# Result Export - Delta Changes

## ADDED Requirements

### Requirement: Image Extraction and Persistence
The OCR system SHALL save extracted images to disk during layout analysis for later use in PDF generation.

#### Scenario: Images extracted by PP-StructureV3 are saved to disk
- **WHEN** OCR processes a document containing images (charts, tables, figures)
- **THEN** system SHALL extract image objects from `markdown_images` dictionary
- **AND** system SHALL create `imgs/` subdirectory in result folder
- **AND** system SHALL save each image object to disk using PIL Image.save()
- **AND** saved file paths SHALL match paths recorded in JSON `images_metadata`
- **AND** system SHALL log warnings for failed image saves but continue processing

#### Scenario: Multi-page documents with images on different pages
- **WHEN** OCR processes multi-page PDF with images on multiple pages
- **THEN** system SHALL save images from all pages to same `imgs/` folder
- **AND** image filenames SHALL include bbox coordinates for uniqueness
- **AND** images SHALL be available for PDF generation after OCR completes

### Requirement: Layout-Preserving PDF Generation
The system SHALL generate PDF files that preserve the original document layout using OCR JSON data.

#### Scenario: PDF generated from JSON with accurate layout
- **WHEN** user requests PDF download for a completed task
- **THEN** system SHALL parse OCR JSON result file
- **AND** system SHALL extract bounding box coordinates for each text region
- **AND** system SHALL determine page dimensions from source file or bbox maximum values
- **AND** system SHALL generate PDF with text positioned at precise coordinates
- **AND** system SHALL use Chinese-compatible font (e.g., Noto Sans CJK)
- **AND** system SHALL embed images from `imgs/` folder using paths in `images_metadata`
- **AND** generated PDF SHALL visually resemble original document layout with images

#### Scenario: PDF download works correctly
- **WHEN** user clicks PDF download button
- **THEN** system SHALL return cached PDF if already generated
- **OR** system SHALL generate new PDF from JSON on first request
- **AND** system SHALL NOT return 403 Forbidden error
- **AND** downloaded PDF SHALL contain task OCR results with layout preserved

#### Scenario: Multi-page PDF generation
- **WHEN** OCR JSON contains results for multiple pages
- **THEN** generated PDF SHALL contain same number of pages
- **AND** each page SHALL display text regions for that page only
- **AND** page dimensions SHALL match original document pages

## MODIFIED Requirements

### Requirement: Export Interface
The Export page SHALL support downloading OCR results in multiple formats using V2 task APIs.

#### Scenario: PDF caching improves performance
- **WHEN** user downloads same PDF multiple times
- **THEN** system SHALL serve cached PDF file on subsequent requests
- **AND** system SHALL NOT regenerate PDF unless JSON changes
- **AND** download response time SHALL be faster than initial generation