Video Overlay Interface

The #GstVideoOverlay interface is used for 2 main purposes :

A plugin drawing video output in a video window will need to have that window at one stage or another. Passive mode simply means that no window has been given to the plugin before that stage, so the plugin created the window by itself. In that case the plugin is responsible of destroying that window when it's not needed any more and it has to tell the applications that a window has been created so that the application can use it. This is done using the have-window-handle message that can be posted from the plugin with the gst_video_overlay_got_window_handle method.

As you probably guessed already active mode just means sending a video window to the plugin so that video output goes there. This is done using the gst_video_overlay_set_window_handle method.

It is possible to switch from one mode to another at any moment, so the plugin implementing this interface has to handle all cases. There are only 2 methods that plugins writers have to implement and they most probably look like that :

static void
gst_my_filter_set_window_handle (GstVideoOverlay *overlay, guintptr handle)
{
  GstMyFilter *my_filter = GST_MY_FILTER (overlay);

  if (my_filter->window)
    gst_my_filter_destroy_window (my_filter->window);
    
  my_filter->window = handle;
}

static void
gst_my_filter_xoverlay_init (GstVideoOverlayClass *iface)
{
  iface->set_window_handle = gst_my_filter_set_window_handle;
}
    

You will also need to use the interface methods to post messages when needed such as when receiving a CAPS event where you will know the video geometry and maybe create the window.

static MyFilterWindow *
gst_my_filter_window_create (GstMyFilter *my_filter, gint width, gint height)
{
  MyFilterWindow *window = g_new (MyFilterWindow, 1);
  ...
  gst_video_overlay_got_window_handle (GST_VIDEO_OVERLAY (my_filter), window->win);
}

/* called from the event handler for CAPS events */
static gboolean
gst_my_filter_sink_set_caps (GstMyFilter *my_filter, GstCaps *caps)
{
  gint width, height;
  gboolean ret;
  ...
  ret = gst_structure_get_int (structure, "width", &width);
  ret &= gst_structure_get_int (structure, "height", &height);
  if (!ret) return FALSE;

  gst_video_overlay_prepare_window_handle (GST_VIDEO_OVERLAY (my_filter));
  
  if (!my_filter->window)
    my_filter->window = gst_my_filter_create_window (my_filter, width, height);

  ...
}