[FFmpeg-trac] #5791(ffmpeg:new): -itsoffset shifts using a different value on some subtitles

[FFmpeg]

#5791: -itsoffset shifts using a different value on some subtitles
--------------------------------+----------------------------------
             Reporter:  gillux  |                     Type:  defect
               Status:  new     |                 Priority:  minor
            Component:  ffmpeg  |                  Version:  3.0.2
             Keywords:          |               Blocked By:
             Blocking:          |  Reproduced by developer:  0
Analyzed by developer:  0       |
--------------------------------+----------------------------------
 I tried to resync subtitles with the -itsoffset option. The resulting
 subtitles had timestamps offset to a different value from what I provided
 on the command line.

 I figured out that the problem has to do with the first subtitle that
 would have a negative timestamp once the offset is applied. I attached a
 simple subtitle file (to_resync.srt) to demonstrate the problem. The
 following command:
 {{{
 $ ffmpeg -itsoffset -15 -i to_resync.srt synched.srt
 ffmpeg version 3.0.2 Copyright (c) 2000-2016 the FFmpeg developers
   built with gcc 5.3.0 (GCC)
   configuration: --prefix=/usr --disable-debug --disable-static --disable-
 stripping --enable-avisynth --enable-avresample --enable-fontconfig
 --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-
 libbluray --enable-libdcadec --enable-libfreetype --enable-libfribidi
 --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-
 libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-
 libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger
 --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora
 --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx
 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid
 --enable-netcdf --enable-shared --enable-version3 --enable-x11grab
   libavutil      55. 17.103 / 55. 17.103
   libavcodec     57. 24.102 / 57. 24.102
   libavformat    57. 25.100 / 57. 25.100
   libavdevice    57.  0.101 / 57.  0.101
   libavfilter     6. 31.100 /  6. 31.100
   libavresample   3.  0.  0 /  3.  0.  0
   libswscale      4.  0.100 /  4.  0.100
   libswresample   2.  0.101 /  2.  0.101
   libpostproc    54.  0.100 / 54.  0.100
 Input #0, srt, from 'to_resync.srt':
   Duration: N/A, bitrate: N/A
     Stream #0:0: Subtitle: subrip
 Output #0, srt, to 'synched.srt':
   Metadata:
     encoder         : Lavf57.25.100
     Stream #0:0: Subtitle: subrip (srt)
     Metadata:
       encoder         : Lavc57.24.102 srt
 Stream mapping:
   Stream #0:0 -> #0:0 (subrip (srt) -> subrip (srt))
 Press [q] to stop, [?] for help
 size=       0kB time=00:00:10.00 bitrate=   0.1kbits/s speed=6.9e+04x
 video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: 144.680847%
 }}}

 produces synched.srt with timestamps shifted to -1 second whereas I asked
 for -15 seconds.

--
Ticket URL: <https://trac.ffmpeg.org/ticket/5791>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker