/usr/share/doc/gstreamer1.0-doc/pwg/html/section-tagging-write.html

<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Writing Tags to Streams</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="GStreamer Plugin Writer's Guide (1.2.4)"><link rel="up" href="chapter-advanced-tagging.html" title="Chapter 22. Tagging (Metadata and Streaminfo)"><link rel="prev" href="section-tagging-read.html" title="Reading Tags from Streams"><link rel="next" href="part-other.html" title="Part IV. Creating special element types"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Writing Tags to Streams</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="section-tagging-read.html">Prev</a> </td><th width="60%" align="center">Chapter 22. Tagging (Metadata and Streaminfo)</th><td width="20%" align="right"> <a accesskey="n" href="part-other.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="section-tagging-write"></a>Writing Tags to Streams</h2></div></div></div><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 <span class="symbol">GST_EVENT_TAG</span> event, which means
      that tags writers should implment an event handler. 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><p>
      Warning, this example is outdated and doesn't work with the 1.0 version
      of <span class="application">GStreamer</span> anymore.
    </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,
				 &amp;tag_setter_info);
[..]
}

static void
gst_my_filter_init (GstMyFilter *filter)
{
[..]
}

/*
 * 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 (&amp;to, G_TYPE_STRING);

  for (n = 0; n &lt; num_values; n++) {
    guint8 * data;
    gsize size;

    from = gst_tag_list_get_value_index (taglist, tagname, n);
    g_value_transform (from, &amp;to);

    data = g_strdup_printf ("%s:%s", tagname,
		g_value_get_string (&amp;to));
    size = strlen (data);

    buf = gst_buffer_new_wrapped (data, size);
    gst_pad_push (filter-&gt;srcpad, buf);
  }

  g_value_unset (&amp;to);
}

static void
gst_my_filter_task_func (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-&gt;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-&gt;sinkpad, event);
        break;
    }
  }

  /* merge tags with the ones retrieved from the application */
  if ((gst_tag_setter_get_tag_list (tagsetter)) {
    gst_tag_list_insert (taglist,
			 gst_tag_setter_get_tag_list (tagsetter),
			 gst_tag_setter_get_tag_merge_mode (tagsetter));
  }

  /* write tags */
  gst_tag_list_foreach (taglist, gst_my_filter_write_tag, filter);

  /* signal EOS */
  gst_pad_push (filter-&gt;srcpad, gst_event_new (GST_EVENT_EOS));
}

    </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></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="section-tagging-read.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="chapter-advanced-tagging.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="part-other.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Reading Tags from Streams </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Part IV. Creating special element types</td></tr></table></div></body></html>
gstreamer1.0-doc 1.2.4-0ubuntu1.1 / usr / share / doc / gstreamer1.0-doc / pwg / html / section-tagging-write.html
