People use their mobile devices anywhere and anytime to run various apps, and the information shown on their device screens can be seen by nearby unauthorized parties, referred to as shoulder surfers. To mitigate this privacy threat, techniques have been developed utilizing human vision and optical system properties to hide the users' on-screen information from the shoulder surfers. Specifically, the proposed techniques discretize the device screen into grid patterns to neutralize the low-frequency components so that the on-screen information will “blend into” the background when viewed from the outside of the designed visible range.