[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
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Reporter: gjdfgh | Type: defect
Status: new | Priority: normal
Component: | Version:
undetermined | unspecified
Keywords: | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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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.
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Ticket URL: <https://trac.ffmpeg.org/ticket/3287>
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