[FFmpeg-trac] #6021(avcodec:closed): tx3g / mov_text subtitles are not encoded correctly in some specific cases

[FFmpeg]

#6021: tx3g / mov_text subtitles are not encoded correctly in some specific cases
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             Reporter:  erikbs         |                    Owner:
                 Type:  defect         |                   Status:  closed
             Priority:  normal         |                Component:  avcodec
              Version:  git-master     |               Resolution:  fixed
             Keywords:  utf8 mov_text  |               Blocked By:
             Blocking:                 |  Reproduced by developer:  0
Analyzed by developer:  0              |
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Comment (by erikbs):

 Replying to [comment:17 CoRoNe]:
 > First time I've been looking at this again in 2 years time. I believe my
 issue is different than erikbs', because this fix didn't do anything for
 me.
 > Whenever I mux subtitles it will always be to a matroska container,
 because muxing mov_text in a mp4 container was problematic and after
 testing today I can say it still is.
 > Ran above command again with a (obviously) newer FFmpeg binary, but now
 my Samsung Smart TV (2014) doesn't show/render any subtitles at all. My tv
 has no problems at all with the video generated by MP4Box and shows all
 subtitle lines.
 > Could it be that FFmpeg doesn't fully adhere to the ISO/IEC 14496-17
 standard? There has to be a reason why my tv doesn't like the FFmpeg
 generated MP4[tx3g] files afterall.

 Absolutely possible (actually I am even convinced that it does not,
 considering how it was overlooked that character and byte count are two
 different things). Through my investigation, I discovered that MP4Box
 writes a subtitle box size while ffmpeg does not (or, more precisely, it
 apparently sets the box size to 0x0). Perhaps that is why the subtitles
 are not shown on the TV?

 Another thing to try is to encode the subtitles into an MP4 file with
 ffmpeg as usual, then use MP4Box to demux and remux the tx3g subtitles
 generated by ffmpeg (`mp4box -ttxt 3 FILENAME.mp4` etc.). If that works,
 then ffmpeg somehow messes up the muxing, not the encoding. If it still
 does not work, then there is something wrong with its tx3g encoder.

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Ticket URL: <https://trac.ffmpeg.org/ticket/6021#comment:18>
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