2-Thumb Gesture: The Design & Evaluation of a Non-Sequential Bi-manual Gesture Based Text Input Technique for Touch Tablets Khai N. Truong1, Sen Hirano2, Gillian R. Hayes2, Karyn Moffatt3 2Donald Bren School of Information 1Department of Computer Science and Computer Sciences 3School of Information Studies University of Toronto University of California, Irvine McGill University Toronto, ON M5S 2E4 Irvine, CA 92697 Montreal, QB H3A 1X1 [email protected] {shirano,gillianrh}@ics.uci.edu [email protected] ABSTRACT two smaller strokes that can be drawn by both thumbs over the keys We present 2-Thumb Gesture (2TG), a non-sequential bi-manual sequentially on their respective sides of the keyboard. gesture-based text input technique for touch tablets, which enables In this paper, we demonstrate that the two-stroke gestures for the user to enter text with both hands by using each thumb to draw entering English words are highly unique and hence each stroke can small strokes over the keys on their respective sides of the keyboard be drawn over the letters non-sequentially by the two thumbs. We without waiting for their turn in the letter sequence of a word. The describe our design and evaluation of a non-sequential bi-manual results of a study comparing 2TG to Swype (a 1-finger word gesture based text input technique for touch tablets, called 2-Thumb drawing method), suggest that the learning and use of the 2TG Gesture or 2TG (see Figure 1). Through a controlled experiment, technique to perform text input is comparable with the commercial we establish the feasibility of learning and using the 2TG technique Swype technique by those who had no prior experience with either. to perform text input alongside the commercial Swype keyboard (a Furthermore, participants were able to hold and use the tablet with 1-finger word drawing software). As expected, those with prior both hands without experiencing the substantial fatigue that results knowledge of Swype were faster using Swype than 2TG; however, from a one-handed approach. Only 60 minutes after being their learning and performance rates with our two-thumb version introduced to the technique, participants were able to use the 2- were not statistically different from those with no prior Swype Thumb Gesture keyboard to enter text at 24.43 wpm, with an knowledge. Furthermore, by the end of the study, participants had uncorrected error rate of 0.65%. begun to learn the gestures for many short words and were able to perform almost one fifth of the input by simultaneously gesturing Categories and Subject Descriptors with both thumbs and touching keys out of order with respect to H5.2 [Information interfaces and presentation]: User Interfaces. – their turn in the actual letter sequence. These results suggest that Input devices and strategies. as participants begin to remember more gestures over time, 2TG General Terms Design, Experimentation, Human Factors Keywords Text entry, soft keyboard, gesture, bi-manual interaction 1. INTRODUCTION As mobile devices have grown in popularity, an increasing number of text-input techniques have been introduced. One such technique is shape writing—a method that allows the user to input text by drawing a stroke with a near unique shape containing points close to all letters found sequentially in a word. Drawing shapes enables faster input than traditional typing techniques that require the individual pressing of each letter in a word. As a result, it is now found commercially in a variety of products, including ShapeWriter [36], Swype [28], and SlideIT [26]. More recently, sales of large mobile touchscreen devices (i.e., those with screens four inches and larger) have increased. The size and weight of these devices make them hard to hold steady with only one hand while interacting with the other for long tasks, especially text input. As a result, commercial products such as Keymonk [10] and research efforts such as Bi et al.’s investigation of bimanual gesture keyboards [2] have begun to explore how words can be drawn on mobile touchscreen devices using both hands. In contrast Figure 1. The 2-Thumb Gesture keyboard. The user enters to previous shape writing techniques, which support text input as text by using both thumbs to perform drag gestures across the drawing of a single stroke containing points near the keys found the letters in a word on their respective half of the keyboard. sequentially in a word, these methods break the input of a word into can potentially extend similar performance benefits found in the 20 wpm with Cirrin after using the technique for over 2 months; traditional Swype technique to large-size touchscreen tablets, while Isokoski and Raisamo [9] showed that participants were able to enabling the user to hold and use the tablet with both hands to reach 16 wpm with Quikwriting using a stylus after 20 15-minute eliminate the substantial fatigue that results from a one handed sessions. In 2003, Zhai and Kristensson began to explore shape approach. writing as a way to allow users to perform shorthand gestures to input words on an optimized keyboard layout [12][13][34][35]. 