## ADDED Requirements

### Requirement: Layout Detection Image Preprocessing

The system SHALL provide optional image preprocessing to enhance layout detection accuracy for documents with faint lines, low contrast, or poor scan quality.

#### Scenario: Preprocessing improves table detection
- **GIVEN** a document with faint table borders that PP-Structure fails to detect
- **WHEN** layout preprocessing is enabled
- **THEN** the system SHALL preprocess the image before layout detection
- **AND** contrast enhancement SHALL make faint lines more visible
- **AND** PP-Structure SHALL receive the preprocessed image for layout detection

#### Scenario: Image element extraction uses original quality
- **GIVEN** an image element detected by PP-Structure from preprocessed input
- **WHEN** the system extracts the image element
- **THEN** the system SHALL crop from the ORIGINAL image, not the preprocessed version
- **AND** the extracted image SHALL maintain original quality and colors

#### Scenario: Preprocessing can be disabled
- **GIVEN** `layout_preprocessing_enabled` is set to false in configuration
- **WHEN** OCR track processing runs
- **THEN** the system SHALL skip preprocessing
- **AND** PP-Structure SHALL receive the original image directly

#### Scenario: CLAHE contrast enhancement
- **WHEN** `layout_preprocessing_contrast` is set to "clahe"
- **THEN** the system SHALL apply Contrast Limited Adaptive Histogram Equalization
- **AND** the enhancement SHALL not over-saturate already bright regions

#### Scenario: Sharpening enhances faint lines
- **WHEN** `layout_preprocessing_sharpen` is enabled
- **THEN** the system SHALL apply unsharp masking to enhance edges
- **AND** faint table borders SHALL become more detectable

#### Scenario: Optional binarization for extreme cases
- **WHEN** `layout_preprocessing_binarize` is enabled
- **THEN** the system SHALL apply adaptive thresholding
- **AND** this SHALL be used only for documents with very poor contrast

### Requirement: Preprocessing Track Isolation

The layout preprocessing feature SHALL only affect layout detection input without impacting other processing components.

#### Scenario: Raw OCR is unaffected
- **GIVEN** layout preprocessing is enabled
- **WHEN** Raw OCR processing runs
- **THEN** Raw OCR SHALL use the original image
- **AND** text detection quality SHALL not be affected by preprocessing

#### Scenario: Preprocessed image is temporary
- **GIVEN** an image is preprocessed for layout detection
- **WHEN** layout detection completes
- **THEN** the preprocessed image SHALL NOT be persisted to storage
- **AND** only the original image and element crops SHALL be saved