10 bit HEVC frame offset (sync)

[emperor06]

Frame accuracy seems wrong when using AegisubDC with a hevc-mkv video. It's offset by 3 frames for me. MPC-HC, mpv, and Subtitle Edit say a particular frame is 3834 while AegisubDC says it's frame 3831.

Here's what I've tried so far:

• Aegisub 3.2.2 (upstream) works fine, no offset.
• Converting the video to AVC with ffmpeg (nvenc) gives the correct frame numbers in AegisubDC.
• I've compiled ffms2.dll myself to include a newer version of ffmpeg (actually 4.4, version 5 doesn't seem to be in vcpkg) but got the same offset issue.
• Aegisub on Manjaro Linux (arch-based) has the same problem (aegisub 3.2.2-49).

I don't know if that affects all HEVC videos, or only 10 bit videos, or that particular hevc encoding. Also, I don't know if the offset is a consistent 3 frames. I've noticed this issue in the past (subs in sync within Aegisub but offset when playing) on some of my videos but I didn't pay too much attention before. It's very probable these videos use the same encoding parameters.

Current workaround for me:

• Use the old Aegisub 3.2.2
• Convert the videos to temporary h264 files and use these to sync the subs.

[BillyBob1354]

I have also had frame sync issues that are not present in aegisub 3.2.2 with some files. It stays in sync if I play it starting 10 seconds before the line in aegisub, but if I skip to the line or start from the end of the line and play it in reverse it goes out of sync.

[emperor06]

I made a little script that generates a mkv+ass where each frame is tagged with its frame number, so it's easier to see when Aegisub skips a frame. The bug doesn't seem to appear when the video is too short. With this generated test file, one of the frames get skipped and everything after that is out of sync by 1 frame. On real videos, it can get as bad as 3 frames.

The script runs on WSL 1 (Windows) with ffmpeg.exe and magick.exe on the path. Works on Linux too (change the default font).

If you want to have a go:

• use ./genframes.sh -l to list available fonts (defaults to Courier New, which is available on Windows)
• use ./genframes.sh -n 20000

For me, with 20k frames, frame number 481 has gone.

I wish I can attach the actual mkv file…

genframes.txt

[arch1t3cht]

This is a seek error in ffms2 that has been fixed only very recently. Until AegisubDC is released with a newer version of ffms2, current alternatives are:

• Use AegisubDC 9213 and replace ffms2.dll with the file in 9208
• Recompile the latest github version of ffms2 yourself