I have an mkv with multiple subtitle streams. I know I can use mkvextract to extract one of those subtitle streams to a file, for example, sub.srt. I can then burn those subtitles into the new video with ffmpeg like so:
ffmpeg -i video.mkv -vf subtitles=sub.srt out.mp4
I also know I can use -map to select video, audio, and subtitle tracks and encode those into the new video, again something like so:
ffmpeg -i video.mkv -map 0:0 -map 0:1 -map 0:4 -c:v copy -c:a copy -c:s copy out.mp4
However, what I can't figure out is how to transcode a video and burn in the subtitles into the new file (not just adding a subtitle track, but burning them into the video) not by reading it from a subtitle file but directly from the old video stream itself.