gstreamer/docs/manual/advanced-dataaccess.xml

<chapter id="chapter-dataaccess">
  <title>Pipeline manipulation</title>
  <para>
    This chapter will discuss how you can manipulate your pipeline in several
    ways from your application on. Parts of this chapter are downright
    hackish, so be assured that you'll need some programming knowledge
    before you start reading this.
  </para>
  <para>
    Topics that will be discussed here include how you can insert data into
    a pipeline from your application, how to read data from a pipeline,
    how to manipulate the pipeline's speed, length, starting point and how
    to listen to a pipeline's data processing.
  </para>

  <sect1 id="section-data-probe">
    <title>Data probing</title>
    <para>
      Probing is best envisioned as a pad listener. Technically, a probe is
      nothing more than a signal callback that can be attached to a pad.
      Those signals are by default not fired at all (since that may have a
      negative impact on performance), but can be enabled by attaching a
      probe using <function>gst_pad_add_buffer_probe ()</function>,
      <function>gst_pad_add_event_probe ()</function>, or
      <function>gst_pad_add_data_probe ()</function>.
      Those functions attach the signal handler and
      enable the actual signal emission. Similarly, one can use the
      <function>gst_pad_remove_buffer_probe ()</function>,
      <function>gst_pad_remove_event_probe ()</function>, or
      <function>gst_pad_remove_data_probe ()</function>
      to remove the signal handlers again.
    </para>
    <para>
      Probes run in pipeline threading context, so callbacks should try to
      not block and generally not do any weird stuff, since this could
      have a negative impact on pipeline performance or, in case of bugs,
      cause deadlocks or crashes. More precisely, one should usually not
      call any GUI-related functions from within a probe callback, nor try
      to change the state of the pipeline.  An application may post custom
      messages on the pipeline's bus though to communicate with the main
      application thread and have it do things like stop the pipeline.
    </para>
    <para>
      In any case, most common buffer operations
      that elements can do in <function>_chain ()</function> functions, can
      be done in probe callbacks as well. The example below gives a short
      impression on how to use them (even if this usage is not entirely
      correct, but more on that below):
    </para>
    <programlisting><!-- example-begin probe.c -->
#include &lt;gst/gst.h&gt;

static gboolean
cb_have_data (GstPad    *pad,
	      GstBuffer *buffer,
	      gpointer   u_data)
{
  gint x, y;
  GstMapInfo info;
  guint16 *ptr, t;
  
  gst_buffer_map (buffer, &amp;info, GST_MAP_WRITE);

  ptr = info.data;
  /* invert data */
  for (y = 0; y &lt; 288; y++) {
    for (x = 0; x &lt; 384 / 2; x++) {
      t = ptr[384 - 1 - x];
      ptr[384 - 1 - x] = ptr[x];
      ptr[x] = t;
    }
    ptr += 384;
  }
  gst_buffer_unmap (buffer, &amp;info);

  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline, *src, *sink, *filter, *csp;
  GstCaps *filtercaps;
  GstPad *pad;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* build */
  pipeline = gst_pipeline_new ("my-pipeline");
  src = gst_element_factory_make ("videotestsrc", "src");
  if (src == NULL)
    g_error ("Could not create 'videotestsrc' element");

  filter = gst_element_factory_make ("capsfilter", "filter");
  g_assert (filter != NULL); /* should always exist */

  csp = gst_element_factory_make ("videoconvert", "csp");
  if (csp == NULL)
    g_error ("Could not create 'videoconvert' element");

  sink = gst_element_factory_make ("xvimagesink", "sink");
  if (sink == NULL) {
    sink = gst_element_factory_make ("ximagesink", "sink");
    if (sink == NULL)
      g_error ("Could not create neither 'xvimagesink' nor 'ximagesink' element");
  }

  gst_bin_add_many (GST_BIN (pipeline), src, filter, csp, sink, NULL);
  gst_element_link_many (src, filter, csp, sink, NULL);
  filtercaps = gst_caps_new_simple ("video/x-raw",
			   "format", G_TYPE_STRING, "RGB16",
			   "width", G_TYPE_INT, 384,
			   "height", G_TYPE_INT, 288,
			   "framerate", GST_TYPE_FRACTION, 25, 1,
			   NULL);
  g_object_set (G_OBJECT (filter), "caps", filtercaps, NULL);
  gst_caps_unref (filtercaps);

  pad = gst_element_get_pad (src, "src");
  gst_pad_add_buffer_probe (pad, G_CALLBACK (cb_have_data), NULL);
  gst_object_unref (pad);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

  /* wait until it's up and running or failed */
  if (gst_element_get_state (pipeline, NULL, NULL, -1) == GST_STATE_CHANGE_FAILURE) {
    g_error ("Failed to go into PLAYING state");
  }

  g_print ("Running ...\n");
  g_main_loop_run (loop);

