The movement of a user's face, easily detected by a smartphone's front camera, is an underexploited input modality for mobile interactions. We introduce three sets of face-engaged interaction techniques for augmenting the traditional mobile inputs, which leverages the combination of the head movements with touch gestures and device motions, all sensed via the phone's built-in sensors. We systematically present the space of design considerations for mobile interactions using one or more of the three input modalities (i.e., touch, motion, and head). The additional affordances of the proposed techniques expand the mobile interaction vocabulary, and can facilitate unique usage scenarios such as one-hand or touch-free interaction. An initial evaluation was conducted and users had positive reactions to the new techniques, indicating the promise of an intuitive and convenient user experience.
@cite_30: The input vocabulary for touch-screen interaction on handhelds is dramatically limited, especially when the thumb must be used. To enrich that vocabulary we propose to discriminate, among thumb gestures, those we call MicroRolls, characterized by zero tangential velocity of the skin relative to the screen surface. Combining four categories of thumb gestures, Drags, Swipes, Rubbings and MicroRolls, with other classification dimensions, we show that at least 16 elemental gestures can be automatically recognized. We also report the results of two experiments showing that the roll vs. slide distinction facilitates thumb input in a realistic copy and paste task, relative to existing interaction techniques.
@cite_5: We introduce two families of techniques, rubbing and tapping, that use zooming to make possible precise interaction on passive touch screens, and describe examples of each. Rub-Pointing uses a diagonal rubbing gesture to integrate pointing and zooming in a single-handed technique. In contrast, Zoom-Tapping is a two-handed technique in which the dominant hand points, while the non-dominant hand taps to zoom, simulating multi-touch functionality on a single-touch display. Rub-Tapping is a hybrid technique that integrates rubbing with the dominant hand to point and zoom, and tapping with the non-dominant hand to confirm selection. We describe the results of a formal user study comparing these techniques with each other and with the well-known Take-Off and Zoom-Pointing selection techniques. Rub-Pointing and Zoom-Tapping had significantly fewer errors than Take-Off for small targets, and were significantly faster than Take-Off and Zoom-Pointing. We show how the techniques can be used for fluid interaction in an image viewer and in Google Maps.
@cite_10: Three studies were conducted comparing speed of performance, error rates and user preference ratings for three selection devices. The devices tested were a touchscreen, a touchscreen with stabilization (stabilization software filters and smooths raw data from hardware), and a mouse. The task was the selection of rectangular targets 1, 4, 16 and 32 pixels per side (0·4 × 0·6, 1·7 × 2·2, 6·9 × 9·0, 13·8 × 17·9 mm respectively). Touchscreen users were able to point at single pixel targets, thereby countering widespread expectations of poor touchscreen resolution. The results show no difference in performance between the mouse and touchscreen for targets ranging from 32 to 4 pixels per side. In addition, stabilization significantly reduced the error rates for the touchscreen when selecting small targets. These results imply that touchscreens, when properly used, have attractive advantages in selecting targets as small as 4 pixels per size (approximately one-quarter of the size of a single character). A variant of Fitts' Law is proposed to predict touchscreen pointing times. Ideas for future research are also presented.
@cite_25: We present the design and evaluation of TapTap and MagStick, two thumb interaction techniques for target acquisition on mobile devices with small touch-screens. These two techniques address all the issues raised by the selection of targets with the thumb on small tactile screens: screen accessibility, visual occlusion and accuracy. A controlled experiment shows that TapTap and MagStick allow the selection of targets in all areas of the screen in a fast and accurate way. They were found to be faster than four previous techniques except Direct Touch which, although faster, is too error prone. They also provided the best error rate of all tested techniques. Finally the paper also provides a comprehensive study of various techniques for thumb based touch-screen target selection.
@cite_20: A study comparing the speed, accuracy, and user satisfaction of three different touch screen strategies was performed. The purpose of the experiment was to evaluate the merits of the more intricate touch strategies that are possible on touch screens that return a continuous stream of touch data. The results showed that a touch strategy providing continuous feedback until a selection was confirmed had fewer errors than other touch strategies. The implications of the results for touch screens containing small, densely-packed targets were discussed.
@cite_11: 
For touch only input techniques, one of the major issues is the imprecision and occlusion of touch interfaces, also known as the “fat finger” problem. For example, Offset Cursor displays the cursor at a fixed distance on the screen to avoid the finger occlusion @cite_20 ; similarly, Shift applies a hybrid approach where a normal coarse direct touch selection is followed by a precise position adjustment @cite_10 . Along this line, Roudaut al proposed TapTap and MagStick that further improve the efficiency and accuracy of small target selection @cite_25 . Another relevant area is that concerning one-handed interaction. Karlson al have shown many cases in which users prefer one-handed use for mobile devices @cite_11 . Some gestures for performing one-handed zooming have been widely used on commercial systems, such as “double tap” on iOS and “tap then slide” in Google Map mobile app. Other techniques such as “rubbing” @cite_5 or “rolling” @cite_30 have also been proposed to facilitate single hand touch manipulations.