Moving subtitles not correctly displayed by webplayer

[denden1088]

Describe The Bug
Playback of moving subtitles seems to lags behind by no more than 1-2 frame when being played.

Steps To Reproduce
Play any video with .ass subtitles on a scene with moving signs

Expected Behavior
The subtitle's location should match exactly to its correct frame and not appear "ghosting"

Logs

Screenshots

System (please complete the following information):

• Platform: Windows
• Browser: Firefox, Chrome (likely more but I have not tested)
• Jellyfin Version: 10.10.3

Additional Context
Looking at each individual frame, it primarily seems that the subtitle stays in its position in intervals of 2 frames (though there will occasionally be an exception) even if there should be movement on every frame.

[dmitrylyzo]

The current JSO implementation forces each event to last at least 1/targetFps seconds. targetFps is hardcoded to 24. Therefore, if the original frame rate is higher, it will cause subtitle frames to be "stretched".

What is the frame rate of the video?

I opened #6462 to make targetFps equal to the original frame rate, but that probably won't fix the problem completely. Also, if the subtitles contain too much animation, JSO may not be able to maintain that frame rate, producing some intermediate state (ghosting?).

[denden1088]

The framerate of my video is also NTSC 24 fps so unfortunately it likely wont affect it. There is the possibility that its just struggling to maintain the frame rate, but the animation didn't seem anything overly significant. I might try to make a test video later on to get more details on the issue.