FFmpeg unintelligently meddled input timestamps during "-c copy"

[hachi]

Summary of the bug:

FFmpeg incorrecly alters timestamps, which results in subtitles not displayed when viewing output file:

[sost#0:6/copy @ 0x127e150b0] Non-monotonic DTS; previous: 1268750, current: 150; changing to 1268750. This may result in incorrect timestamps in the output file.
[sost#0:6/copy @ 0x127e150b0] Non-monotonic DTS; previous: 1268750, current: 2020; changing to 1268750. This may result in incorrect timestamps in the output file.
[sost#0:6/copy @ 0x127e150b0] Non-monotonic DTS; previous: 1268750, current: 5070; changing to 1268750. This may result in incorrect timestamps in the output file.

I have not found an option to preserve original timestamps and bypass this error.

How to reproduce:

% cat input_*.mkv > input.mkv
% ffmpeg -i input.mkv -c copy -map 0 output.mkv

ffmpeg version 7.0.2 Copyright (c) 2000-2024 the FFmpeg developers
built with Apple clang version 15.0.0 (clang-1500.3.9.4)

[hachi]

If needed I can attach a 10MB file to see how subtitles are affected

[Balling]

Please do.

[MasterQuestionable]

͏    Try "-copyts" whatsoever..?
͏    ​https://trac.ffmpeg.org/ticket/11034#comment:1

͏    Such peculiar timestamps indicated input seemingly broken.
͏    Caution that FFmpeg currently may not handle non-perfectly-valid files in the most sensible way:
͏    ​https://trac.ffmpeg.org/ticket/11056#comment:18

[MasterQuestionable]

͏    Also, the "DTS" here likely refers presentation timestamp ("PTS"). (misuse)
͏    Non-monotonic DTS (count in presentation order) is valid involving B-frames.

͏    See also: ​https://trac.ffmpeg.org/ticket/11055#comment:16

[Balling]

"Also, the "DTS" here likely refers presentation timestamp ("PTS"). (misuse)"

Mkv has no DTS. But here it appears that ffmpeg is generating DTS that are wrong. Each next DTS must be strictly bigger than previous one.

[hachi]

I have uploaded a 10 seconds mkv split into 3 parts, because of the 2.5M upload limit.
To concatenate the parts simply do:
cat input_*.mkv > input.mkv

[MasterQuestionable]

͏    "input.mkv" is a blob of attachments... (mostly TTF, subtitles, and metadata)

͏    Same bothersome... ​https://trac.ffmpeg.org/ticket/10891#comment:5
͏    Anyway, I extracted some of the subtitles best-effort:

[MasterQuestionable]

͏    I guess this is the one of interest:
͏    Tagged default; also had alike error messages during extraction.

[MasterQuestionable]

͏    ffprobe -hide_banner -threads 0 -show_entries "frame" -select_streams v:0 -of "xml" "input.mkv" -o "0.h265.xml"