Subtitles with FFmpeg: Burn‑In, Soft Subs, and Format Conversion

FFmpeg can embed, burn, and convert subtitles. Use burn‑in when you need captions visible everywhere; use soft subs to keep tracks switchable.

Subtitle Types

• SRT: common text format; simple styling.
• ASS: advanced styling.
• VTT: web‑friendly.
• Embedded: tracks stored inside containers like MP4/MKV.

Workflows

# Burn-in SRT
ffmpeg -i video.mp4 -vf "subtitles=subs.srt" -c:v libx264 -crf 22 -c:a copy out_burn.mp4

# Add soft subs to MP4 (convert to mov_text)
ffmpeg -i video.mp4 -i subs.srt -c copy -c:s mov_text out_soft.mp4

# Convert SRT → WebVTT
ffmpeg -i subs.srt subs.vtt

# Extract first subtitle stream from MKV as SRT
ffmpeg -i in.mkv -map 0:s:0 -c:s srt subs.srt

Pitfalls

• MP4 stores text subs as mov_text; SRT isn’t valid in MP4.
• Non‑UTF‑8 files may require -sub_charenc UTF-8.

Pick burn‑in when compatibility is critical; prefer soft subs when you want multiple languages or editability.