Interaction with 3D objects and scenes is becoming increasingly important on mobile devices. We explore 3D object rotation as a fundamental interaction task. We propose an extension of the virtual trackball metaphor, which is typically restricted to a half sphere and single-sided interaction, to actually use a full sphere. The extension is enabled by a hardware setup called the ÃÂÿiPhone Sandwich,ÃÂÿ which allows for simultaneous front-and-back touch input. This setup makes the rear part of the virtual trackball accessible for direct interaction and thus achieves the realization of the virtual trackball metaphor to its full extent. We conducted a user study that shows that a back-of-device virtual trackball is as effective as a front-of-device virtual trackball and that both outperform an implementation of tilt-based input.
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