The XOverlay interface is used for 2 main purposes :

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-xwindow-id' element messages
 if (GST_MESSAGE_TYPE (message) != GST_MESSAGE_ELEMENT)
   return GST_BUS_PASS;
 
 if (!gst_structure_has_name (message->structure, "prepare-xwindow-id"))
   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_x_overlay_set_xwindow_id (GST_X_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);
...
}

GstXOverlay

typedef struct _GstXOverlay GstXOverlay;

Opaque GstXOverlay data structure.

GstXOverlayClass

typedef struct {
  GTypeInterface klass;

  /* virtual functions */
  void (* set_xwindow_id) (GstXOverlay *overlay,
                           gulong       xwindow_id);

  void (* expose)         (GstXOverlay *overlay);
  
  void (* handle_events)  (GstXOverlay *overlay,
                           gboolean     handle_events);  
} GstXOverlayClass;

GstXOverlay interface

gst_x_overlay_set_xwindow_id ()

void                gst_x_overlay_set_xwindow_id        (GstXOverlay *overlay,
                                                         gulong xwindow_id);

This will call the video overlay's set_xwindow_id method. You should use this method to tell to a XOverlay to display video output to a specific XWindow. Passing 0 as the xwindow_id will tell the overlay to stop using that window and create an internal one.

gst_x_overlay_got_xwindow_id ()

void                gst_x_overlay_got_xwindow_id        (GstXOverlay *overlay,
                                                         gulong xwindow_id);

This will post a "have-xwindow-id" element message on the bus.

This function should only be used by video overlay plugin developers.

gst_x_overlay_prepare_xwindow_id ()

void                gst_x_overlay_prepare_xwindow_id    (GstXOverlay *overlay);

This will post a "prepare-xwindow-id" element message on the bus to give applications an opportunity to call gst_x_overlay_set_xwindow_id() before a plugin creates its own window.

This function should only be used by video overlay plugin developers.

gst_x_overlay_expose ()

void                gst_x_overlay_expose                (GstXOverlay *overlay);

Tell an overlay that it has been exposed. This will redraw the current frame in the drawable even if the pipeline is PAUSED.

gst_x_overlay_handle_events ()

void                gst_x_overlay_handle_events         (GstXOverlay *overlay,
                                                         gboolean handle_events);

Tell an overlay that it should handle events from the window system. These events are forwared upstream as navigation events. In some window system, events are not propagated in the window hierarchy if a client is listening for them. This method allows you to disable events handling completely from the XOverlay.