[FFmpeg-user] MKV to MP4 subtitle issue

[Steve Zemlicka]

The question I have is similar to the following post:
https://superuser.com/questions/932730/ffmpeg-mkv-to-mp4-conversion-loses-subtitles

I am using ffmpeg version n5.1.2.  I have an mkv that has two subtitle
tracks.  I am trying to convert it to mp4.

    mkvmerge -i input.mkv
    File 'input.mkv': container: Matroska
    Track ID 0: video (HEVC/H.265/MPEG-H)
    Track ID 1: audio (E-AC-3)
    Track ID 2: subtitles (HDMV PGS)
    Track ID 3: subtitles (HDMV PGS)
    Chapters: 10 entries

I believe Track ID 2 is for foreign dialog only and is the one I would
prefer to use.  I also prefer to use hardsubs but would take hard or soft
subs at this point.  I've tried a number of approaches with little
success.  The closest I've gotten is to issue the subtitle=input.mkv option.

    ffmpeg -i input.mkv -c:v hecv_nvenc -crf 26 -preset fast -c:a aac -b:a
128k subtitles=input.mkv output.mp4

I expected this to burn the subs to the video but oddly enough, it seemed
to make an mp4 with a subtitle track to toggle.  Unfortunately the
subtitles are just the chapter names and not the actual subtitles.  I've
also tried to manually select different subtitle tracks by using
"subtitles=input.mkv:si=1" as per
http://ffmpeg.org/ffmpeg-filters.html#subtitles.  Unfortunately this
results in an error:
"Unable to find a suitable output format for 'subtitles=input.mkv:si1'"
followed by:
"subtitles=input.mkv:si=1: Invalid argument"

I've also tried several other options including "-c:s copy" which didn't
work due to the nature of the original subtitles (PGS) I suspect.

Am I doing something wrong or missing something?