[FFmpeg-trac] #9001(ffmpeg:new): concat demuxed ASS subtitles stop working with subtitles filter

[FFmpeg]

#9001: concat demuxed ASS subtitles stop working with subtitles filter
-------------------------------------+-------------------------------------
             Reporter:  Gregory      |                    Owner:  (none)
  Beauregard                         |
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:  ffmpeg
              Version:  git-master   |               Resolution:
             Keywords:  ass libass   |               Blocked By:
  subtitles concat                   |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Description changed by Gregory Beauregard:

Old description:

> Summary of the bug: The subtitles filter fails to reliably display concat
> demuxed ASS subtitles
> How to reproduce:
> Download test files (3.6 MB): [https://gably.net/testcase/test1.mkv]
> [https://gably.net/testcase/test2.mkv]
> Each test file has 20 seconds of video with ASS subtitles. Note in
> particular the last section of subtitles in the second clip. Concat demux
> the subtitles and videos as follows. Once by converting first to text,
> and once by leaving as ASS subtitles.
> {{{
> #!/bin/bash
> rm -f playlist.txt
> echo "file '$(readlink -f "test1.mkv")'" >> playlist.txt
> echo "file '$(readlink -f "test2.mkv")'" >> playlist.txt
> ffmpeg -f concat -safe 0 -i playlist.txt -ss 00:00:00 -c:s copy -map
> 0:s:0 -map 0:t? -y subs.mkv
> ffmpeg -f concat -safe 0 -i playlist.txt -ss 00:00:00 -c:s text -map
> 0:s:0 -y textsubs.mkv
> ffmpeg -f concat -safe 0 -ss 00:00:00 -i playlist.txt -vf
> subtitles=subs.mkv -c:v libx264 -b:v 1M -c:a libopus -b:a 96k -map 0:v
> -map 0:a -y concat.mkv
> ffmpeg -f concat -safe 0 -ss 00:00:00 -i playlist.txt -vf
> subtitles=textsubs.mkv -c:v libx264 -b:v 1M -c:a libopus -b:a 96k -map
> 0:v -map 0:a -y textconcat.mkv
> }}}
> Observe that `concat.mkv` is missing some of the ASS subtitles at the end
> of the second clip that `textconcat.mkv` is not missing. The subs are
> present in `test2.mkv`. This bug was originally observed when
> concatenating entire files as the ASS subtitles suddenly failing to
> render entirely. Various insertions of `-fix_sub_duration` don't change
> the situation. Note not including the `-ss` arguments produce other odd
> timing problems, but this may need to be filed as a separate bug.
>
> ffmpeg version: master
> built on 2020-11-22 5 pm UTC

New description:

 Summary of the bug: The subtitles filter fails to reliably display concat
 demuxed ASS subtitles
 How to reproduce:
 Download test files (3.6 MB): [https://gably.net/testcase/test1.mkv]
 [https://gably.net/testcase/test2.mkv]
 Each test file has 20 seconds of video with ASS subtitles. Note in
 particular the last section of subtitles in the second clip. Concat demux
 the subtitles and videos as follows. Once by converting first to text, and
 once by leaving as ASS subtitles.
 {{{
 #!/bin/bash
 rm -f playlist.txt
 echo "file '$(readlink -f "test1.mkv")'" >> playlist.txt
 echo "file '$(readlink -f "test2.mkv")'" >> playlist.txt
 ffmpeg -f concat -safe 0 -i playlist.txt -ss 00:00:00 -c:s copy -map 0:s:0
 -map 0:t? -y subs.mkv
 ffmpeg -f concat -safe 0 -i playlist.txt -ss 00:00:00 -c:s text -map 0:s:0
 -y textsubs.mkv
 ffmpeg -f concat -safe 0 -ss 00:00:00 -i playlist.txt -vf
 subtitles=subs.mkv -c:v libx264 -b:v 1M -c:a libopus -b:a 96k -map 0:v
 -map 0:a -y concat.mkv
 ffmpeg -f concat -safe 0 -ss 00:00:00 -i playlist.txt -vf
 subtitles=textsubs.mkv -c:v libx264 -b:v 1M -c:a libopus -b:a 96k -map 0:v
 -map 0:a -y textconcat.mkv
 }}}
 Observe that `concat.mkv` is missing some of the ASS subtitles at the end
 of the second clip that `textconcat.mkv` is not missing. The subs are
 present in `test2.mkv`. This bug was originally observed when
 concatenating entire files as the ASS subtitles suddenly failing to render
 entirely. Various insertions of `-fix_sub_duration` don't change the
 situation. Note not including the `-ss` arguments produce other odd timing
 problems, but this may need to be filed as a separate bug.

 ffmpeg version: master
 built on 2020-11-22 5 pm UTC

 In files I haven't clipped like this the concat demuxed subtitles stopped
 working exactly on the file boundaries.

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Ticket URL: <https://trac.ffmpeg.org/ticket/9001#comment:7>
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