adding captions.txt

say you have some files, maybe youtube-dl output, thats includes an mpeg 4 video file and captions in ttml format like this:

$ ls
Destroy the Technostructure.description	
Destroy the Technostructure.en.ttml	
Destroy the Technostructure.jpg
Destroy the Technostructure.mp4

and you want to prepare a nice mp4 file to use in Home Sharing with that subtitle track available like a real movie.

ffmpeg doesn't support the ttml format, so you need to convert to `srt` with this purpose-built utility: https://github.com/Cryental/Kayla.NET

$ ./Kayla.NET-macOS -f SubRip -i Destroy\ the\ Technostructure.en.ttml -o Destroy\ the\ Technostructure.en.srt

and then use ffmpeg to combine the tracks:

$ ffmpeg -i Destroy\ the\ Technostructure.mp4 -i Destroy\ the\ Technostructure.en.srt -c copy -c:s mov_text -metadata:s:2 language=eng Destroy\ the\ Technostructure-CC.mp4

...and you'll have a video file with the option for "English" captions!


post script: the ffmpeg invocation

$ ffmpeg \ 
    -i Destroy\ the\ Technostructure.mp4 \     # first input
    -i Destroy\ the\ Technostructure.en.srt \  # second input
    -c copy -c:s mov_text \                    # copy viewo and convert caption to mov_text
    -metadata:s:2 language=eng \               # set the language for the second track (the captions) to english
    Destroy\ the\ Technostructure-CC.mp4       # output filename
    

you can convert all the ttml files in a folder at once with

$ ./Kayla.NET-macOS -f SubRip -b -i ./ -o ./