How to rip subtitles from a BluRay disc or m2ts file

I’m looking for information on ripping subtitles from HD video. I am trying to gain space by converting my videos from 1080P m2ts files to 780P mkv. I have tried the popular video conversion packages with no luck. The only info that might be helpful is that VLC sees the subtitles as this:

Orig ID: 4608
BD subtitles (bdpg)

I have read of Windows programs to do the job, but nothing for Linux. Can anyone make suggestions?