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

FFmpeg trac at avcodec.org
Fri Nov 16 22:20:29 CET 2012

#1924: ASS subtitles are demuxed inconsistently
             Reporter:  gjdfgh    |                     Type:  defect
               Status:  new       |                 Priority:  normal
            Component:  avformat  |                  Version:  unspecified
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
 There's a difference how subtitles are demuxed with the "matroska" and the
 "ass" demuxers.

 Consider an even like this in the source file:
 Dialogue: 0,0:00:00.00,0:00:01.00,,,0000,0000,0000,,text

 "matroska" outputs as packet:

 while "ass" outputs:
 Dialogue: 0,0:00:00.00,0:00:01.00,,,0000,0000,0000,,text

 Matroska produces output that libass' ass_process_chunk() can consume.
 That means an application using the ffmpeg demuxer interface to read ASS
 subtitles (possibly for uniformity with the matroska case) has to add
 extra workarounds. Further, the "ass" output doesn't include the event
 index, while "matroska" does. (The event index is the first number, called
 "ReadOrder" in libass.) Additionally, the "ass" output does not seem to
 seem to set the packet duration correctly.

 The "ass" decoder in libavcodec is indifferent to that, but libass can't
 handle it. This makes the "ass" demuxer useless. Please fix.

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