AI Sub is a command-line tool that leverages Google's Gemini models to generate high-quality, audio-synchronized subtitles. It is designed to produce precise English and Japanese subtitles by analyzing both audio and visual cues.

Key Features:

• Multimodal Understanding: Utilizes video frames for context (e.g., identifying speakers, reading on-screen text) and audio for precise timing.
• Dual-Language Support: Generates verbatim transcriptions and translations for English and Japanese.
• Automatic Segmentation: Automatically splits long videos into smaller segments for efficient processing.

Here's an example of subtitles generated by AI Sub:

For more examples, please visit ai-sub-showcase.

1. Preprocessing: The input video is segmented into smaller chunks to fit within API context windows and file size limits.
2. AI Processing: Each segment is sent to Google Gemini. The AI analyzes the audio for speech and the video for context, following strict prompting rules to generate subtitles.
3. Compilation: Generated subtitles from all segments are merged into a final, chronologically sorted SRT file.

Prerequisites: Python 3.10 or higher.

1. Set up a Python virtual environment:

   python -m venv venv
   source venv/bin/activate  # On Windows, use `venv\Scripts\activate.bat`

2. Install AI Sub:

   pip install --upgrade ai-sub

You can use AI Sub with either a Google AI Studio API Key or the Gemini CLI.

1. Obtain your API Key:

   • Sign in to Google AI Studio.
   • Click "Create API Key".
   • Copy and securely store your key. Never disclose your API key publicly.

2. Run the application:

   ai-sub --ai.google.key YOUR_API_KEY --ai.model=google-gla:gemini-3-flash-preview "path/to/your/video.mp4"

   Note: Replace YOUR_API_KEY with your actual key and "path/to/your/video.mp4" with the video file path.

1. Install and Authenticate Gemini CLI:

   • Install: npm install -g @google/gemini-cli
   • Authenticate: Follow instructions at gemini-cli.

2. Run the application:

   ai-sub --ai.model=gemini-cli:gemini-3-pro-preview --split.re-encode.enabled=True --thread.subtitles=1 "path/to/your/video.mp4"

   Important Notes for CLI Mode:

   • No API key is required; the tool uses your authenticated Gemini CLI instance.
   • Additional arguments are required to split and re-encode the video because the Gemini CLI has a 20MB upload limit per chunk.
   • Re-encoding is resource-intensive and will increase processing time.

1. Timestamp Accuracy: Subtitle timestamps may occasionally be inaccurate. This is an inherent characteristic of the Gemini AI model. Shorter video segments generally yield better accuracy.
2. AI Hallucinations: Like all LLMs, Gemini may occasionally produce "hallucinations" or inaccurate information.

If you encounter issues, consider re-processing specific video segments as detailed below.

Intermediate files are stored in a temporary directory (default: tmp_<input_file_name>). You can customize this location using the --dir.tmp flag.

To re-process a specific segment:

1. Navigate to the temporary directory.
2. Locate and delete the corresponding part_XXX.json file.
3. Re-run the script. It will automatically detect missing files and re-process only those segments.