We present HiLight, a new form of unobtrusive screen-camera communication for off-theshelf smart devices. HiLight hides information underlying any images shown on an LED or OLED screen, and camera-equipped smart devices can fetch the information by turning their cameras to the screen. HiLight achieves this by leveraging the transparency (alpha) channel, a well-known concept in computer graphics, to encode bits into pixel translucency changes without modifying pixel color (RGB) values. We demonstrated HiLight’s feasibility using smartphones. By offering an unobtrusive, flexible, and lightweight communication channel between screens and cameras, HiLight allows new HCI and context-aware applications to emerge.