Linux: Improve Rendering Performance

[trin94]

The rendering performance issue is caused by us rendering into the same framebuffer as Qt renders its UI. When playing 24 fps videos, this reduces the framerate being rendered to 24 fps. And therefore menu animations and GUI interactions result in a bad user experience.

We tried playing around with mpv not blocking for the next frame but that leads to frame drops mpvqc/mpvQC-dev#61.

We need to find a different way to improve performance.

Ideas:

• Use two framebuffers (1x onscreen for Qt, 1x offscreen for mpv)

[trin94]

Check #27 if this issue is necessary.

[trin94]

https://github.com/vlc-qt/vlc-qt/tree/master/src/qml

[trin94]

Maybe using something like this helps: https://doc-snapshots.qt.io/qt6-6.5/qtquick-itemvariablerefreshrate-example.html

[trin94]

Asked for help in the Qt forum: https://forum.qt.io/topic/149728/improve-rendering-performance-while-integrating-libmpv-with-qtquick-python

[barribarrier]

Hi @trin94

I've tested this with 24fps and 5fps videos but the UI seems unaffected? I don't notice any lag or slowdowns when interacting with the UI.

Tested on linux.

Edit: nvm you're right

[trin94]

This rendering issue only applies for Linux on both x11 and Wayland. On Windows, we now (f6e7cde) do hWnd embedding instead of libmpv rendering. Unless someone has a clever idea, I don't think we can do much about it until there's a Wayland protocol including common compositor support for this.

[trin94]

For Linux users, a temporary fix is to adjust the video-timing-offset setting in mpv (Options → Edit mpv.conf...):

By default, video-timing-offset is set to 0.050. On a 120 Hz monitor, we want the video to match the refresh rate, which is 120 frames per second (fps). This means each frame should appear every 1000 / 120 ≈ 8.3 ms. By lowering the video-timing-offset (e.g., to 0.008), mpvQC will reduce the maximum frame delay from 50 ms (default) to about 8 ms, keeping video and monitor refresh rates in sync.

Here are suggested values for different frame rates:

Target Frame Rate	Frame Time Calculation	Recommended Offset
60 fps	1000 ms / 60 ≈ 16.6 ms	video-timing-offset=0.016
120 fps	1000 ms / 120 ≈ 8.3 ms	video-timing-offset=0.008
240 fps	1000 ms / 240 ≈ 4.2 ms	video-timing-offset=0.004

Note: This requires powerful hardware, and even with adjustments, you might still see frame drops or audio sync issues. You can check for any audio-video drift in mpvQC by pressing i to display video stats. If you experience frame drops, consider lowering the target frame rate.