Optical Character Recognition (OCR) Filter

The FilterOpticalCharacterRecognition is a pluggable filter that extracts text from image frames using Optical Character Recognition (OCR). It supports multiple OCR backends and offers flexible configuration for language support, output, and debug logging.

Features

• Dual OCR Engine Support
  Choose between:

  • tesseract
  • easyocr
    Configure with the ocr_engine parameter.

• Multi-language OCR
  Use the ocr_language option to specify one or more language codes (e.g., en,fr).

• Topic-based Processing

  • Filter frames by topic using topic_pattern regex
  • Exclude specific topics using exclude_topics list
  • Support for exact topic names or regex patterns in exclusions

• Flexible Output Options

  • Write results to JSON file (configurable via write_output_file)
  • Forward OCR results in frame metadata (configurable via forward_ocr_texts)
  • Results are written to output_json_path as newline-delimited JSON

• Debug Mode
  Enabling debug: true will increase logging verbosity for troubleshooting and transparency.

• Frame-level Skipping
  Add the metadata flag skip_ocr: true to individual frames to bypass OCR processing.

• Custom Tesseract Path
  You can specify a custom tesseract_cmd binary path if using the Tesseract engine (defaults to a bundled AppImage).

• Safe Streaming Output
  Results are flushed to disk immediately after processing each frame.

  Note

  This may lead to heavy I/O operations. A configurable flushing strategy is planned for future releases.

Example Output

Each processed frame will produce a JSON line similar to:

{
  "topic": "camera",
  "frame_id": "abc123",
  "texts": ["Detected text line 1", "Detected text line 2"]
}

When forwarding results in metadata, they are stored under the ocr_texts key in the frame metadata, with topics as keys:

{
  "meta": {
    "ocr_texts": {
      "camera": ["Detected text line 1", "Detected text line 2"],
      "thermal": ["Temperature: 25°C"]
    }
  }
}

When to Use

This filter is ideal for any pipeline that requires reading printed or handwritten text from images, such as:

• Scanned documents
• Signboards or product packaging in photos
• Scene text in videos
• Multi-camera systems with different text sources

Configuration Reference

Key	Type	Default	Description
ocr_engine	string	"easyocr"	OCR engine to use: "tesseract" or "easyocr"
ocr_language	string[]	["en"]	List of language codes for OCR
output_json_path	string	"./output/ocr_results.json"	Path to save output results
debug	boolean	false	Enable debug logging
tesseract_cmd	string	Packaged AppImage path	Path to Tesseract binary
forward_ocr_texts	boolean	true	Whether to forward OCR results in frame metadata
write_output_file	boolean	false	Whether to write results to output file
topic_pattern	string	null	Regex pattern to match topic names
exclude_topics	string[]	[]	List of topics to exclude from OCR processing

Environment Variables

All configuration options can be overridden using environment variables with the prefix FILTER_. For example:

• FILTER_OCR_ENGINE
• FILTER_OCR_LANGUAGE
• FILTER_DEBUG
• FILTER_TOPIC_PATTERN
• FILTER_EXCLUDE_TOPICS

Boolean values should be set to "true" or "false" (case-insensitive). List values should be comma-separated strings.