1. Extract from mp4
2. Extract from mkv
3. Convert from ass (lol) to srt
4. Add subtitles into container
   1. add to mp4
   2. burn the subtitles into the video

There was a time where I had a raspberry-pi (I think a 2b) set up as a media player. Hardware acceleration was only possible through the OMX player. However, the OMX player could only read subtitles from external srt files.

Here some scripts to extract the subtitles from mkv and mp4 containers:

Extract from mp4

# Remove spaces in filenames
for filename in *.mp4 ;
do   
    echo "${filename// /_}"
    mv "$filename" ${filename// /_} ; 
done

# Extract subtitles
for filename in *.mp4 ; 
do
    echo "${filename/mp4/srt}"
    MP4Box -srt 3 "$filename"
done

# Rename subtitles
for filename in *.srt ; 
do
  echo "${filename/_3_text/}"
  mv "$filename" ${filename/_3_text/} ;   
done

Extract from mkv

for filename in *.mkv ; 
do
    echo "${filename/mkv/srt}"
    if [ -e "${filename/mkv/srt}" ]
    then
        echo "already exists"
    else
        mkvextract tracks "$filename" 2:"${filename/mkv/srt}"
    fi
done

Convert from ass (lol) to srt

for i in *.ass ; 
    do ffmpeg -i "$i" "$i.srt" ; 
done

Add subtitles into container

add to mp4

ffmpeg -i infile.mp4 -i infile.srt -c copy -c:s mov_text outfile.mp4

burn the subtitles into the video

First convert the subtitles to .ass format:

ffmpeg -i subtitles.srt subtitles.ass

Then add them using a video filter:

ffmpeg -i mymovie.mp4 -vf ass=subtitles.ass mysubtitledmovie.mp4
ffmpeg -i infile.mp4 -f srt -i subtitles.srt -c:v copy -c:a copy -c:s srt -metadata:s:s:0 language=nor outfile.mkv