[FFmpeg-trac] #3287(undetermined:new): Can't correctly time frame-based subtitle formats

FFmpeg trac at avcodec.org
Thu Jan 9 23:17:00 CET 2014

#3287: Can't correctly time frame-based subtitle formats
             Reporter:  gjdfgh       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Some subtitle formats, like MicroDVD, do not always use absolute
 timestamps. Instead, they use something like frame numbers.

 This makes subtitle timestamps dependend on video FPS. A subtitle demuxer
 can't return correct timestamps, because it (of course) does not know the
 video FPS. libavformat's MicroDVD demuxer just assumes 23.976 FPS.

 To further complicate the situation, MicroDVD subtitles _can_ declare a
 video FPS, in which case libavformat will use that, instead of the 23.976
 FPS fallback.

 This makes it impossible for an application to determine the subtitle
 timestamps correctly: the timestamps could be using the fallback (in which
 case the application would have to recompute all subtitle timestamps by
 reversing the fallback, and using the video's actual FPS instead), or the
 timestamps could be correct (in which case the application should not
 touch them).

 I propose adding a flag that tells the application whether the timestamps
 are correct. I would also consider this issue relatively critical, because
 it makes the MicroDVD demuxer more or less unusable.

Ticket URL: <https://trac.ffmpeg.org/ticket/3287>
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