Ffmpeg Subtitles

Ffmpeg Subtitles

Context: $ARGUMENTS

Quick start

• Burn (hardcode) SRT into MP4: → Step 1 (mode = burn-in), Step 3 example A
• Soft-embed SRT into MP4: → Step 1 (mode = soft-mux), Step 3 example B
• Extract embedded track from MKV: → Step 1 (mode = extract), Step 3 example C
• Convert SRT → ASS or VTT: → Step 1 (mode = convert), Step 3 example D
• Style caption fonts / colors: → Step 3 example A + references/formats.md force_style section
• Fix garbled accented characters: → Gotchas (charset) + Troubleshooting

When to use

• You have a video and an external subtitle file (.srt, .ass, .vtt, .sub) and need them merged or displayed.
• You need to extract a subtitle track from an MKV/MP4 to edit or re-use.
• You need to convert between subtitle formats while preserving timing.
• You need to sync / shift subtitle timing to match a re-cut video.
• You need to restyle captions (font, size, color, outline, margin) before burning in.

Step 1 — Pick mode

Decide one of the four modes. Every decision downstream depends on this:

Step 2 — Match container to subtitle codec

Containers only accept certain subtitle codecs. Pick the right -c:s for the output extension:

See references/formats.md for the full matrix.

Step 3 — Run the command

All examples produce output you can verify with ffprobe -v error -show_streams -select_streams s <file>.

A) Burn-in SRT with styling

ffmpeg -i in.mp4 \
  -vf "subtitles=subs.srt:force_style='Fontname=Arial,Fontsize=24,PrimaryColour=&H00FFFFFF,OutlineColour=&H80000000,BorderStyle=1,Outline=2,MarginV=60'" \
  -c:v libx264 -crf 20 -preset medium -c:a copy out.mp4

Burn-in always re-encodes video — -c:v copy is not possible. Use the ass filter instead when the subtitle file is ASS/SSA (it respects the file's own styling):

ffmpeg -i in.mp4 -vf "ass=subs.ass" -c:v libx264 -crf 20 -c:a copy out.mp4

B) Soft-mux

MP4 (SRT source → mov_text):

ffmpeg -i in.mp4 -i subs.srt -c:v copy -c:a copy -c:s mov_text \
  -metadata:s:s:0 language=eng -disposition:s:0 default out.mp4

MKV (keep original sub format):

ffmpeg -i in.mkv -i subs.srt -c copy -c:s srt -metadata:s:s:0 language=eng out.mkv

WebM:

ffmpeg -i in.webm -i subs.srt -c:v copy -c:a copy -c:s webvtt out.webm

Multiple tracks at once:

ffmpeg -i in.mp4 -i en.srt -i fr.srt \
  -map 0 -map 1 -map 2 -c:v copy -c:a copy -c:s mov_text \
  -metadata:s:s:0 language=eng -metadata:s:s:1 language=fra out.mp4

C) Extract embedded subtitle track

ffmpeg -i in.mkv -map 0:s:0 -c:s copy subs.srt          # copy if already SRT
ffmpeg -i in.mkv -map 0:s:0 -c:s srt subs.srt            # transcode ASS → SRT
ffmpeg -i in.mkv -map 0:s:0 -c:s webvtt subs.vtt         # transcode → VTT

D) Convert format

ffmpeg -i subs.srt subs.ass                              # SRT → ASS
ffmpeg -i subs.ass subs.vtt                              # ASS → VTT
ffmpeg -sub_charenc CP1252 -i subs.srt -c:s srt out.srt  # re-save non-UTF-8 as UTF-8

Step 4 — Verify

ffprobe -v error -show_entries stream=index,codec_type,codec_name:stream_tags=language \
  -of csv=p=0 out.mp4

For burn-in, visually spot-check one segment:

ffmpeg -ss 00:00:30 -i out.mp4 -frames:v 1 frame.png

Available scripts

• scripts/subs.py — stdlib-only wrapper over the four modes (burn, mux, extract, convert). Picks the correct -c:s from the output extension, escapes filter paths for burn-in, supports --dry-run and --verbose.

Workflow

# Burn SRT into MP4 with styling
python3 ${CLAUDE_SKILL_DIR}/scripts/subs.py burn \
  --video in.mp4 --subs subs.srt --output out.mp4 \
  --font Arial --size 28 --color white

# Soft-mux an English SRT into MP4
python3 ${CLAUDE_SKILL_DIR}/scripts/subs.py mux \
  --video in.mp4 --subs subs.srt --output out.mp4 --lang eng

# Extract first embedded subtitle stream
python3 ${CLAUDE_SKILL_DIR}/scripts/subs.py extract \
  --input movie.mkv --stream 0 --output movie.srt

# Convert SRT → ASS (extension-driven)
python3 ${CLAUDE_SKILL_DIR}/scripts/subs.py convert \
  --input subs.srt --output subs.ass

Reference docs

• Read references/formats.md for the container × codec matrix, the full force_style parameter list, ASS color hex format, per-format feature comparison, common fontfile paths, and filter-path escaping rules.

