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 render through the managed Noto Sans CJK font. The font directory
is supplied through OPENSQUILLA_MEDIA_FONTS_DIR by the managed toolchain.
An empty/whitespace-only SRT is a valid no-subtitle request: the input video is
probed, copied to the requested output, and reported as
SUBTITLES_SKIPPED: empty without invoking libass.
Inputs (with:)
| key | required | default | notes |
|---|---|---|---|
input | yes | — | Source MP4 path. |
subtitles | yes | — | .srt path (UTF-8). |
output | yes | — | Output MP4 path. Parent dir created if missing. |
font | no | Noto Sans CJK SC | One libass FontName; comma-separated names are not a fallback chain. |
fonts_dir | no | managed environment | Directory containing subtitle fonts, normally supplied by OpenSquilla. |
font_size | no | 42 | Font size. When play_res=auto this is in source-video pixels. |
margin_v | no | 80 | Bottom margin in source-video pixels (because play_res=auto sets PlayRes to the input W×H). |
play_res | no | auto | auto 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. |
crf | no | 20 | x264 CRF (0-51, lower = better quality). |
preset | no | medium | x264 preset. |
Output
Prints the absolute path of the subtitled MP4 on stdout. Empty SRT input first
prints SUBTITLES_SKIPPED: empty. Both paths stage the result in the output
directory, require ffprobe to confirm a decodable positive-duration video
stream, and atomically replace the destination only after validation. Non-zero
exit on any encoding, copy, probe, or output-installation failure; stderr tails
the last 2.5 KB of the encoder log for diagnosis.
Dependencies
- ffmpeg ≥ 5.0 with libass, libx264, AAC, xfade, and zoompan support.
- ffprobe from the matching ffmpeg distribution.
- Noto Sans CJK Regular (managed by OpenSquilla; OFL-1.1).
- 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.
