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subtitle-burner

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Burn an SRT subtitle file into an MP4 via ffmpeg's subtitles filter (libass). Single-pass re-encode of video; audio copied as-is. CJK-friendly font fallback chain (Microsoft YaHei → SimHei → Arial Unicode MS → Arial). Used by meta-short-drama as the final subtitling step after merge.

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Source SKILL.md: https://github.com/opensquilla/opensquilla/blob/HEAD/src/opensquilla/skills/bundled/subtitle-burner/SKILL.md

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subtitle-burner

Burns an SRT subtitle stream into an MP4. The video is re-encoded (H.264 + faststart), the audio is copied untouched. libass renders the text per ASS-style override flags, so Chinese / Japanese / Korean characters survive when the host has any of the listed fallback fonts installed (on Windows, Microsoft YaHei and SimHei ship with the OS).

Inputs (with:)

keyrequireddefaultnotes
inputyes—Source MP4 path.
subtitlesyes—.srt path (UTF-8).
outputyes—Output MP4 path. Parent dir created if missing.
fontnoMicrosoft YaHei,SimHei,Arial Unicode MS,Ariallibass FontName fallback chain. First wins.
font_sizeno42Font size. When play_res=auto this is in source-video pixels.
margin_vno80Bottom margin in source-video pixels (because play_res=auto sets PlayRes to the input W×H).
play_resnoautoauto probes the input MP4 for resolution; or pass WxH like 720x1280. Setting this makes FontSize/MarginV act in source pixels rather than libass's 384×288 default.
crfno20x264 CRF (0-51, lower = better quality).
presetnomediumx264 preset.

Output

Prints the absolute path of the subtitled MP4 on stdout. Non-zero exit on any ffmpeg failure; stderr tails the last 2.5 KB of the encoder log for diagnosis.

Dependencies

  • ffmpeg ≥ 5.0 (libass support is standard in any modern build).
  • Python 3.8+.

The script auto-locates ffmpeg via PATH; on Windows it falls back to the winget Gyan.FFmpeg / scoop / chocolatey install paths if PATH inheritance failed (matches the resolution logic in video-merger and video-still-animator).

Path-escaping notes

ffmpeg's subtitles= filter is picky on Windows:

  • Drive-letter colons (C:/…) must be backslash-escaped (C\:/…).
  • The path uses forward slashes regardless of host OS.
  • Single quotes inside the path get backslash-escaped.

The script applies these rules so callers don't have to.

Style chain

The force_style defaults render white text with a 2-px black outline on a transparent background (BorderStyle=3), bottom-centred, 80 px above the frame edge. Override any of the --* flags via with.* if you want a different look.