Copy of my answer to the question “Is there an option in ffmpeg to specify a subtitle track that should be shown by default?” on superuser:

No, there isn’t an option in ffmpeg to specify a subtitle track that should be shown by default.

(ffmpeg version 2.6.1 built with gcc 4.9.2 (GCC) 20150304 (prerelease) as default on Arch, see end of answer for compile flags)

Resources highlighting the lack of command line options for setting default/forced subtitles stream in ffmpeg include the following:

Finally, this question on StackOverflow asks the same as you but for mkv, and highlights the fact that ffmpeg is the culprit since mkvpropedit (part of mkvtoolnix) can do it.

However, there is a potential workaround

This March 2013 ffmpeg bug report (still open) deals with mkv and default flag for subtitles and explains something that we can use to circumvent this limitation in some scenarios:

The Matroska specification - ​http://matroska.org/technical/specs/index.html (search for FlagDefault) - specifies that the value for the default flag is “default” if nothing is specified, the demuxer therefore marks all subtitle tracks as “default”.

So ffmpeg may write the default flag on our subtitles stream in some cases; I tried it with different input video files and got mixed results:

  • with the Big Buck Bunny AVI Mpeg4 and a random .srt file, the remuxed mkv subtitles stream did have the default flag:

      $ ffmpeg -i big_buck_bunny_480p_surround-fix.avi -i subtitle.srt -c copy test.mkv
  $ ffmpeg -i test.mkv
  Input #0, matroska,webm, from 'test.mkv':
    Metadata:
      ENCODER         : Lavf56.25.101
    Duration: 00:28:31.40, start: 0.000000, bitrate: 1028 kb/s
      Stream #0:0: Video: mpeg4 (Simple Profile), yuv420p, 854x480 [SAR 1:1 DAR 427:240], 24 fps, 24 tbr, 1k tbn, 24 tbc (default)
      Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
      Stream #0:2: Subtitle: subrip (default)

  • however I couldn’t reproduce this with a sample mp4-contained H264 sample neither with a personal mp4, so I guess mp4 --> mkv doesn’t work. But what if…?

      $ ffmpeg -i sample.mp4 -c copy sample.avi
  $ ffmpeg -i sample.avi -i subtitle.srt -c copy sample.mkv
  $ ffmpeg -i sample.mkv
  ...
Input #0, matroska,webm, from 'sample.mkv':
  Metadata:
	ENCODER         : Lavf56.25.101
  Duration: 00:28:31.40, start: 0.000000, bitrate: 2 kb/s
	Stream #0:0: Video: h264 (Constrained Baseline), yuv420p(tv, bt709), 560x320, SAR 1:1 DAR 7:4, 60 fps, 60 tbr, 1k tbn, 60 tbc (default)
	Stream #0:1: Audio: aac (LC), 48000 Hz, mono, fltp (default)
	Stream #0:2: Subtitle: subrip (default)

VICTORY! Doing mp4 --> avi and then avi + srt --> mkv gets the subtitles stream on by default and VLC displays them correctly.

Sure that’s very roundabout and I’d rather not think of the information loss in the process, but at least it works.

Summary

  • ffmpeg is definitely not adapted right now for setting streams as default or forced,
  • mkv is probably better than mp4 as an origin container, since it has a known tool doing the job fine (mkvpropedit from mkvtoolnix).

ffmpeg version details:

ffmpeg version 2.6.1 built with gcc 4.9.2 (GCC) 20150304 (prerelease) configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-runtime-cpudetect --enable-shared --enable-swresample --enable-vdpau --enable-version3 --enable-x11grab