[gst-devel] Summer Of Code Project Help

Mario Torre neugens at limasoftware.net
Fri Apr 13 12:27:08 CEST 2007


My project for the Google Summer Of Code was accepted as part of the GNU

As I said in an earlier mail, my task is to hook GStreamer into GNU

Now, I've found a problem that is blocking me from a couple days and is
taking me away all my sleep :)

I need to write a (Java) class that displays this kind of information
about an audio file:

* Encoding type
* Sample rate
* Sample size in bits
* Channels
* Frame size
* Frame rate
* Endiannes
* Header size

Of course, I could just add a reader for any given kind of audio file,
but then this is pointless to have a gstreamer hook that can recognize
(almost) any type given the right plugin.

My problem is that I can't find an example of how to parse a file and
extract this data (I need to do that in C, not Java). The program should
return as soon as all these info are retrieved (if possible). I've see
the Discover class from the python bindings (which seems to be what I'm
looking for), but I hate to admit that I was unable to recreate its
behaviour, though I think I'm on the correct way. What I'm doing is this
(to see what capabilities an element has), I get a typefind element and
attach a signal handler to it, but this seems to be not enough, and all
the output I get is:

size of caps: 1
fields in structure: 0

g_signal_connect (G_OBJECT (typefind), "have-type",
                    G_CALLBACK (have_type_callback), loop);

gboolean have_type_callback(GstElement *typefind,
                            guint probability,
                            const GstCaps *caps,
                            gpointer data)
  GstStructure *st = NULL;
  int size = 0;
  g_fprintf (stderr, "print_pad\n");

  size = gst_caps_get_size (caps);
  g_fprintf (stderr, "size of caps: %d\n", size);
  for (int j = 0; j < size; j++) {
    st = gst_caps_get_structure (caps, j);
    if (st != NULL) {
      size = gst_structure_n_fields (st);
      g_fprintf (stderr, "fields in structure: %d\n", size);
      for (int i = 0; i < size; i++) {
        g_fprintf (stderr, "field name: %s\n",
                   gst_structure_nth_field_name(st, i));

I'm a bit new to all that, so forgive me if I'm doing some incredibly
stupid mistake :)

Thanks for all the help,
Lima Software - http://www.limasoftware.net/
GNU Classpath Developer - http://www.classpath.org/
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