Convert MKV (DVD rip) with VobSub to MP4 with burned-in bitmap subtitles

DVD rips often include VobSub (dvdsub) subtitles, image-based (bitmap) tracks¹ that aren't supported in MP4 containers. Rather than extracting and OCRing (which can be slow and error-prone), burn them directly into the MP4 with ffmpeg:

1. Identify the subtitle stream:

   Use ffprobe to confirm that the MKV contains VobSub (dvd_subtitle) subtitles:

   ffprobe -v error -select_streams s -show_entries stream=index,codec_name:stream_tags=language -of csv=p=0 foo.mkv
   3,dvd_subtitle,eng

   Stream 3 is dvd_subtitle (VobSub), in English.

2. Convert & burn-in with ffmpeg:

   This command burns in the bitmap subtitles, retains all audio, preserves chapters, and encodes the video using Apple's hardware-accelerated hevc_videotoolbox:

   ffmpeg -i foo.mkv \
     -filter_complex "[0:v][0:s:0]overlay,format=nv12[v]" \
     -map "[v]" \
     -map 0:a \
     -map_chapters 0 \
     -c:v hevc_videotoolbox -b:v 6000k -tag:v hvc1 \
     -c:a copy \
     -movflags +faststart \
     foo.mp4

Flag exegesis

• -filter_complex "[0:v][0:s:0]overlay,format=nv12[v]" defines a filtergraph to burn in subtitles and prepare the video for encoding:

  • [0:v] selects the video stream from input 0.
  • [0:s:0] selects the first subtitle stream from input 0.
  • overlay burns the subtitle onto the video frame-by-frame.
  • format=nv12 sets a pixel format compatible with hardware encoders like videotoolbox.
  • [v] names the final video stream for later mapping.

• -map "[v]" maps the filtered (burned-in subtitle) video stream ([v]) to the output.

• -map 0:a includes all audio streams from input 0.

• -map_chapters 0 preserves chapter metadata from the source MKV.

• -c:v hevc_videotoolbox uses Apple silicon's hardware-accelerated HEVC (H.265) encoder.

• -b:v 6000k sets bitrate to match 480p DVD quality, close to typical on-disc MPEG-2 rates.

• -tag:v hvc1 ensures compatibility with Apple players (QuickTime, Safari, iOS) by setting the correct codec identifier.

• -c:a copy copies audio tracks without re-encoding.

• -movflags +faststart moves metadata to the beginning of the file for faster playback or streaming.

Footnote

1. Blu-ray subtitles use PGS, an image-based format like VobSub that's also best burned in for MP4 compatibility and to skip OCR. ↩

❧ 2025-08-06