[Interest] Video, Animations and Vsync on Windows

Dominik R. inikofdoom at googlemail.com
Mon Mar 13 09:45:31 CET 2023

 Hi everybody,

I have a quite serious problem and maybe someone here can help.

Qt 6.4 (and 6.5 beta as well) on Windows seems to have some serious issues
regarding VSync if video playback and animations are combined (simple
example: video playing and a rotating rectangle on top).

If the animation is running, it seems that the video frames are presented
in wrong order. The video jumps and stutters. It does not if I disable
vsync by setting the sync interval to 0 and can be improved using Nvidia
settings (Fast / Low Latency etc., happens on Intel HW as well, though) but
is always notable. While the video runs perfectly when Vsync is off, nearly
all animations (especially those changing x / y positions) look horrible
then, so this is not a solution.

A similar effect can be observed when multiple videos are playing in
parallel, in particular if they have different frame rates.

I digged a bit deeper into QQuickVideoOutput and QSGVideoMaterial and I'm
confident that all frames arrive at the correct time in the correct order
for presentation but "something" later messes things up. If I force the use
of hardware video textures off which "solved" the issue, but of course is
pretty much unusable from a performance perspective.

This happens using OpenGL and Direct3D. There are some bug reports that
contain some extra info (videos to demonstrate the effect, logfiles, etc):

Every fix, workaround or just general idea where to have a look is highly
appreciated (deadline for important project is coming nearer).

Thanks and kind regards
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