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subextract

Extract hardcoded (burned-in) subtitles from video files using Apple Vision OCR on macOS.

subextract reads a video file, samples frames at a configurable rate, crops the subtitle region, uses Apple's Vision framework to OCR the text, deduplicates consecutive frames, and outputs a standard SRT subtitle file.

Requirements

  • macOS (uses Apple Vision framework)
  • Python 3.9–3.12
  • ffmpeg (optional, for videos OpenCV can't decode)

Installation

pip install subextract

Quick Start

# Extract subtitles with defaults
subextract video.mp4 -o subtitles.srt

# Use accurate OCR mode (slower but better)
subextract video.mp4 -o subtitles.srt --recognition-level accurate

# Extract a specific time range
subextract video.mp4 -o subtitles.srt --start 14:30 --end 15:30

Configuration

subextract can be configured via CLI flags, a YAML config file, or both. Priority: CLI flags > config file > defaults.

Generate a config file

subextract --init-config

This creates config.yaml with all options and their defaults:

# Subtitle crop region (pixels from top of frame)
crop:
  y: 400              # Top of crop region
  height: 105         # Height of crop region

# OCR settings
ocr:
  recognition_level: fast   # "fast" or "accurate"
  language: en-US           # OCR language code

# Frame sampling
sampling:
  fps: 3.0                   # Frames per second to sample
  variance_threshold: 100.0  # Skip frames below this variance

# Text processing
text:
  similarity_threshold: 0.75  # Merge consecutive subs above this
  min_duration: 0.3           # Discard subs shorter than this (seconds)
  brightness_threshold: 200   # White-text masking threshold (0-255)

Use a config file

subextract video.mp4 --config config.yaml -o subtitles.srt

CLI Reference

subextract video.mp4 [options]

positional arguments:
  video                       Path to video file

options:
  -o, --output PATH           Output SRT path (default: output.srt)
  --config PATH               Path to YAML config file
  --init-config               Write config.yaml with defaults and exit
  --version                   Show version and exit

  --crop-y INT                Crop region top Y (pixels)
  --crop-height INT           Crop region height (pixels)
  --sample-fps FLOAT          Frames per second to sample
  --recognition-level MODE    "fast" or "accurate"
  --language CODE             OCR language code (e.g. en-US)
  --variance-threshold FLOAT  Skip frames below this variance
  --similarity-threshold FLOAT Text similarity for dedup (0-1)
  --min-duration FLOAT        Min subtitle duration in seconds
  --brightness-threshold INT  White-text masking threshold (0-255)
  --no-mask                   Disable brightness masking (for subs with drop shadow)
  --start TIME                Start time (HH:MM:SS or MM:SS)
  --end TIME                  End time (HH:MM:SS or MM:SS)

How It Works

  1. Frame sampling — Reads the video and samples frames at the configured rate (default 3 fps)
  2. Crop — Extracts the subtitle region from each frame
  3. White-text masking — Thresholds the crop to isolate bright white subtitle text, removing background noise
  4. Variance check — Skips frames with low variance (no text present)
  5. OCR — Runs Apple Vision text recognition on each frame via ocrmac
  6. Deduplication — Merges consecutive frames with similar text into single subtitle entries
  7. SRT output — Writes standard SRT format with sequential numbering and timestamps

Limitations

  • macOS only — Requires Apple Vision framework (no Linux/Windows support)
  • White text assumed — The brightness masking works best with white or light-colored subtitles on darker backgrounds. For subtitles with a drop shadow or outline, use --no-mask to skip masking and let Vision OCR the raw crop directly
  • OCR quality varies — Low resolution video, unusual fonts, or complex backgrounds may produce errors that need manual cleanup
  • No auto-detection — You need to specify the subtitle crop region manually (use a screenshot to determine the Y position and height)

Contributing

Contributions are welcome. Please open an issue first to discuss what you'd like to change.

License

GPL v3