GstVideoOverlay

The GstVideoOverlay interface is used for 2 main purposes :

• To get a grab on the Window where the video sink element is going to render. This is achieved by either being informed about the Window identifier that the video sink element generated, or by forcing the video sink element to use a specific Window identifier for rendering.
• To force a redrawing of the latest video frame the video sink element displayed on the Window. Indeed if the GstPipeline is in GST_STATE_PAUSED state, moving the Window around will damage its content. Application developers will want to handle the Expose events themselves and force the video sink element to refresh the Window's content.

Using the Window created by the video sink is probably the simplest scenario, in some cases, though, it might not be flexible enough for application developers if they need to catch events such as mouse moves and button clicks.

Setting a specific Window identifier on the video sink element is the most flexible solution but it has some issues. Indeed the application needs to set its Window identifier at the right time to avoid internal Window creation from the video sink element. To solve this issue a GstMessage is posted on the bus to inform the application that it should set the Window identifier immediately. Here is an example on how to do that correctly:

 static GstBusSyncReply
 create_window (GstBus * bus, GstMessage * message, GstPipeline * pipeline)
 {
  // ignore anything but 'prepare-window-handle' element messages
  if (!gst_is_video_overlay_prepare_window_handle_message (message))
    return GST_BUS_PASS;

  win = XCreateSimpleWindow (disp, root, 0, 0, 320, 240, 0, 0, 0);

  XSetWindowBackgroundPixmap (disp, win, None);

  XMapRaised (disp, win);

  XSync (disp, FALSE);

  gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (GST_MESSAGE_SRC (message)),
      win);

  gst_message_unref (message);

  return GST_BUS_DROP;
 }
 ...
 int
 main (int argc, char **argv)
 {
 ...
  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_set_sync_handler (bus, (GstBusSyncHandler) create_window, pipeline,
        NULL);
 ...
 }

Two basic usage scenarios

There are two basic usage scenarios: in the simplest case, the application uses playbin or playsink or knows exactly what particular element is used for video output, which is usually the case when the application creates the videosink to use (e.g. xvimagesink, ximagesink, etc.) itself; in this case, the application can just create the videosink element, create and realize the window to render the video on and then call gst_video_overlay_set_window_handle directly with the XID or native window handle, before starting up the pipeline. As playbin and playsink implement the video overlay interface and proxy it transparently to the actual video sink even if it is created later, this case also applies when using these elements.

In the other and more common case, the application does not know in advance what GStreamer video sink element will be used for video output. This is usually the case when an element such as autovideosink is used. In this case, the video sink element itself is created asynchronously from a GStreamer streaming thread some time after the pipeline has been started up. When that happens, however, the video sink will need to know right then whether to render onto an already existing application window or whether to create its own window. This is when it posts a prepare-window-handle message, and that is also why this message needs to be handled in a sync bus handler which will be called from the streaming thread directly (because the video sink will need an answer right then).

As response to the prepare-window-handle element message in the bus sync handler, the application may use gst_video_overlay_set_window_handle to tell the video sink to render onto an existing window surface. At this point the application should already have obtained the window handle / XID, so it just needs to set it. It is generally not advisable to call any GUI toolkit functions or window system functions from the streaming thread in which the prepare-window-handle message is handled, because most GUI toolkits and windowing systems are not thread-safe at all and a lot of care would be required to co-ordinate the toolkit and window system calls of the different threads (Gtk+ users please note: prior to Gtk+ 2.18 GDK_WINDOW_XID was just a simple structure access, so generally fine to do within the bus sync handler; this macro was changed to a function call in Gtk+ 2.18 and later, which is likely to cause problems when called from a sync handler; see below for a better approach without GDK_WINDOW_XID used in the callback).

