# Tasks: Add Image Preprocessing for Layout Detection

## 1. Configuration

- [ ] 1.1 Add preprocessing configuration to `backend/app/core/config.py`
  - `layout_preprocessing_enabled: bool = True` - Enable/disable preprocessing
  - `layout_preprocessing_contrast: str = "clahe"` - Options: none, histogram, clahe
  - `layout_preprocessing_sharpen: bool = True` - Enable sharpening for faint lines
  - `layout_preprocessing_binarize: bool = False` - Optional binarization (aggressive)

## 2. Preprocessing Service

- [ ] 2.1 Create `backend/app/services/preprocessing_service.py`
  - Image loading utility (supports PIL, OpenCV)
  - Contrast enhancement methods (histogram equalization, CLAHE)
  - Sharpening filter for line enhancement
  - Optional adaptive binarization
  - Return preprocessed image as numpy array or PIL Image

- [ ] 2.2 Implement `enhance_for_layout_detection()` function
  - Input: Original image path or PIL Image
  - Output: Preprocessed image (same format as input)
  - Steps: contrast → sharpen → (optional) binarize

## 3. Integration with OCR Service

- [ ] 3.1 Update `backend/app/services/ocr_service.py`
  - Import preprocessing service
  - Before `_run_ppstructure()`, preprocess image if enabled
  - Pass preprocessed image to PP-Structure for layout detection
  - Keep original image reference for image extraction

- [ ] 3.2 Ensure image element extraction uses original
  - Verify `saved_path` and `img_path` in elements reference original
  - Bbox coordinates from preprocessed detection applied to original crop

## 4. Testing

- [ ] 4.1 Unit tests for preprocessing_service
  - Test contrast enhancement methods
  - Test sharpening filter
  - Test binarization
  - Test with various image formats (PNG, JPEG)

- [ ] 4.2 Integration tests
  - Test OCR track with preprocessing enabled/disabled
  - Verify image element quality is preserved
  - Test with known problematic documents (faint table borders)

## 5. Documentation

- [ ] 5.1 Update API documentation
  - Document new configuration options
  - Explain preprocessing behavior