ffmpeg -i input.mkv -filter_complex [0:v][0:s]overlay[v] -map [v] -map 0:a -acodec libvo_aacenc -ac 2 -ar 48000 -ab 320k output.mp4
参考 http://ffmpeg.org/trac/ffmpeg/wiki/How%20to%20burn%20subtitles%20into%20the%20video#picturebasedsubtitles
ffmpeg -i input.mkv -filter_complex [0:v][0:s]overlay[v] -map [v] -map 0:a -acodec libvo_aacenc -ac 2 -ar 48000 -ab 320k output.mp4