Understanding back-to-front pinching for eyes-free mobile touch input

Using a smartphone touchscreen to control apps mirrored to a distant display is hard, since the user cannot see where she is touching while looking at the distant screen. Tactile landmarks at the back of the phone can mitigate this problem, especially in landscape mode [3]: By moving a finger across these landmarks, the user can haptically estimate the finger position in proportion to the touchscreen. Upon pinching the thumb resting above the touchscreen towards that finger at the back, the finger position is transferred to the front and registered as a touch. However, despite proprioception, this technique leads to a shift between back and front position, denoted as pinch error. We investigated this error using different target locations, device thicknesses, and tilt angles to derive target sizes that can be acquired at a 96% success rate.