I'm transcoding some videos from .mkv (h.264 / AC3 / subtitles) to .webm.
Excuse me if I get some terminology wrong.
The "soft" subtitles I'm not that worried about, the subtitles that are optional. But some subtitles are necessary, such as English audio track, the actor or actress is speaking klingon, and an English subtitle is provided.
I understand these are called "forced" or "hard" subtitles and that to get them in webm, they need to become part of the video stream when it is transcoded to vp8.
The format they are in is PGS which looks like an image subtitle format and not a text format, so using libass is not possible.
If I mkvextract these tracts to a .sup file, how do I include it as a source to ffmpeg and tell ffmpeg to impose them on the video during transcode?