BiPad - Designing Bimanual Interaction for Hand-held Tablets
Figure 1: Bimanual interaction with BiPad. The non-dominant support hand can tap, make gestures or perform chords, thus modifying interaction by the dominant hand.
Bipad is a research project by Julie Wagner, Stéphane Huot, and Wendy Mackay. It has been presented at CHI 2012.
Despite the demonstrated benefits of bimanual interaction, most tablets use just one hand for interaction, to free the other for support. We designed Bipad, a toolkit that supports interaction designers in creating bimanual interfaces with the support hand's thumb or fingers for iPad tablets. We identified five interactive zones (Bipad zones) at the screen borders that permit simultaneous support and interaction and provide them in Bipad. Another finding from a preliminary study is that people frequently change position to combat fatigue. Designers can therefore define multiple bipad interactions for each Bipad zone. Depending on users hold, they activate the next reachable Bipad area to interact with. Bipad provides three types of Bipad interactions: Bipad taps, chords and gestures.
BiPanZoom allows user to zoom-in and zoom-out using BiPad buttons, while the dominant hand's index finger sets the center of zoom.
BiKeyboard allows the user to type with their dominant hand on a traditional iPad keyboard. The non-dominant hand can trigger frequent commands, e.g. the space key, or mode switches, e.g. the shift key, using BiPad buttons.
BiPDF is an example application where the non-dominant hand triggers a mode switch: when the dominant hand's finger is placed on top of the PDF while a BiPad button is pressed, a circular menu appears that let's users select the desired function.
BiDraw provides a simple drawing area for the dominant hand. Bipad menu buttons provide the possibility to change stroke properties (e.g. color) while the dominant hand keeps drawing.
Guiard's Kinematic Chain analysis of bimanual interaction emphasizes the asymmetric relationship commonly observed between the two hands. He proposes the kinematic chain as a general model, in which the shoulder, elbow, wrist and fingers work together as a series of abstract motors. Each consists of a proximal element, e.g. the elbow, and a distal element, e.g. the wrist, which together make up a specific link, e.g. the forearm. In this case, the distal wrist must organize its movement relative to the output of the proximal elbow, since the two are physically attached.
Guiard argues that the relationships between the non-dominant and dominant hands are similar to those between proximal and distal elements: the former provides the spatial frame of reference for the detailed action of the latter. In addition, the movements of the proximal element or non-dominant hand are generally less frequent and less precise and usually precede the movements of the higher frequency, more detailed actions of the distal element or dominant hand.
Figure 2: The user creates a spatial frame, supports the device, and interacts with it. Different holds offer different trade-offs with respect to interactive power and comfort.
We see the kinematic chain in action when users interact with hand-held tablets: the non-dominant hand usually supports the tablet, leaving the fingers and thumb of the dominant hand free to interact. Fig. 2 shows three bimanual alternatives, based on the location of tablet support within the kinematic chain: the palm or forearm of the non-dominant arm (Fig. 2a, 2b); shared equally between the palms of both hands (Fig. 2c). In each case, the most proximal links control the spatial frame of reference; support links are always intermediate between framing and interaction links; and the most distal links use whatever remains of the thumb and fingers to interact.
Our preliminary study highlighted ten user-generated support holds that permit the thumb or fingers to reach the interactive area. Each poses trade-offs between comfort and degrees of freedom available for interaction. For example, supporting the tablet with the forearm (Fig. 2b) provides a secure, stable hold but forces the fingers to curl around the tablet, leaving little room for movement. In contrast, holding the tablet in the palm (Fig. 2a) gives the thumb its full range of movement, but is tiring and less stable.
We studied BiPad in a controlled experiment. Please have a look at our paper for further design guidelines.
Julie Wagner, Stéphane Huot and Wendy Mackay (2012) BiTouch and BiPad: Designing Bimanual Interaction for Hand-held Tablets. In CHI '12: Proceedings of the 30th international conference on Human factors in computing systems. ACM, pages 2317-2326. (google)