[gst-devel] g_signal_connect question

[Michael Smith]

On Wed, Feb 24, 2010 at 9:52 AM, wanting2learn
<wanting2learn at hotmail.co.uk> wrote:
>
> I am woking on a gstreamer project and have a problem.
>
> I have pipeline set up and I have used the following to monitor the data
> passing through it:
>
> static void onVideoHandoff( GstElement *object, GstBuffer *buffer, gpointer
> user_data )
> {
>    //blah..blah
> }
>
> GstElement *identity = gst_element_factory_make("identity", "identity");
> g_signal_connect( identity, "handoff", G_CALLBACK(onVideoHandoff), this);
>
> The problem I'm having is that in my onVideoHandoff(..) function when I get
> the size of the 'buffer' (thisPacketSize = GST_BUFFER_SIZE(buffer);)  it is
> always the same size no matter what type of video frame is going through the
> pipeline. (I would expect it to be smaller for delta frames etc but its
> always about 2MB).

Most likely you put your identity element after your decoder - so
you're looking at decoded video frames, which are (for a given
height/width/format) obviously fixed size. You didn't tell us anything
about your pipeline, so there's no way for us to know - but this
sounds like a plausible guess.


>
> I'm thinking that this is because the onVideoHandoff function is called
> after the video has been decoded and that the buffer size is always the same
> as it is about to be writen on screen.
>
> is this correct??
>
> I think I need to monitor the data when it is going through my decoder
> element.

Well, if you want to look at things before the decoder, you could just
put your identity element before the decoder.

You can also use pad probes for this - see the documentation for details.

>
> My decoder element is set up like so depending in the type needed:
> decoder = gst_element_factory_make("ffdec_mjpeg", "decoder"); OR
> decoder = gst_element_factory_make("ffdec_mpeg4", "decoder"); OR
> decoder = gst_element_factory_make("ffdec_h264", "decoder"); OR
>
> I'm trying to attach an identity element like above but whatever I have
> tried has failed.  I have tried the following:
> g_signal_connect (decoder, "new-decoded-pad", G_CALLBACK (cb_newpad),
> NULL);....
> g_signal_connect( decoder, "handoff", G_CALLBACK(cb_handoff), this)
>
> etc but I do not really know the proper way to do this. (the footprint of
> the callback function etc)

Decoders won't normally have a handoff or new-decoded-pad signal; if
you run "gst-inspect ffdec_h264" (or other decoders) it'll tell you
what signals it has.

You should probably just use a pad probe.

Mike