[mythtv] OpenGL vsync issues
matt.doran at papercut.biz
Mon Jun 4 06:43:56 UTC 2007
I recently had an issue where a DVD had very poor playback - jerky
(almost like it was missing frames) and there was visible tearing. When
I changed the video sync from progressive to interlaced the problem
disappeared. I originally thought Myth was incorrectly detecting the
DVD's scan type, but after some experimentation and help of Stanley
Kamithi I've come to the conclusion that the problem is with OpenGL
vsync (and possibly it's interaction with the bob-deinterlacer??).
I have a nvidia 6150 and I'm using bob deinterlacing. I also have all
of the vblank setting disabled in "nvidia-settings".
Below is a summary of the behavior I see with combinations of settings:
* bob deint, video scan->progressive, opengl vsync enabled: jerky
* bob deint, video scan->interlaced, opengl vsync enabled: good
* bob deint, video scan->progressive, opengl vsync disabled: good
* bob deint, video scan->interlaced, opengl vsync disabled: good
Firstly, before I go any further in trying to diagnose/fix this. Is it
worth it? Is OpenGL vsync is the preferred option and does it provide
better playback/performance. If I'm not using OpenGL vsync, then I fall
back to RTC or usleep vsync. Are there downsides to these fallback
I spent a few hours on the weekend reading through the code in
NuppelVideoPlayer.cpp, and in vsync.cpp and also adding some debugging
to better understand what's going on. I can't see anything obviously
wrong. But given the awful output and obvious tearing, there is
definitely something wrong.
My current thinking is that the OpenGL vsync works correctly if it only
has to wait for a single display refresh. I have a display set to 50Hz
and my video is 25fps, so when bob is enabled the display is updated
every refresh. When the interlacer is disabled (e.g. by setting video
scan to progressive), the display is only updated every second refresh,
so the vsync code needs to wait for 2 refreshes.
The relevant code is below (vsync.cpp:715
It appears to wait for the next refresh, then if we need to wait further
(m_delay > 0) calculate the number of refreshes to wait in a single
// Always sync to the next retrace execpt when we are very late.
if ((m_delay = CalcDelay()) > -(m_refresh_interval/2))
err = m_imp->glXWaitVideoSyncSGI(2, (frameNum+1)%2 ,&frameNum);
m_delay = CalcDelay();
// Wait for any remaining retrace intervals in one pass.
if (m_delay > 0)
uint n = m_delay / m_refresh_interval + 1;
err = m_imp->glXWaitVideoSyncSGI((n+1), (frameNum+n)%(n+1), &frameNum);
m_delay = CalcDelay();
The logic seems reasonable, and can't see anything obviously wrong (but
I don't understand all the intricacies of the video timing).
Does anyone have any suggestions as to why OpenGL vsync is resulting in
so poor output in these circumstances? Am I looking in the right
area? What else could cause poor playback and tearing only when OpenGL
vsync is enabled?
This code is a bit tricky to debug, because it's called so often and
very timing dependent. It would be interesting to know how the above 2
code blocks behave in both circumstances.
Should I just give up and use RTC vsync? ;)
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