subcap 0.2.2

Burn precisely-timed captions into video using forced alignment.

subcap

A one-command captioning pipeline built around WhisperX's forced alignment. Give it a video and a transcript — it handles audio extraction, alignment, subtitle segmentation, styling, and burn-in encoding.

Why this exists

WhisperX solves the hard problem: using wav2vec2 to map each word of a known transcript to its exact position in audio. But WhisperX outputs raw word timestamps — turning those into readable, styled, burned-in captions is still a non-trivial amount of glue code per video.

subcap is that glue code, packaged as a CLI:

• Subtitle segmentation — groups aligned words into readable chunks with sentence-boundary breaks, line wrapping, duration caps, and proper gaps between cues
• Styled ASS generation — four presets (modern, outline, minimal, bold), auto-adapted for landscape vs portrait video
• ffmpeg burn-in — re-encodes to H.264, H.265, or ProRes with a single --quality flag
• SRT bypass — if you already have timed subtitles, it skips alignment and goes straight to styling + burn-in

Without subcap, getting from video + transcript to video with burned-in captions requires chaining WhisperX, writing your own segmentation logic, hand-crafting ASS files, and orchestrating ffmpeg. subcap is subcap video.mov transcript.txt.

Install

pip install subcap

Requires Python 3.10–3.12 and ffmpeg with libass support.

On first run, subcap downloads the wav2vec2 alignment model (~360 MB).

Usage

# Align a transcript and burn captions in
subcap video.mov transcript.txt -o output.mp4

# Use an existing SRT file (skips alignment)
subcap video.mov subtitles.srt -o output.mp4

# Choose a style
subcap video.mov transcript.txt --style outline

# ProRes output for editing
subcap video.mov transcript.txt --quality studio -o output.mov

# Portrait/vertical video (auto-detected)
subcap shorts.mp4 transcript.txt -o shorts_captioned.mp4

Options

subcap <video> <transcript> [options]

  -o, --output          Output path (default: <input>_captioned.mp4)
  --style               modern | outline | minimal | bold (default: modern)
  --quality             standard | high | studio (default: standard)
  --max-lines           Max lines per subtitle (default: 2)
  --max-chars           Max characters per line (default: auto)
  --line-spacing        Gap between lines in px (default: auto)
  --position            bottom | center | top (default: bottom)

Styles

Preset	Look
modern	White bold text, semi-transparent dark box
outline	White text with black outline
minimal	Lighter weight, subtle shadow
bold	Large text, opaque dark box

Quality

Preset	Codec	Use case
standard	H.264	Sharing, uploading
high	H.265	Smaller files
studio	ProRes 422	Editing, broadcast

Pipeline

1. Extract audio — mono 16 kHz WAV via ffmpeg
2. Force-align (WhisperX / wav2vec2) — map each word of the transcript to its exact position in the audio
3. Segment (subcap) — group words into readable subtitle cues, break at sentence boundaries, wrap long lines, enforce min/max display duration, insert gaps
4. Style (subcap) — generate ASS with the selected preset, adapted to aspect ratio
5. Burn in (ffmpeg) — re-encode with hardcoded subtitles

Steps 1, 2, and 5 are wrappers around existing tools. Steps 3 and 4 are what subcap adds. Because the transcript text is fixed and only timing is being solved, alignment stays precise even for fast speech, accents, or noisy audio — conditions that break speech-to-text approaches.

Transcript notes

Your transcript must match what's actually said in the audio. Small edits are tolerated, but missing or extra sentences will cause alignment failures. If the speaker ad-libs or skips text, update the transcript to match the final delivery.

Acknowledgments

Built on:

• WhisperX — Phoneme-level forced alignment using wav2vec2
• wav2vec2 — Self-supervised speech model used as the acoustic backbone for alignment
• ffmpeg — Video encoding and subtitle rendering via libass

License

MIT