BackXPress lets users create pressure input with six fingers at the back of landscape-held smartphones to augment their frontal touchscreen interaction. We present design guidelines derived from three studies.
When people hold their smartphone in landscape orientation, they use their thumbs for input on the frontal touchscreen, while their remaining fingers rest on the back of the device (BoD) to stabilize the grip. We present BackXPress, a new interaction technique that lets users create BoD pressure input with these remaining fingers to augment their interaction with the touchscreen on the front: Users can apply various pressure levels with each of these fingers to enter different temporary “quasi-modes” that are only active as long as that pressure is applied. Both thumbs can then interact with the frontal screen in that mode. We illustrate the practicality of BackXPress with several sample applications, and report our results from three user studies: Study 1 investigated which fingers can be used to exert BoD pressure and found index, middle, and ring finger from both hands to be practical. Study 2 revealed how pressure touches from these six fingers are distributed across the BoD. Study 3 examined user performance for applying BoD pressure (a) during single touches at the front and (b) for 20 seconds while touching multiple consecutive frontal targets. Participants achieved up to 92% pressure accuracy for three separate pressure levels above normal resting pressure, with the middle fingers providing the highest accuracy. BoD pressure did not affect frontal touch accuracy. We conclude with design guidelines for BoD pressure input.
Marten Junga. Combining Back-of-Device Finger Pressure with Multi-Touch Thumb-Gestures on Smartphones. Bachelor's Thesis, RWTH Aachen University, Aachen, February 2018.
Christian Corsten, Bjoern Daehlmann, Simon Voelker and Jan Borchers. BackXPress: Using Back-of-Device Finger Pressure to Augment Touchscreen Input on Smartphones. In Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems, CHI '17, pages 4654–4666, ACM, New York, NY, USA, May 2017.