[FFmpeg-trac] #9018(ffmpeg:new): Issue with disposition "forced" for a subtitles stream (mov_tex) in a mp4 container

[FFmpeg]

#9018: Issue with disposition "forced" for a subtitles stream (mov_tex) in a mp4
container
-------------------------------------+-------------------------------------
             Reporter:  andreaerdna  |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:
                                     |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Using ffmpeg (4.3.1-2020-11-19-full_build-www.gyan.dev) on Windows 7 to
 mux video, audio and subtitles streams in a mp4 container, I use the
 "-disposition:s:0 forced" option in order to force the subtitles stream to
 be automatically displayed.

 However ffmpeg fails to properly set the "disposition" option for the
 subtitles stream to "forced" so I need to manually enable the subtitle
 when the video is played by VLC media player 3.0.11.

 The exact command is:
 ffmpeg -i Video.mp4 -i Audio.mp4 -i Sub.vtt -map 0 -map 1 -map 2
 -metadata:s:a:0 language=jpn -metadata:s:s:0 language=ita -c:v copy -c:a
 copy -c:s mov_text -disposition:a:0 default -disposition:s:0 forced
 Video_Audio_Sub.mp4

 The relevant part of the command output for the subtitle stream is:

     Stream #0:2(ita): Subtitle: mov_text (tx3g / 0x67337874) (forced)

 While the subtitles stream is tagged as "(forced)" in the command output,
 the subtitle stream is only tagged as "default" in the resulting muxed
 video  as reported by ffprobe and mediainfo and is not automatically
 displayed in VLC:

 ffprobe Video_Audio_Sub.mp4
 [...]
     Stream #0:2(ita): Subtitle: mov_text (tx3g / 0x67337874), 0 kb/s
 (default)

 mediainfo Video_Audio_Sub.mp4
 [...]
 Text
 ID                                       : 3
 Format                                   : Timed Text
 Muxing mode                              : sbtl
 Codec ID                                 : tx3g
 Default                                  : Yes
 Forced                                   : No

 For comparison, using mp4box -add Video_Audio.mp4 -add
 Sub.srt:txtflags=0xC0000000 -new Video_Audio_Sub.mp4 the subtitles stream
 is correctly recognized as "forced" by mediainfo (and incorrectly only as
 "default" by ffprobe) and it is automatically displayed in VLC:

 mediainfo Video_Audio_Sub.mp4
 Text
 ID                                       : 3
 Format                                   : Timed Text
 Codec ID                                 : tx3g
 Forced                                   : Yes

--
Ticket URL: <https://trac.ffmpeg.org/ticket/9018>
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