Gotchas

• MP4 only supports mov_text for subs. -c:s copy from SRT/ASS to MP4 fails with Subtitle codec X not supported by output format. Always use -c:s mov_text.
• WebM only supports WebVTT. Same class of error — use -c:s webvtt.
• Burn-in always re-encodes video. You cannot use -c:v copy with -vf subtitles=....
• Filter path escaping is three-level (shell → filter graph → libavfilter). A Windows path like C:\subs\foo.srt must become subtitles='C\\:\\\\subs\\\\foo.srt'. On POSIX, wrap paths with colons or commas in single quotes: -vf "subtitles='my:file.srt'". The included subs.py does this for you.
• force_style uses ASS colors in &HAABBGGRR order — alpha + BGR, not RGB. Pure red is &H000000FF. Half-transparent black outline is &H80000000. Omit the &H at your peril.
• Styling inside an SRT file is ignored. SRT tags like <font> / {\b1} are not reliably rendered; convert to ASS first (ffmpeg -i x.srt x.ass) and edit there, then -vf ass=x.ass.
• Non-UTF-8 SRTs render as mojibake. Western European files are often CP1252 or ISO-8859-1. Prepend input with -sub_charenc CP1252 (only works on -i file.srt, not on the -vf subtitles= filter). For burn-in with a legacy charset, first re-save the SRT as UTF-8.
• fontconfig must be available at runtime. Without it, Fontname=Arial silently falls back to a bundled default and Unicode glyphs may drop. Either install fontconfig or use fontfile= with an absolute path.
• stream_index selects a track when the subtitle file itself has multiple streams: subtitles=movie.mkv:stream_index=2 burns in the third embedded track.
• Default subtitle disposition: players auto-enable the track with disposition default. Set explicitly with -disposition:s:0 default (or -disposition:s:0 0 to clear).
• -itsoffset does not shift the subtitles= filter. -itsoffset shifts an input; the subtitles filter re-reads the file from scratch and ignores demuxer offsets. To shift a burn-in SRT, either (a) rewrite the SRT timestamps, or (b) soft-mux with -itsoffset 2 -i subs.srt and re-extract.
• subrip and srt are the same codec — ffmpeg accepts either name for -c:s.
• MKV auto-selects on first matching language tag. If you mux two English tracks, the second one won't be picked by default — use -disposition:s:N default and clear the other.

Examples

Example 1: YouTube upload with burned-in captions

ffmpeg -i raw.mp4 \
  -vf "subtitles=captions.srt:force_style='Fontname=Arial,Fontsize=22,PrimaryColour=&H00FFFFFF,OutlineColour=&H80000000,BorderStyle=1,Outline=2,Shadow=0,MarginV=80,Alignment=2'" \
  -c:v libx264 -crf 18 -preset slow -pix_fmt yuv420p \
  -c:a aac -b:a 192k out.mp4

Result: YouTube-safe H.264/AAC MP4 with always-visible bottom-center captions (Alignment=2 = bottom-center).

Example 2: Archival MKV with three soft subtitle tracks

ffmpeg -i film.mkv -i en.srt -i es.srt -i fr.srt \
  -map 0:v -map 0:a -map 1 -map 2 -map 3 \
  -c:v copy -c:a copy -c:s srt \
  -metadata:s:s:0 language=eng -metadata:s:s:0 title="English" \
  -metadata:s:s:1 language=spa -metadata:s:s:1 title="Español" \
  -metadata:s:s:2 language=fra -metadata:s:s:2 title="Français" \
  -disposition:s:0 default out.mkv

Example 3: Extract, shift by +2 s, re-mux

ffmpeg -i in.mkv -map 0:s:0 -c:s srt orig.srt
# shift SRT timestamps with any tool, then:
ffmpeg -i in.mkv -i shifted.srt -map 0:v -map 0:a -map 1 \
  -c copy -c:s srt out.mkv

Troubleshooting

Error: Subtitle codec 94213 [0][0][0][0] / 0x17F05 is not supported by this version of FFmpeg

Cause: you tried -c:s copy into an MP4 with a non-mov_text subtitle. Solution: use -c:s mov_text.

Error: Unable to parse option value "&H00FFFFFF" as image size

Cause: missing quotes around force_style — the comma inside it is being parsed as a filter separator. Solution: wrap the whole force_style block in single quotes: force_style='Fontname=Arial,Fontsize=24'.

Error: No such filter: 'subtitles'

Cause: ffmpeg built without --enable-libass. Solution: install a full build (brew install ffmpeg, apt install ffmpeg) — the "minimal" / ffmpeg-lite builds omit libass.

Symptom: accented characters display as é, è, etc.

Cause: SRT is CP1252/Latin-1 but ffmpeg is reading it as UTF-8. Solution: add -sub_charenc CP1252 before the -i subs.srt. For the subtitles= filter (burn-in), convert the file to UTF-8 first: iconv -f CP1252 -t UTF-8 in.srt > out.srt.

Symptom: subtitles render in the wrong font or drop Unicode glyphs

Cause: fontconfig missing, or the named font not installed. Solution: either install the font / fontconfig, or switch to fontfile=: subtitles=subs.srt:fontsdir=/System/Library/Fonts:force_style='Fontname=Helvetica'.

Symptom: soft-muxed subs don't show up in QuickTime / iOS

Cause: only mov_text works in MP4 on Apple players, and the track needs a language tag + default disposition. Solution: -c:s mov_text -metadata:s:s:0 language=eng -disposition:s:0 default.