18.2. Manually adding or removing data from/to a pipeline

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. However, there is no support for those methods. 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 guanranteed 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.

After all those disclaimers, let's start. There's three possible elements that you can use for the above-mentioned purposes. Those are called "fakesrc" (an imaginary source), "fakesink" (an imaginary sink) and "identity" (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.

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.

18.2.1. Inserting or grabbing data

The three before-mentioned elements (fakesrc, fakesink and identity) each have a "handoff" signal that will be called in the _get ()- (fakesrc) or _chain ()-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 "sizemax" property. For both fakesrc and fakesink, you also have to set the "signal-handoffs" property for this method to work.

Note that your handoff function should not 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.

18.2.2. Forcing a format

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 GstCaps on the pipeline, which is possible by using filtered caps. You can set a filtered caps on a link by using the "capsfilter" element in between the two elements, and specifying a GstCaps as "caps" property on this element. It will then only allow types matching that specified capability set for negotiation.

18.2.3. Example application

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.


#include <string.h> /* for memset () */
#include <gst/gst.h>

static void
cb_handoff (GstElement *fakesrc,
	    GstBuffer  *buffer,
	    GstPad     *pad,
	    gpointer    user_data)
{
  static gboolean white = FALSE;

  /* this makes the image black/white */
  memset (GST_BUFFER_DATA (buffer), white ? 0xff : 0x0,
	  GST_BUFFER_SIZE (buffer));
  white = !white;
}

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

  /* init GStreamer */
  gst_init (&argc, &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 ("ffmpegcolorspace", "conv");
  videosink = gst_element_factory_make ("xvimagesink", "videosink");

  /* setup */
  g_object_set (G_OBJECT (flt), "caps",
  		gst_caps_new_simple ("video/x-raw-rgb",
				     "width", G_TYPE_INT, 384,
				     "height", G_TYPE_INT, 288,
				     "framerate", GST_TYPE_FRACTION, 1, 1,
				     "bpp", G_TYPE_INT, 16,
				     "depth", G_TYPE_INT, 16,
				     "endianness", G_TYPE_INT, G_BYTE_ORDER,
				     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;
}