  /* exit */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);

  return 0;
}
    <!-- example-end probe.c --></programlisting>
    <para>
      Compare that output with the output of <quote>gst-launch-0.10
      videotestsrc ! xvimagesink</quote>, just so you know what you're
      looking for.
    </para>
    <para>
      The above example is not really correct though.  Strictly speaking, a
      pad probe callback is only allowed to modify the buffer content if the
      buffer is writable, and it is only allowed to modify buffer metadata like
      timestamps, caps, etc. if the buffer metadata is writable.  Whether this
      is the case or not depends a lot on the pipeline and the elements
      involved.  Often enough, this is the case, but sometimes it is not,
      and if it is not then unexpected modification of the data or metadata
      can introduce bugs that are very hard to debug and track down. You can
      check if a buffer and its metadata are writable with
      <function>gst_buffer_is_writable ()</function> and
      <function>gst_buffer_is_metadata_writable ()</function>.  Since you
      can't pass back a different buffer than the one passed in, there is no
      point of making a buffer writable in the callback function.
    </para>
    <para>
      Pad probes are suited best for looking at data as it passes through
      the pipeline. If you need to modify data, you should write your own
      GStreamer element. Base classes like GstAudioFilter, GstVideoFilter or
      GstBaseTransform make this fairly easy.
    </para>
    <para>
      If you just want to inspect buffers as they pass through the pipeline,
      you don't even need to set up pad probes. You could also just insert
      an identity element into the pipeline and connect to its "handoff"
      signal. The identity element also provides a few useful debugging tools
      like the "dump" property or the "last-message" property (the latter is
      enabled by passing the '-v' switch to gst-launch).
    </para>
  </sect1>

  <sect1 id="section-data-spoof">
    <title>Manually adding or removing data from/to a pipeline</title>
    <para>
      Many people have expressed the wish to use their own sources to inject
      data into a pipeline. Some people have also expressed the wish to grab
      the output in a pipeline and take care of the actual output inside
      their application. While either of these methods are stongly
      discouraged, &GStreamer; offers hacks to do this. <emphasis>However,
      there is no support for those methods.</emphasis> If it doesn't work,
      you're on your own. Also, synchronization, thread-safety and other
      things that you've been able to take for granted so far are no longer
      guaranteed if you use any of those methods. It's always better to
      simply write a plugin and have the pipeline schedule and manage it.
      See the Plugin Writer's Guide for more information on this topic. Also
      see the next section, which will explain how to embed plugins statically
      in your application.
    </para>
    <note><para>
	<ulink type="http"
	  url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gstreamer-app.html">New
	  API</ulink> was developed to make data insertion and extraction easy
	for applications. It can be found as GstAppSrc and GstAppSink in the
	<ulink type="http"
	  url="http://gstreamer.freedesktop.org/modules/gst-plugins-base.html">
	  gst-plugins-base</ulink> module.
    </para></note>
    <para>
      After all those disclaimers, let's start. There's three possible
      elements that you can use for the above-mentioned purposes. Those are
      called <quote>fakesrc</quote> (an imaginary source),
      <quote>fakesink</quote> (an imaginary sink) and <quote>identity</quote>
      (an imaginary filter). The same method applies to each of those
      elements. Here, we will discuss how to use those elements to insert
      (using fakesrc) or grab (using fakesink or identity) data from a
      pipeline, and how to set negotiation.
    </para>
    <para>
      Those who're paying close attention will notice that the purpose
      of identity is almost identical to that of probes. Indeed, this is
      true. Probes allow for the same purpose, and a bunch more, and
      with less overhead plus dynamic removing/adding of handlers, but
      apart from those, probes and identity have the same purpose, just
      in a completely different implementation type.
    </para>

    <sect2 id="section-spoof-handoff">
      <title>Inserting or grabbing data</title>
      <para>
        The three before-mentioned elements (fakesrc, fakesink and identity)
        each have a <quote>handoff</quote> signal that will be called in
        the <function>_get ()</function>- (fakesrc) or <function>_chain
        ()</function>-function (identity, fakesink). In the signal handler,
        you can set (fakesrc) or get (identity, fakesink) data to/from the
        provided buffer. Note that in the case of fakesrc, you have to set
        the size of the provided buffer using the <quote>sizemax</quote>
        property. For both fakesrc and fakesink, you also have to set the
        <quote>signal-handoffs</quote> property for this method to work.
      </para>
      <para>
        Note that your handoff function should <emphasis>not</emphasis>
        block, since this will block pipeline iteration. Also, do not try
        to use all sort of weird hacks in such functions to accomplish
        something that looks like synchronization or so; it's not the right
        way and will lead to issues elsewhere. If you're doing any of this,
        you're basically misunderstanding the &GStreamer; design.
      </para>
    </sect2>

