A design element is displayed on a display screen. The design element is modulated over time to encode data in the design element. The design element is presented by a merchant. A camera component of a mobile device is used to capture images over time of the design element. A payment application in the mobile device analyzes the captured images to obtain the encoded data. The payment application uses the encoded data to establish a communication channel with a merchant device operated by the merchant. As a result of communication over the communication channel, a purchase transaction is implemented, including transmission of payment credentials to the merchant from the payment application.