GstVideoOverlay and Gtk+

 #include <gst/video/videooverlay.h>
 #include <gtk/gtk.h>
 #ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>  // for GDK_WINDOW_XID
 #endif
 #ifdef GDK_WINDOWING_WIN32
 #include <gdk/gdkwin32.h>  // for GDK_WINDOW_HWND
 #endif
 ...
 static guintptr video_window_handle = 0;
 ...
 static GstBusSyncReply
 bus_sync_handler (GstBus * bus, GstMessage * message, gpointer user_data)
 {
  // ignore anything but 'prepare-window-handle' element messages
  if (!gst_is_video_overlay_prepare_window_handle_message (message))
    return GST_BUS_PASS;

  if (video_window_handle != 0) {
    GstVideoOverlay *overlay;

    // GST_MESSAGE_SRC (message) will be the video sink element
    overlay = GST_VIDEO_OVERLAY (GST_MESSAGE_SRC (message));
    gst_video_overlay_set_window_handle (overlay, video_window_handle);
  } else {
    g_warning ("Should have obtained video_window_handle by now!");
  }

  gst_message_unref (message);
  return GST_BUS_DROP;
 }
 ...
 static void
 video_widget_realize_cb (GtkWidget * widget, gpointer data)
 {
 #if GTK_CHECK_VERSION(2,18,0)
   // Tell Gtk+/Gdk to create a native window for this widget instead of
   // drawing onto the parent widget.
   // This is here just for pedagogical purposes, GDK_WINDOW_XID will call
   // it as well in newer Gtk versions
   if (!gdk_window_ensure_native (widget->window))
     g_error ("Couldn't create native window needed for GstVideoOverlay!");
 #endif

 #ifdef GDK_WINDOWING_X11
   {
     gulong xid = GDK_WINDOW_XID (gtk_widget_get_window (video_window));
     video_window_handle = xid;
   }
 #endif
 #ifdef GDK_WINDOWING_WIN32
   {
     HWND wnd = GDK_WINDOW_HWND (gtk_widget_get_window (video_window));
     video_window_handle = (guintptr) wnd;
   }
 #endif
 }
 ...
 int
 main (int argc, char **argv)
 {
   GtkWidget *video_window;
   GtkWidget *app_window;
   ...
   app_window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
   ...
   video_window = gtk_drawing_area_new ();
   g_signal_connect (video_window, "realize",
       G_CALLBACK (video_widget_realize_cb), NULL);
   gtk_widget_set_double_buffered (video_window, FALSE);
   ...
   // usually the video_window will not be directly embedded into the
   // application window like this, but there will be many other widgets
   // and the video window will be embedded in one of them instead
   gtk_container_add (GTK_CONTAINER (ap_window), video_window);
   ...
   // show the GUI
   gtk_widget_show_all (app_window);

   // realize window now so that the video window gets created and we can
   // obtain its XID/HWND before the pipeline is started up and the videosink
   // asks for the XID/HWND of the window to render onto
   gtk_widget_realize (video_window);

   // we should have the XID/HWND now
   g_assert (video_window_handle != 0);
   ...
   // set up sync handler for setting the xid once the pipeline is started
   bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
   gst_bus_set_sync_handler (bus, (GstBusSyncHandler) bus_sync_handler, NULL,
       NULL);
   gst_object_unref (bus);
   ...
   gst_element_set_state (pipeline, GST_STATE_PLAYING);
   ...
 }

GstVideoOverlay and Qt

 #include <glib.h>;
 #include <gst/gst.h>;
 #include <gst/video/videooverlay.h>;

 #include <QApplication>;
 #include <QTimer>;
 #include <QWidget>;

 int main(int argc, char *argv[])
 {
   if (!g_thread_supported ())
     g_thread_init (NULL);

   gst_init (&argc, &argv);
   QApplication app(argc, argv);
   app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit ()));

   // prepare the pipeline

   GstElement *pipeline = gst_pipeline_new ("xvoverlay");
   GstElement *src = gst_element_factory_make ("videotestsrc", NULL);
   GstElement *sink = gst_element_factory_make ("xvimagesink", NULL);
   gst_bin_add_many (GST_BIN (pipeline), src, sink, NULL);
   gst_element_link (src, sink);

   // prepare the ui

   QWidget window;
   window.resize(320, 240);
   window.show();

   WId xwinid = window.winId();
   gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (sink), xwinid);

   // run the pipeline

   GstStateChangeReturn sret = gst_element_set_state (pipeline,
       GST_STATE_PLAYING);
   if (sret == GST_STATE_CHANGE_FAILURE) {
     gst_element_set_state (pipeline, GST_STATE_NULL);
     gst_object_unref (pipeline);
     // Exit application
     QTimer::singleShot(0, QApplication::activeWindow(), SLOT(quit()));
   }

   int ret = app.exec();

   window.hide();
   gst_element_set_state (pipeline, GST_STATE_NULL);
   gst_object_unref (pipeline);

   return ret;
 }