[FFmpeg-trac] #1924(avformat:closed): ASS subtitles are demuxed inconsistently

FFmpeg trac at avcodec.org
Fri Nov 30 23:13:49 CET 2012

#1924: ASS subtitles are demuxed inconsistently
             Reporter:  gjdfgh       |                    Owner:
                 Type:  enhancement  |                   Status:  closed
             Priority:  normal       |                Component:  avformat
              Version:  unspecified  |               Resolution:  invalid
             Keywords:  ass          |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |

Comment (by gjdfgh):

 This is not an issue with the ASS demuxer, but an issue with the ffmpeg
 ASS packet format. It loses information. You can't distinguish multiple
 identical events from each other. This happens always when getting ASS
 subtitles from a ffmpeg demuxer.

 It should happen when seeking back, so that the same subtitle events are
 demuxed and displayed again. This assumes that mplayer doesn't remove all
 events from to the ASS_Track on seeking.

 Turns out it does.

 Look at mplayer commit 31293, where this was explicitly "fixed", in
 exchange for a worse user experience: you lose the nice behavior that
 subtitles will display immediately if you seek back to somewhere you
 already have watched, but it also compensates for ffmpeg's not so great
 packet format.

 So you can't demonstrate this issue with mplayer. In fact, ffmpeg pretty
 much forces you to use ass_flush_events() on seeking to delete all
 existing subtitle events, because otherwise subtitle events would

 (Note: depending on how the file is muxed, subtitles can appear
 immediately anywaqy when seeking back. Just increase the duration of the
 subtitle events until it breaks.)

Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/1924#comment:10>
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