For all the talk of mobile-marketing tech, there remains a pretty wide gap between the potential and the practicality of QR codes. That's why it's nice to see this case study from Korea, where a retailer increased lunchtime sales by 25 percent with a shadow-based QR code that's only scannable in the middle of the day. Emart's "Sunny Sale" codes are created with three-dimensional displays outside several dozen locations in Seoul. When the sun is at its zenith, the shadows line up, allowing the code to be scanned for access to coupons and online ordering. It's a smart idea that, in the short term at least, has generated plenty of strong PR and sales. While the wow factor is sure to fade quickly, it's still a great example of a marketer finding a way to turn QR codes into something actually worth scanning.