backend_design.txt

The backend will be a Python Flask server with the following key components:

1. Routes:
   - POST /upload_image: Handle image upload
   - POST /generate_invoice: Generate invoice based on user input
   - GET /invoices: Retrieve list of generated invoices
   - GET /invoice/<id>: Retrieve a specific invoice
   - GET /download/<id>: Download invoice as PDF or PNG

2. Database:
   - Use SQLite to store invoice data and metadata

3. Libraries:
   - Pillow: For image processing
   - ReportLab: For PDF generation
   - Flask-SQLAlchemy: For database operations

4. Key functions:
   - process_image(): Resize and optimize uploaded image
   - generate_invoice(): Create invoice based on user input
   - create_pdf(): Generate PDF version of invoice
   - create_png(): Generate PNG version of invoice
   - save_invoice(): Save invoice data to database

5. Data models:
   - Invoice: Store invoice details (items, prices, company info, image path)

The server will handle image uploads, invoice generation, and storage. It will provide APIs for the frontend to interact with, including invoice creation, retrieval, and download functionality.