mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-09-21 11:30:18 +00:00
178 lines
6 KiB
Markdown
178 lines
6 KiB
Markdown
---
|
|
title: Metadata
|
|
...
|
|
|
|
# Metadata
|
|
|
|
GStreamer makes a clear distinction between the two types of metadata it
|
|
supports: Stream tags, which describe the content of a stream in a non-technical
|
|
way; and Stream-info, which is a somewhat technical description of the
|
|
properties of a stream. Stream tags examples include the author of a song, the
|
|
song's title or the album it belongs to. Examples for stream-info include video
|
|
size, audio samplerate, codecs used and so on.
|
|
|
|
Tags are handled using the GStreamer tagging system. Stream-info can be retrieved
|
|
from a `GstPad` by getting the current (negotiated) `GstCaps` for that pad.
|
|
|
|
## Metadata reading
|
|
|
|
Stream information can most easily be obtained by reading it from a `GstPad`.
|
|
Note that this requires access to all pads of which you want stream information.
|
|
This approach has already been discussed in [Using capabilities for metadata](application-development/basics/pads.md#using-capabilities-for-metadata).
|
|
Therefore, we will skip it here.
|
|
|
|
Tag reading is done through a bus in GStreamer. You can listen for
|
|
`GST_MESSAGE_TAG` messages and handle them as you wish. This has been discussed
|
|
previously in [Bus](application-development/basics/bus.md).
|
|
|
|
Note, however, that the `GST_MESSAGE_TAG` message may be emitted multiple
|
|
times and it is the application's responsibility to aggregate and display the tags
|
|
in a coherent way. This can be done using `gst_tag_list_merge ()` but make sure
|
|
to empty the cache when loading a new song, or after every few minutes when
|
|
listening to internet radio. Also, make sure you use `GST_TAG_MERGE_PREPEND` as
|
|
merging mode, so that a new title (which came in later) has precedence over
|
|
the old one.
|
|
|
|
The following example shows how to extract tags from a file and print them:
|
|
|
|
``` c
|
|
/* compile with:
|
|
* gcc -o tags tags.c `pkg-config --cflags --libs gstreamer-1.0` */
|
|
#include <gst/gst.h>
|
|
|
|
static void
|
|
print_one_tag (const GstTagList * list, const gchar * tag, gpointer user_data)
|
|
{
|
|
int i, num;
|
|
|
|
num = gst_tag_list_get_tag_size (list, tag);
|
|
for (i = 0; i < num; ++i) {
|
|
const GValue *val;
|
|
|
|
/* Note: when looking for specific tags, use the gst_tag_list_get_xyz() API,
|
|
* we only use the GValue approach here because it is more generic */
|
|
val = gst_tag_list_get_value_index (list, tag, i);
|
|
if (G_VALUE_HOLDS_STRING (val)) {
|
|
g_print ("\t%20s : %s\n", tag, g_value_get_string (val));
|
|
} else if (G_VALUE_HOLDS_UINT (val)) {
|
|
g_print ("\t%20s : %u\n", tag, g_value_get_uint (val));
|
|
} else if (G_VALUE_HOLDS_DOUBLE (val)) {
|
|
g_print ("\t%20s : %g\n", tag, g_value_get_double (val));
|
|
} else if (G_VALUE_HOLDS_BOOLEAN (val)) {
|
|
g_print ("\t%20s : %s\n", tag,
|
|
(g_value_get_boolean (val)) ? "true" : "false");
|
|
} else if (GST_VALUE_HOLDS_BUFFER (val)) {
|
|
GstBuffer *buf = gst_value_get_buffer (val);
|
|
guint buffer_size = gst_buffer_get_size (buf);
|
|
|
|
g_print ("\t%20s : buffer of size %u\n", tag, buffer_size);
|
|
} else if (GST_VALUE_HOLDS_DATE_TIME (val)) {