    <sect2 id="section-spoof-format">
      <title>Forcing a format</title>
      <para>
        Sometimes, when using fakesrc as a source in your pipeline, you'll
        want to set a specific format, for example a video size and format
        or an audio bitsize and number of channels. You can do this by
        forcing a specific <classname>GstCaps</classname> on the pipeline,
        which is possible by using <emphasis>filtered caps</emphasis>. You
        can set a filtered caps on a link by using the
        <quote>capsfilter</quote> element in between the two elements, and
        specifying a <classname>GstCaps</classname> as
        <quote>caps</quote> property on this element. It will then
        only allow types matching that specified capability set for
	negotiation.  See also <xref linkend="section-caps-filter"/>.
      </para>
    </sect2>

    <sect2 id="section-spoof-example">
      <title>Example application</title>
      <para>
        This example application will generate black/white (it switches
        every second) video to an X-window output by using fakesrc as a
        source and using filtered caps to force a format. Since the depth
        of the image depends on your X-server settings, we use a colorspace
        conversion element to make sure that the output to your X server
        will have the correct bitdepth. You can also set timestamps on the
        provided buffers to override the fixed framerate.
      </para>
      <programlisting><!-- example-begin fakesrc.c -->
#include &lt;string.h&gt; /* for memset () */
#include &lt;gst/gst.h&gt;

static void
cb_handoff (GstElement *fakesrc,
	    GstBuffer  *buffer,
	    GstPad     *pad,
	    gpointer    user_data)
{
  static gboolean white = FALSE;
  GstMapInfo info;
  
  gst_buffer_map (buffer, &amp;info, GST_MAP_WRITE);

  /* this makes the image black/white */
  memset (info.data, white ? 0xff : 0x0, info.size);
  white = !white;

  gst_buffer_unmap (buffer, &amp;info);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GstElement *pipeline, *fakesrc, *flt, *conv, *videosink;
  GMainLoop *loop;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* setup pipeline */
  pipeline = gst_pipeline_new ("pipeline");
  fakesrc = gst_element_factory_make ("fakesrc", "source");
  flt = gst_element_factory_make ("capsfilter", "flt");
  conv = gst_element_factory_make ("videoconvert", "conv");
  videosink = gst_element_factory_make ("xvimagesink", "videosink");

  /* setup */
  g_object_set (G_OBJECT (flt), "caps",
  		gst_caps_new_simple ("video/x-raw",
				     "format", G_TYPE_STRING, "RGB16",
				     "width", G_TYPE_INT, 384,
				     "height", G_TYPE_INT, 288,
				     "framerate", GST_TYPE_FRACTION, 1, 1,
				     NULL), NULL);
  gst_bin_add_many (GST_BIN (pipeline), fakesrc, flt, conv, videosink, NULL);
  gst_element_link_many (fakesrc, flt, conv, videosink, NULL);

  /* setup fake source */
  g_object_set (G_OBJECT (fakesrc),
		"signal-handoffs", TRUE,
		"sizemax", 384 * 288 * 2,
		"sizetype", 2, NULL);
  g_signal_connect (fakesrc, "handoff", G_CALLBACK (cb_handoff), NULL);

  /* play */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* clean up */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}
      <!-- example-end fakesrc.c --></programlisting>
    </sect2>
  </sect1>

  <sect1 id="section-data-manager">
    <title>Embedding static elements in your application</title>
    <para>
      The <ulink type="http"
      url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/index.html">Plugin
      Writer's Guide</ulink> describes in great detail how to write elements
      for the &GStreamer; framework. In this section, we will solely discuss
      how to embed such elements statically in your application. This can be
      useful for application-specific elements that have no use elsewhere in
      &GStreamer;.
    </para>
    <para>
      Dynamically loaded plugins contain a structure that's defined using
      <function>GST_PLUGIN_DEFINE ()</function>. This structure is loaded
      when the plugin is loaded by the &GStreamer; core. The structure
      contains an initialization function (usually called
      <function>plugin_init</function>) that will be called right after that.
      It's purpose is to register the elements provided by the plugin with
      the &GStreamer; framework. If you want to embed elements directly in
      your application, the only thing you need to do is to replace
	  <function>GST_PLUGIN_DEFINE ()</function> with
	  <function>GST_PLUGIN_DEFINE_STATIC ()</function>. This will cause the
      elements to be registered when your application loads, and the elements
      will from then on be available like any other element, without them
      having to be dynamically loadable libraries. In the example below, you
      would be able to call <function>gst_element_factory_make
      ("my-element-name", "some-name")</function> to create an instance of the
      element.
    </para>

    <programlisting>
/*
 * Here, you would write the actual plugin code.
 */

[..]

static gboolean
register_elements (GstPlugin *plugin)
{
  return gst_element_register (plugin, "my-element-name",
			       GST_RANK_NONE, MY_PLUGIN_TYPE);
}

GST_PLUGIN_DEFINE_STATIC (
  GST_VERSION_MAJOR,
  GST_VERSION_MINOR,
  "my-private-plugins",
  "Private elements of my application",
  register_elements,
  VERSION,
  "LGPL",
  "my-application",
  "http://www.my-application.net/"
)
    </programlisting>
  </sect1>
</chapter>