[FFmpeg-trac] #8825(undetermined:new): Corrupt timestamps on WebVTT subtitles

[FFmpeg]

#8825: Corrupt timestamps on WebVTT subtitles
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             Reporter:  paralucent   |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  git-master   |               Resolution:
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by giwiniswut):

 I am also affected by this issue. German TV station ZDF for example serves
 their livestream with WebVTT subtitles served via HLS.

 I found that there is a patch for X-TIMESTAMP-MAP support on
 [https://patchwork.ffmpeg.org/project/ffmpeg/patch/20170205194344.333-1-franklinphillips@gmx.com/
 patchwork.ffmpeg.org].

 It is rather old (from 2017-02-05) but i changed it a little bit so that
 it applies to the current master branch. I also made it so that it treats
 all timestamps relative to the first fragment. See the attached patches.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8825#comment:2>
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