|
|
GstDateTime *dt = g_value_get_boxed (val);
|
|
gchar *dt_str = gst_date_time_to_iso8601_string (dt);
|
|
|
|
g_print ("\t%20s : %s\n", tag, dt_str);
|
|
g_free (dt_str);
|
|
} else {
|
|
g_print ("\t%20s : tag of type '%s'\n", tag, G_VALUE_TYPE_NAME (val));
|
|
}
|
|
}
|
|
}
|
|
|
|
static void
|
|
on_new_pad (GstElement * dec, GstPad * pad, GstElement * fakesink)
|
|
{
|
|
GstPad *sinkpad;
|
|
|
|
sinkpad = gst_element_get_static_pad (fakesink, "sink");
|
|
if (!gst_pad_is_linked (sinkpad)) {
|
|
if (gst_pad_link (pad, sinkpad) != GST_PAD_LINK_OK)
|
|
g_error ("Failed to link pads!");
|
|
}
|
|
gst_object_unref (sinkpad);
|
|
}
|
|
|
|
int
|
|
main (int argc, char ** argv)
|
|
{
|
|
GstElement *pipe, *dec, *sink;
|
|
GstMessage *msg;
|
|
gchar *uri;
|
|
|
|
gst_init (&argc, &argv);
|
|
|
|
if (argc < 2)
|
|
g_error ("Usage: %s FILE or URI", argv[0]);
|
|
|
|
if (gst_uri_is_valid (argv[1])) {
|
|
uri = g_strdup (argv[1]);
|
|
} else {
|
|
uri = gst_filename_to_uri (argv[1], NULL);
|
|
}
|
|
|
|
pipe = gst_pipeline_new ("pipeline");
|
|
|
|
dec = gst_element_factory_make ("uridecodebin", NULL);
|
|
g_object_set (dec, "uri", uri, NULL);
|
|
gst_bin_add (GST_BIN (pipe), dec);
|
|
|
|
sink = gst_element_factory_make ("fakesink", NULL);
|
|
gst_bin_add (GST_BIN (pipe), sink);
|
|
|
|
g_signal_connect (dec, "pad-added", G_CALLBACK (on_new_pad), sink);
|
|
|
|
gst_element_set_state (pipe, GST_STATE_PAUSED);
|
|
|
|
while (TRUE) {
|
|
GstTagList *tags = NULL;
|
|
|
|
msg = gst_bus_timed_pop_filtered (GST_ELEMENT_BUS (pipe),
|
|
GST_CLOCK_TIME_NONE,
|
|
GST_MESSAGE_ASYNC_DONE | GST_MESSAGE_TAG | GST_MESSAGE_ERROR);
|
|
|
|
if (GST_MESSAGE_TYPE (msg) != GST_MESSAGE_TAG) /* error or async_done */
|
|
break;
|
|
|
|
gst_message_parse_tag (msg, &tags);
|
|
|
|
g_print ("Got tags from element %s:\n", GST_OBJECT_NAME (msg->src));
|
|
gst_tag_list_foreach (tags, print_one_tag, NULL);
|
|
g_print ("\n");
|
|
gst_tag_list_unref (tags);
|
|
|
|
gst_message_unref (msg);
|
|
}
|
|
|
|
if (GST_MESSAGE_TYPE (msg) == GST_MESSAGE_ERROR) {
|
|
GError *err = NULL;
|
|
|
|
gst_message_parse_error (msg, &err, NULL);
|
|
g_printerr ("Got error: %s\n", err->message);
|
|
g_error_free (err);
|
|
}
|
|
|
|
gst_message_unref (msg);
|
|
gst_element_set_state (pipe, GST_STATE_NULL);
|
|
gst_object_unref (pipe);
|
|
g_free (uri);
|
|
return 0;
|
|
}
|
|
|
|
```
|
|
|
|
## Tag writing
|
|
|
|
Tag writing is done using the
|
|
[`GstTagSetter`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/GstTagSetter.html)
|
|
interface. All that's required is a tag-set-supporting element in your
|
|
pipeline.
|
|
|
|
In order to see if any of the elements in your pipeline supports tag writing,
|
|
you can use the function `gst_bin_iterate_all_by_interface (pipeline, GST_TYPE_TAG_SETTER)`.
|
|
On the resulting element, usually an encoder or muxer, you can use
|
|
`gst_tag_setter_merge ()` with a taglist or `gst_tag_setter_add ()` with
|
|
individual tags, to set tags on it.
|
|
|
|
A nice extra feature in GStreamer's tag support is that tags are preserved
|
|
in pipelines. This means that if you transcode one file containing tags
|
|
into another tag-supporting media type, then the tags will be handled as part of
|
|
the data stream and will be so merged into the newly written media file.
|