captions.js Server Side Rendering

maskin25/captions.js-render is a ready-to-run container that burns captions onto a video using the captions.js renderer plus ffmpeg. This page shows how to pull the image, feed it your assets, and customize the render through CLI flags.

1. Pull the image

docker pull maskin25/captions.js-render:latest

Tags follow the npm package version (0.1.0, 0.2.0, …). Use a fixed tag in production.

2. Provide inputs

The container expects:

Flag	Description	Example
--preset <name>	Name of the captions style preset (e.g. Lovly, From). See the stylePresets.	--preset Lovly
--video <path or URL>	Source video file. Supports @/path for mounted files or full HTTP(S) URLs.	--video @/data/margo.mp4
--captions <path, JSON, or URL>	Captions payload. Accepts JSON string, mounted file, or remote URL.	--captions @/data/margo-dg.json
--output <path>	Where to save the rendered video inside the container. Defaults to /app/output.mp4.	--output @/data/output.mp4
--rootDir <path>	Convenience flag: if set, any argument starting with @/ will be resolved relative to this directory.	--rootDir /data

All paths that start with @/ are interpreted relative to the bind mount supplied via -v. This lets you keep videos/captions on the host.

3. Typical local run

docker run --rm \
  -v /absolute/path/to/assets:/data \
  maskin25/captions.js-render:latest \
  burnCaptions \
    --preset Lovly \
    --rootDir /data \
    --video @/video.mp4 \
    --captions @/captions.json \
    --output @/output.mp4

• /absolute/path/to/assets must contain both video.mp4 and captions.json.
• The resulting output.mp4 is written back into the same host folder.

4. Remote assets

The CLI can download files directly:

docker run --rm maskin25/captions.js-render \
  burnCaptions \
    --preset From \
    --rootDir /data \
    --video https://<path-to-video> \
    --captions https://<path-to-captions> \
    --output /app/output.mp4

After the run, copy the result out with docker cp <container>:/app/output.mp4 ./output.mp4 or mount a host directory via -v so the file lands on disk automatically.