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>Chapter 19. Tagging (Metadata and Streaminfo)</TD
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><A
NAME="section-tagging-write"
>19.2. Writing Tags to Streams</A
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><P
> Tag writers are the opposite of tag readers. Tag writers only take
metadata tags into account, since that's the only type of tags that have
to be written into a stream. Tag writers can receive tags in three ways:
internal, application and pipeline. Internal tags are tags read by the
element itself, which means that the tag writer is - in that case - a tag
reader, too. Application tags are tags provided to the element via the
TagSetter interface (which is just a layer). Pipeline tags are tags
provided to the element from within the pipeline. The element receives
such tags via the <CODE
CLASS="classname"
>GST_EVENT_TAG</CODE
> event, which means
that tags writers should automatically be event aware. The tag writer is
responsible for combining all these three into one list and writing them
to the output stream.
</P
><P
> The example below will receive tags from both application and pipeline,
combine them and write them to the output stream. It implements the tag
setter so applications can set tags, and retrieves pipeline tags from
incoming events.
</P
><PRE
CLASS="programlisting"
>
GType
gst_my_filter_get_type (void)
{
[..]
static const GInterfaceInfo tag_setter_info = {
NULL,
NULL,
NULL
};
[..]
g_type_add_interface_static (my_filter_type,
GST_TYPE_TAG_SETTER,
&tag_setter_info);
[..]
}
static void
gst_my_filter_init (GstMyFilter *filter)
{
GST_FLAG_SET (filter, GST_ELEMENT_EVENT_AWARE);
[..]
}
/*
* Write one tag.
*/
static void
gst_my_filter_write_tag (const GstTagList *taglist,
const gchar *tagname,
gpointer data)
{
GstMyFilter *filter = GST_MY_FILTER (data);
GstBuffer *buffer;
guint num_values = gst_tag_list_get_tag_size (list, tag_name), n;
const GValue *from;
GValue to = { 0 };
g_value_init (&to, G_TYPE_STRING);
for (n = 0; n < num_values; n++) {
from = gst_tag_list_get_value_index (taglist, tagname, n);
g_value_transform (from, &to);
buf = gst_buffer_new ();
GST_BUFFER_DATA (buf) = g_strdup_printf ("%s:%s", tagname,
g_value_get_string (&to));
GST_BUFFER_SIZE (buf) = strlen (GST_BUFFER_DATA (buf));
gst_pad_push (filter->srcpad, GST_DATA (buf));
}
g_value_unset (&to);
}
static void
gst_my_filter_loopfunc (GstElement *element)
{
GstMyFilter *filter = GST_MY_FILTER (element);
GstTagSetter *tagsetter = GST_TAG_SETTER (element);
GstData *data;
GstEvent *event;
gboolean eos = FALSE;
GstTagList *taglist = gst_tag_list_new ();
while (!eos) {
data = gst_pad_pull (filter->sinkpad);
/* We're not very much interested in data right now */
if (GST_IS_BUFFER (data))
gst_buffer_unref (GST_BUFFER (data));
event = GST_EVENT (data);
switch (GST_EVENT_TYPE (event)) {
case GST_EVENT_TAG:
gst_tag_list_insert (taglist, gst_event_tag_get_list (event),
GST_TAG_MERGE_PREPEND);
gst_event_unref (event);
break;
case GST_EVENT_EOS:
eos = TRUE;
gst_event_unref (event);
break;
default:
gst_pad_event_default (filter->sinkpad, event);
break;
}
}
/* merge tags with the ones retrieved from the application */
if (gst_tag_setter_get_list (tagsetter)) {
gst_tag_list_insert (taglist,
gst_tag_setter_get_list (tagsetter),
gst_tag_setter_get_merge_mode (tagsetter));
}
/* write tags */
gst_tag_list_foreach (taglist, gst_my_filter_write_tag, filter);
/* signal EOS */
gst_pad_push (filter->srcpad, GST_DATA (gst_event_new (GST_EVENT_EOS)));
gst_element_set_eos (element);
}
</PRE
><P
> Note that normally, elements would not read the full stream before
processing tags. Rather, they would read from each sinkpad until they've
received data (since tags usually come in before the first data buffer)
and process that.
</P
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