We present Portallax, a clip-on technology to retrofit mobile devices with 3D display capabilities. Available technologies (e.g. Nintendo 3DS or LG Optimus) and clip-on solutions (e.g. 3DeeSlide and Grilli3D) force users to have a fixed head and device positions. This is contradictory to the nature of a mobile scenario, and limits the usage of interaction techniques such as tilting the device to control a game. Portallax uses an actuated parallax barrier and face tracking to realign the barrier's position to the user's position. This allows us to provide stereo, motion parallax and perspective correction cues in 60 degrees in front of the device. Our optimized design of the barrier minimizes colour distortion, maximizes resolution and produces bigger view-zones, which support ~81% of adults' interpupillary distances and allow eye tracking implemented with the front camera. We present a reference implementation, evaluate its key features and provide example applications illustrating the potential of Portallax.