LiteParse OCR API Specification

This document defines the standard HTTP API that OCR servers must implement to work with LiteParse.

Overview

LiteParse expects a simple HTTP endpoint that accepts an image and returns text with bounding boxes. Your OCR server can internally use any OCR engine (EasyOCR, PaddleOCR, Tesseract, Cloud APIs, etc.) as long as it conforms to this API.

Endpoint

POST /ocr

Request Format

Content-Type: multipart/form-data

Fields:

Field	Type	Required	Description
file	binary	Yes	Image file (PNG, JPG, etc.)
language	string	No	Language code (default: en)

Language Codes

Use ISO 639-1 two-letter codes:

• en - English
• zh - Chinese
• ja - Japanese
• ko - Korean
• fr - French
• de - German
• es - Spanish
• ar - Arabic
• etc.

Your server should map these to whatever format your underlying OCR engine expects.

Response Format

Content-Type: application/json

Structure:

{
  "results": [
    {
      "text": "recognized text",
      "bbox": [x1, y1, x2, y2],
      "confidence": 0.95,
      "polygon": [[x1, y1], [x2, y2], [x3, y3], [x4, y4]]
    }
  ]
}

Fields:

Field	Type	Description
results	array	Array of text detection results
results[].text	string	Recognized text content
results[].bbox	[number, number, number, number]	Axis-aligned bounding box [x1, y1, x2, y2] where (x1,y1) is top-left and (x2,y2) is bottom-right
results[].confidence	number	Confidence score between 0.0 and 1.0
results[].polygon	[[number, number], ×4]	Optional. 4-point detection polygon ordered top-left → top-right → bottom-right → bottom-left in the glyphs' upright reading frame. Lets LiteParse recover rotation for vertical/sideways text.

Example

Request

curl -X POST http://localhost:8080/ocr \
  -F "file=@document.png" \
  -F "language=en"

Response

{
  "results": [
    {
      "text": "Hello",
      "bbox": [10, 20, 60, 40],
      "confidence": 0.98
    },
    {
      "text": "World",
      "bbox": [70, 20, 130, 40],
      "confidence": 0.97
    }
  ]
}

Error Handling

Return appropriate HTTP status codes:

• 200 OK - Success
• 400 Bad Request - Invalid request (missing file, invalid language, etc.)
• 500 Internal Server Error - OCR processing failed

Error response format:

{
  "error": "Description of the error"
}

Implementation Notes

Coordinate System

• Origin (0,0) is at the top-left of the image
• X increases to the right
• Y increases downward
• All coordinates are in pixels

Bounding Box Format

Always return axis-aligned bounding boxes as [x1, y1, x2, y2]:

• x1, y1 = top-left corner
• x2, y2 = bottom-right corner
• x2 > x1 and y2 > y1

If your OCR engine returns rotated boxes or polygon coordinates, convert them to axis-aligned boxes by taking min/max coordinates. Additionally, you are encouraged to forward the raw 4-point polygon as polygon (TL → TR → BR → BL in the upright reading frame) — LiteParse uses it to detect vertical/sideways text (e.g. legal-document sidebars) and route it through its rotation reading-order handler instead of flattening it into body lines.

Confidence Scores

• Normalize to range 0.0 to 1.0
• 1.0 = 100% confident
• 0.0 = 0% confident
• If your OCR engine doesn't provide confidence, use 1.0

Text Ordering

Results should be ordered by reading order (top-to-bottom, left-to-right for most languages).

Example Implementations

See the /ocr directory for reference implementations:

• ocr/easyocr/ - Wrapper for EasyOCR
• ocr/paddleocr/ - Wrapper for PaddleOCR
• ocr/suryaocr/ - Wrapper for Surya OCR 2 (multilingual)

Testing Your Server

Quick test:

# 1. Start your server
python server.py

# 2. Test with curl
curl -X POST http://localhost:8080/ocr \
  -F "file=@test.png" \
  -F "language=en" \
  | jq .

# 3. Expected output:
# {
#   "results": [
#     {
#       "text": "...",
#       "bbox": [x1, y1, x2, y2],
#       "confidence": 0.xx
#     }
#   ]
# }

Use with LiteParse:

lit parse document.pdf --ocr-server-url http://localhost:8080/ocr

FAQ

Q: What if my OCR returns rotated bounding boxes?

Convert to axis-aligned boxes:

def polygon_to_bbox(polygon):
    """Convert polygon [[x1,y1], [x2,y2], ...] to [x1, y1, x2, y2]"""
    xs = [point[0] for point in polygon]
    ys = [point[1] for point in polygon]
    return [min(xs), min(ys), max(xs), max(ys)]

Q: What if my OCR doesn't return confidence scores?

Just return 1.0 for all results.

Q: Can I return empty results?

Yes, return {"results": []} if no text is detected.

Q: Should I filter low-confidence results?

You can, but LiteParse will also handle filtering based on its own thresholds.

Q: What image formats should I accept?

At minimum: PNG, JPG. Optionally: TIFF, WebP, BMP, GIF.

Q: Should I handle rotation correction?

Optional. If your OCR engine supports it, you can auto-correct rotation before processing.

Q: What about multi-page documents?

LiteParse handles page splitting. Your server only needs to process single images.

Q: Performance considerations?

• Keep server response time under 10 seconds per image
• Support concurrent requests
• Consider GPU acceleration for better performance
• Cache OCR models in memory (don't reload per request)

Compliance Checklist

• Accepts POST /ocr endpoint
• Accepts file and language form fields
• Returns JSON with results array
• Each result has text, bbox, and confidence
• Bounding boxes in [x1, y1, x2, y2] format
• (Optional but recommended) polygon field with 4-point TL→TR→BR→BL polygon for rotated detections
• Confidence normalized to 0.0-1.0 range
• Returns 200 status on success
• Returns appropriate error codes and messages
• Handles common image formats (PNG, JPG)
• Processes images in under 10 seconds

Support

Questions? Open an issue on GitHub or refer to the example implementations in /ocr.