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178 lines
6 KiB
Markdown
178 lines
6 KiB
Markdown
---
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title: Metadata
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...
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# Metadata
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GStreamer makes a clear distinction between the two types of metadata it
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supports: Stream tags, which describe the content of a stream in a non-technical
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way; and Stream-info, which is a somewhat technical description of the
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properties of a stream. Stream tags examples include the author of a song, the
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song's title or the album it belongs to. Examples for stream-info include video
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size, audio samplerate, codecs used and so on.
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Tags are handled using the GStreamer tagging system. Stream-info can be retrieved
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from a `GstPad` by getting the current (negotiated) `GstCaps` for that pad.
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## Metadata reading
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Stream information can most easily be obtained by reading it from a `GstPad`.
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Note that this requires access to all pads of which you want stream information.
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This approach has already been discussed in [Using capabilities for metadata](application-development/basics/pads.md#using-capabilities-for-metadata).
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Therefore, we will skip it here.
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Tag reading is done through a bus in GStreamer. You can listen for
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`GST_MESSAGE_TAG` messages and handle them as you wish. This has been discussed
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previously in [Bus](application-development/basics/bus.md).
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Note, however, that the `GST_MESSAGE_TAG` message may be emitted multiple
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times and it is the application's responsibility to aggregate and display the tags
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in a coherent way. This can be done using `gst_tag_list_merge ()` but make sure
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to empty the cache when loading a new song, or after every few minutes when
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listening to internet radio. Also, make sure you use `GST_TAG_MERGE_PREPEND` as
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merging mode, so that a new title (which came in later) has precedence over
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the old one.
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The following example shows how to extract tags from a file and print them:
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``` c
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/* compile with:
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* gcc -o tags tags.c `pkg-config --cflags --libs gstreamer-1.0` */
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#include <gst/gst.h>
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static void
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print_one_tag (const GstTagList * list, const gchar * tag, gpointer user_data)
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{
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int i, num;
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num = gst_tag_list_get_tag_size (list, tag);
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for (i = 0; i < num; ++i) {
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const GValue *val;
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/* Note: when looking for specific tags, use the gst_tag_list_get_xyz() API,
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* we only use the GValue approach here because it is more generic */
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val = gst_tag_list_get_value_index (list, tag, i);
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if (G_VALUE_HOLDS_STRING (val)) {
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g_print ("\t%20s : %s\n", tag, g_value_get_string (val));
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} else if (G_VALUE_HOLDS_UINT (val)) {
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g_print ("\t%20s : %u\n", tag, g_value_get_uint (val));
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} else if (G_VALUE_HOLDS_DOUBLE (val)) {
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g_print ("\t%20s : %g\n", tag, g_value_get_double (val));
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} else if (G_VALUE_HOLDS_BOOLEAN (val)) {
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g_print ("\t%20s : %s\n", tag,
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(g_value_get_boolean (val)) ? "true" : "false");
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} else if (GST_VALUE_HOLDS_BUFFER (val)) {
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GstBuffer *buf = gst_value_get_buffer (val);
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guint buffer_size = gst_buffer_get_size (buf);
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g_print ("\t%20s : buffer of size %u\n", tag, buffer_size);
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} else if (GST_VALUE_HOLDS_DATE_TIME (val)) {
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GstDateTime *dt = g_value_get_boxed (val);
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gchar *dt_str = gst_date_time_to_iso8601_string (dt);
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g_print ("\t%20s : %s\n", tag, dt_str);
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g_free (dt_str);
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} else {
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g_print ("\t%20s : tag of type '%s'\n", tag, G_VALUE_TYPE_NAME (val));
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}
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}
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}
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static void
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on_new_pad (GstElement * dec, GstPad * pad, GstElement * fakesink)
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{
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GstPad *sinkpad;
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sinkpad = gst_element_get_static_pad (fakesink, "sink");
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if (!gst_pad_is_linked (sinkpad)) {
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if (gst_pad_link (pad, sinkpad) != GST_PAD_LINK_OK)
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g_error ("Failed to link pads!");
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}
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gst_object_unref (sinkpad);
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}
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int
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main (int argc, char ** argv)
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{
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GstElement *pipe, *dec, *sink;
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GstMessage *msg;
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gchar *uri;
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gst_init (&argc, &argv);
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if (argc < 2)
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g_error ("Usage: %s FILE or URI", argv[0]);
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if (gst_uri_is_valid (argv[1])) {
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uri = g_strdup (argv[1]);
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} else {
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uri = gst_filename_to_uri (argv[1], NULL);
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}
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pipe = gst_pipeline_new ("pipeline");
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dec = gst_element_factory_make ("uridecodebin", NULL);
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g_object_set (dec, "uri", uri, NULL);
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gst_bin_add (GST_BIN (pipe), dec);
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sink = gst_element_factory_make ("fakesink", NULL);
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gst_bin_add (GST_BIN (pipe), sink);
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g_signal_connect (dec, "pad-added", G_CALLBACK (on_new_pad), sink);
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gst_element_set_state (pipe, GST_STATE_PAUSED);
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while (TRUE) {
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GstTagList *tags = NULL;
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msg = gst_bus_timed_pop_filtered (GST_ELEMENT_BUS (pipe),
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GST_CLOCK_TIME_NONE,
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GST_MESSAGE_ASYNC_DONE | GST_MESSAGE_TAG | GST_MESSAGE_ERROR);
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if (GST_MESSAGE_TYPE (msg) != GST_MESSAGE_TAG) /* error or async_done */
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break;
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gst_message_parse_tag (msg, &tags);
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g_print ("Got tags from element %s:\n", GST_OBJECT_NAME (msg->src));
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gst_tag_list_foreach (tags, print_one_tag, NULL);
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g_print ("\n");
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gst_tag_list_unref (tags);
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gst_message_unref (msg);
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}
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if (GST_MESSAGE_TYPE (msg) == GST_MESSAGE_ERROR) {
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GError *err = NULL;
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gst_message_parse_error (msg, &err, NULL);
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g_printerr ("Got error: %s\n", err->message);
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g_error_free (err);
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}
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gst_message_unref (msg);
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gst_element_set_state (pipe, GST_STATE_NULL);
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gst_object_unref (pipe);
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g_free (uri);
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return 0;
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}
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```
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## Tag writing
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Tag writing is done using the
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[`GstTagSetter`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/GstTagSetter.html)
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interface. All that's required is a tag-set-supporting element in your
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pipeline.
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In order to see if any of the elements in your pipeline supports tag writing,
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you can use the function `gst_bin_iterate_all_by_interface (pipeline, GST_TYPE_TAG_SETTER)`.
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On the resulting element, usually an encoder or muxer, you can use
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`gst_tag_setter_merge_tags ()` with a taglist or `gst_tag_setter_add_tags ()`
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with individual tags, to set tags on it.
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A nice extra feature in GStreamer's tag support is that tags are preserved
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in pipelines. This means that if you transcode one file containing tags
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into another tag-supporting media type, then the tags will be handled as part of
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the data stream and will be so merged into the newly written media file.
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