2. RELATED WORK This concept has since been adopted by several commercial The 2-Thumb Gesture keyboard was designed to allow the user to products for small mobile touchscreen devices, such as hold the tablet with both hands and draw two short stroke gestures ShapeWriter [36], Swype [28], and SlideIT [26]. These virtual with both thumbs simultaneously, touching keys non-sequentially. keyboards have been used on a variety of platforms, including small Bi-manual word level gestures for entering text on a soft-keyboard devices, tabletops and tablets. Expert performance of shape writing has been previously introduced in commercial products such as using a QWERTY layout on tabletops have been computed to reach Keymonk [10] and research efforts such as Bi et al.’s bimanual 40.7 wpm [24]; but have not been computed for the tablet form gesture keyboard [2]. Bi et al. [2] showed that a bi-manual gesture- factor. In Bi et al.’s study [2], participants reached an average of 30 based method can reduce the length of the stroke used to draw a wpm using the unimanual shape writing technique on a tablet. word into two shorter strokes which are drawn by the thumbs. Our However, tablet users often describe concerns of fatigue in the hand work builds upon these systems by allowing both thumbs to draw holding the device as well as in the finger performing the gestures small strokes over the keys on their side of the keyboard without back and forth across the large touchscreens [5][15]. waiting their turn in the letter sequence of a word. Though Bi et al. mention the possibility of concurrent input [p.144, 2], this was not 2.2 Thumb-Based Text Input explored; the user touches the keys in a word sequentially with both Although very few techniques have been altered or designed Bi et al.’s bimanual keyboard and Keymonk. specifically to support thumb-based text-input, users often are able Below, we discuss how our method builds upon other works. We to effectively employ their thumbs to perform input. For example, review prior research in gesture-based text input, thumb-based text Silfverberg et al.’s study shows that two-handed index finger and input, and two-handed text input methods for mobile devices. one-handed thumb use of multi-tap are ~27 wpm and ~25 wpm respectively [25]; their study also shows that two-handed index 2.1 Gesture-Based Text Input finger and one-handed thumb of T9 is ~46 wpm and 41 wpm. One Virtual keyboards can enable text-input on mobile devices that do notable exception is Gonzalez et al.’s work [7] which explored not have a physical mini-QWERTY keyboard. Methods that different ways of supporting text-entry while the user is driving and support text-entry using gestures—either through touch or stylus— her hands are on a steering wheel. Their work showed that enable users to enter text either at the character-level or at the word- EdgeWrite [33] can be adapted to work on a Synaptics Stampad level. A detailed review of such techniques can be found elsewhere that is embedded on steering wheel. The focus of these previous [19]. works has been to examine how use of the thumb to support one- handed interaction affects performance. Results from these works Character-level input techniques allow users to type a word by show that performance is minimally affected, but provides other making multiple stroke gestures that correspond to different letters, benefits (such as it frees up the other hand, and it requires less one a time. The most common character-level technique is Graffiti, visual attention because the targets are in fixed location relative to created by Palm, Inc. The Graffiti alphabet resembles the Roman the thumb’s anchor position). An important implication when letters, which makes it easy for users to recognize and learn [4]. designing thumb-based interactions, however, is to make target Likewise, unistroke maps a single stroke to a single character [6]. sizes large enough that they can be accurately selected by the thumb Unistroke gestures do not resemble Roman letters, but are designed [22]. to be well distinguishable from one another. One key distinction of Graffiti and unistroke from other character-level input techniques 2.3 Two-Handed Text Input is that the user does not need to perform target selection as a part of Because typing on a desktop QWERTY keyboard is typically their text-entry. Castellucci and MacKenzie’s evaluation [4] performed as a two-handed task, it is not surprising that users can comparing Graffiti and unistroke shows that participants were able effectively employ both hands with many existing text-entry to input 15.8 wpm using unistroke and 11.4 wpm using Graffiti; methods.