pipe, GST_STATE_NULL);
148 gst_object_unref (pipe);
154 <sect1 id="section-tags-write">
155 <title>Tag writing</title>
158 Tag writing is done using the <ulink type="http"
159 url="&URLAPI;GstTagSetter.html"><classname>GstTagSetter</classname></ulink>
160 interface. All that's required is a tag-set-supporting element in
161 your pipeline. In order to see if any of the elements in your
162 pipeline supports tag writing, you can use the function
163 <function>gst_bin_iterate_all_by_interface (pipeline,
164 GST_TYPE_TAG_SETTER)</function>. On the resulting element, usually
165 an encoder or muxer, you can use <function>gst_tag_setter_merge
166 ()</function> (with a taglist) or <function>gst_tag_setter_add
167 ()</function> (with individual tags) to set tags on it.
170 A nice extra feature in &GStreamer; tag support is that tags are
171 preserved in pipelines. This means that if you transcode one file
172 containing tags into another media type, and that new media type
173 supports tags too, then the tags will be handled as part of the
174 data stream and be merged into the newly written media file, too.