| name | subtitle-burner |
| description | 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. |
| provenance | {"origin":"agentos-original","license":"MIT"} |
| metadata | {"agentos":{"risk":"medium","capabilities":["filesystem-write","process-control"],"requires":{"bins":["ffmpeg"],"anyBins":["python","python3"]}}} |
| entrypoint | {"command":"python {baseDir}/scripts/burn.py","args":["--input","{{ with.input }}","--subtitles","{{ with.subtitles }}","--output","{{ with.output }}","--font","{{ with.font | default('Microsoft YaHei,SimHei,Arial Unicode MS,Arial') }}","--font-size","{{ with.font_size | default(42) }}","--margin-v","{{ with.margin_v | default(80) }}","--play-res","{{ with.play_res | default('auto') }}","--crf","{{ with.crf | default(20) }}","--preset","{{ with.preset | default('medium') }}"],"parse":"text","timeout":600} |
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:)
| 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 | Microsoft YaHei,SimHei,Arial Unicode MS,Arial | libass FontName fallback chain. First wins. |
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. 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.
