You can see from Figure 8.1, “Visualisation of a GstBin element without ghost pads” how a bin has no pads of its own. This is where "ghost pads" come into play.

Figure 8.1. Visualisation of a GstBin element without ghost pads

A ghost pad is a pad from some element in the bin that can be accessed directly from the bin as well. Compare it to a symbolic link in UNIX filesystems. Using ghost pads on bins, the bin also has a pad and can transparently be used as an element in other parts of your code.

Figure 8.2. Visualisation of a GstBin element with a ghost pad

Figure 8.2, “Visualisation of a GstBin element with a ghost pad” is a representation of a ghost pad. The sink pad of element one is now also a pad of the bin. Because ghost pads look and work like any other pads, they can be added to any type of elements, not just to a GstBin, just like ordinary pads.

A ghostpad is created using the function gst_ghost_pad_new ():

#include <gst/gst.h>

int
main (int   argc,
      char *argv[])
{
  GstElement *bin, *sink;
  GstPad *pad;

  /* init */
  gst_init (&argc, &argv);

  /* create element, add to bin */
  sink = gst_element_factory_make ("fakesink", "sink");
  bin = gst_bin_new ("mybin");
  gst_bin_add (GST_BIN (bin), sink);

  /* add ghostpad */
  pad = gst_element_get_static_pad (sink, "sink");
  gst_element_add_pad (bin, gst_ghost_pad_new ("sink", pad));
  gst_object_unref (GST_OBJECT (pad));

[..]

}
    

In the above example, the bin now also has a pad: the pad called “sink” of the given element. The bin can, from here on, be used as a substitute for the sink element. You could, for example, link another element to the bin.