Multiple gnloperation elements?

Lorenzo Miniero lminiero at
Mon Jul 8 02:05:52 PDT 2013


I'm trying to build an application to post-process some recorded media
using gstreamer. In particular, audio and video files are separated, and
timestamps are used to specify when they appeared in the original session.
What I'm trying to do is replicating the session using dynamic audio and
video mixing, by attaching the streams only when they're supposed to be

At the beginning I tried to do this "manually", that is, attaching the
streams to an adder/videomixer2 when some time had passed: this had mixed
results (actually, quite bad results, especially in terms of A/V
synchronization) so I tried a different approach, using the gnonlin plugins
for the purpose.

I found a couple of old posts on the mailing list that helped me understand
how I could make use of them for my scenario:

and basing on them I at least managed to have the streams (both audio and
video) dynamically appear at the same time. I still have some issues (e.g.,
when streams end or get stuck for any reason, which I guess I'll have to
handle accordingly by detaching the related pads) but I'll move to this
only when I'll have the scenario sorted out, so that's not what I'd like to
ask you.

Right now, I managed to implement the dynamic videomixing functionality
following the suggestions in the mentioned posts: using a gnloperation
containing the videomixer2, the 'input-priority-changed' signal and
properly set priority values on the video streams, I'm able to set the
xpos/ypos/zorder of the video in the videomixer2 element just fine. The
problem is that I'd like to do some manipulation on the videos before
passing them to the videomixer2 element, specifically cropping the video
(they are 320x240 while I need them to be 240x240) and adding a simple
textoverlay on top of them. This is something I could do in my original
"naive" attempt, but I can't seem to be able to do it using gnonlin.

As far as I've understood, anything in a gnlcomposition must be a compliant
non-linear bin element, meaning I cannot just place those filters cascaded
after a gnlfilesource. I guess the proper way to do so would be to have
further gnloperation elements to take care of those operations, but I
haven't managed so far to understand whether or not a gnlcomposition can
contain more than a single gnloperation at all. If so, how can I have a
specific gnlfilesource attach to a specific gnloperation taking care of it,
and then have this be the source of the data to feed to the videomixing
gnloperation itself? I tried playing a bit with the 'pad-added' signals but
I couldn't do anything in that sense.

Any hint on the best way to proceed?
Thanks in advance for any help you'll be able to provide me!

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