Allow converting subtitle codec

[soloturn]

when cutting an mp4 with mov_text subtitles it does it, the result has the stream, but neither vlc or mpv are able to display it. more reliable is matroska. trying to convert to matroska produced an error though:

/usr/share/losslesscut/resources/node_modules/ffmpeg-static/ffmpeg -hide_banner -ss 316.05468 -i m.mp4 -t 4953.48456 -avoid_negative_ts make_zero -c copy -map 0:0 -map 0:1 -map 0:2 -map 0:3 -map 0:4 -map_metadata 0 -movflags use_metadata_tags -ignore_unknown -f matroska -y m-00.05.16.054-01.27.49.539.mkv

    Stream #0:4(deu): Subtitle: mov_text (tx3g / 0x67337874), 0 kb/s (default)
    Metadata:
      handler_name    : SubtitleHandler
[matroska @ 0x6a95440] Subtitle codec 94213 is not supported.

https://newspaint.wordpress.com/2018/06/24/subtitle-codec-94213-is-not-supported/, says: Or, to retain the subtitle information in the Matroska container, it can be converted to the subrip format using the -c:s srt option.

and this actually makes it work:

/usr/share/losslesscut/resources/node_modules/ffmpeg-static/ffmpeg -hide_banner -ss 316.05468 -i m.mp4 -t 4953.48456 -avoid_negative_ts make_zero -c copy -map 0:0 -map 0:1 -map 0:2 -map 0:3 -map 0:4 -map_metadata 0 -movflags use_metadata_tags -ignore_unknown -f matroska -y -c:s srt m-00.05.16.054-01.27.49.539.mkv

[mifi]

Ohh. Yea I guess subtitle conversion is not necessary a lossy operation, and is probably very fast even though it is a re-encoding, so we might look into that.

[soloturn]

@mifi where in the code is this best done? i was wondering how one could get the command line executed so that their duration is set to 0. this is an ffmpeg bug?

[mifi]

@soloturn I'm not sure I understand what you mean by duration set to 0

[soloturn]

the subtitle is displayed for 0 seconds. so it is there but not displayed.

[mifi]

I trhink you can do something like this https://superuser.com/questions/1090989/changing-subtitle-format-with-ffmpeg
to convert subtitle format

[soloturn]

cool thank you! it works like this. first convert:

ffmpeg -i in.mp4 -c copy -map 0 -c:s srt out.mkv

then cut with losslesscut. all audio tracks are preserved, and subtitles have correct duration and thus show up.