Synchronization precision

Nicolas Dufresne nicolas at
Sun Jul 7 16:28:05 UTC 2019

Le dim. 7 juill. 2019 08 h 25, RiccardoCagnasso <riccardo at> a
écrit :

> I took some time to read your answer, the discussion you linked and some of
> the code in your branch. I think I managed to understand what you are
> trying
> to do and what problems you are facing, at least to an extent (which is an
> achievement by itself). I'm not sure that I can give any meaningful insight
> about that though. Except for maybe the fact that synchronizing the
> gstreamer clock with the wayland presentation layer that way doesn't sound
> right to me. It feels like the process of synchronizing with vblanks shoud
> be confined to the video sink and not have a larger impact on the pipeline
> (and clock) as a whole. But this is probably a stupid observation since I
> don't really know much about gstreamer internals, so meh.

The slight thing you have missed is that this is doing the exact opposite.
The Wayland clock is being slaved to the pipeline clock, hence have no
impact on the pipeline. So it is confined. That being said, if that clock
was the only one available, it would make sense to provide it, that wasn't
anything I wanted to achieve, specially that I'd need an hybrid clock in
order to increase precision.

> What I also did, possibly less pointless, is to make some tests with
> glimagesink. I'll link some results here because they are puzzling.
> 1)
> This is my laptop, with an Intel video card, vaapidecode/vaapisink and both
> videos on the same monitor. You can see random desyncs consistent with what
> we discussed.

The desync is likely not random though, you'd need a better test.

> 2)
> This is one of my workstations with a Quadro P400. Vsync is enabled and
> full
> composition pipeline. nvdec and glimagesink. Both videos on one monitor.
> The
> sync is perfect as far as I can see.
> 3)
> This is the same workstation but with one video on each monitor. One video
> seems exactly one frame later than the second.

I'm not sure if we can with this test see the difference between .5 frame
delay and 1 frame delay.

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