BackTap: robust four-point tapping on the back of an off-the-shelf smartphone.

We present BackTap, an interaction technique that extends the input modality of a smartphone to add four distinct tap locations on the back case of a smartphone. The BackTap interaction can be used eyes-free with the phone in a user's pocket, purse, or armband while walking, or while holding the phone with two hands so as not to occlude the screen with the fingers. We employ three common built-in sensors on the smartphone (microphone, gyroscope, and accelerometer) and feature a lightweight heuristic implementation. In an evaluation with eleven participants and three usage conditions, users were able to tap four distinct points with 92% to 96% accuracy.