Discussion

Based on result of user evaluation, two main concerns of design mobile bimanual interaction are discussed in following section. To let the design patterns to fulfill users’ need, the three design patterns are modified. The modifications would become the recommendations of the design patterns.

6.3.1 Concerns of Designing Mobile Bimanual Interaction

Relieving mental load of each hand’s manipulation

When people interact with mobile device with two hands, the mental load of each hand is added together. Two hands interrupt with each other more easily because the device is small. The result of the interview shows that participants have high mental load when they did the concurrent relocation task. They feel more nervous than in real world when they tilt the device and press the icon simultaneously because the physical feedback is weaker than the real objects. Designers need to relieve the load of each hand. The manipulation could be easy or the feedback could be obvious. Or designers could apply sequent bimanual interaction. Because the task is step by step, the mental load wouldn’t increase.

Also supporting single-handed manipulation

When designers develop a bimanual interaction design, they must let the design also support single hand manipulation. Some participants expressed that they are used to manipulate the cell

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phone by one hand. And in many daily life situations, people sometimes only use one hand to manipulate the mobile device. The system could provide a way of manipulation that both support single-handed and bimanual interaction, such as our relocation and placing prototype. Or the system could provide some function to assist the single-handed interaction such as automatic flipping page.

6.3.2 Modification of Design Patterns

6.3.2.1 Pattern 1 3D Manipulation with Jig

First type of jig is the device itself. The key issue of this kind of jig is sensitivity. The active angle of tilting must be concerned carefully. The system could add deceleration when the cube rotates. The design of continuous rotation also causes users to rotate the cube too far and increases the mental load and errors. When the task is complicated, the design of continuous rotation could be canceled.

The cube only rotates one time when users tilt the device and turn back once. This manipulation is more definite. Last, the system should provide visual cue or tutorial to help users realize the cube could be rotated.

Figure 6.7 Placing the jig to the side of the screen.

The second type of jig is a graphic jig on the screen. The first issue is about the position that two hands interrupt with each other. The position could be changed to the left or right side of the screen. The movement of the thumb is more natural and convenient (Figure 6.7). The second issue is the support of single-handed manipulation. The cube could rotate automatically when users move the item to special position. The third issue is the feedback. Although it is hard to provide the physical feedback, the visual feedback should be stronger and more interesting.

59 6.3.2.2 Pattern 2 Bimanual Relocation of Object

The first type is the concurrent relocation. The result of user evaluation shows it is hard to tilt device and press the item simultaneously. The graphic jig may be appropriate in this design. The most important issue is to reduce the mental load because users need to use their both hands in the meanwhile. The manipulation of each hand should be very simple and light. That the feedback should be very obvious and definite could relieve users’ nervousness.

The second type is the sequent relocation. There is a valuable finding in the user evaluation.

When users relocate the item step by step, they easily do the wrong gesture that is unnatural to them. For example, some users always tap the item inadvertently without dragging the icon after they rotate the cube. These subtasks couldn’t group into a well “chunk” when they are unnatural to users.

Buxton (1986) indicated that designer could build a chunk of subtasks to let novice learn the task fast. The visual effect of the floated item should be stronger to remind user that the item is still floated and movable and make the whole task more natural. In this design, the distinction between the cube and the upper layer where icon float is very clear, and users would see them as two independent layers. So they considered that the icon is moveable in both layers.

6.3.2.3 Pattern 3 Bimanual Object-Placing

The main issue is about the differentiation between upper and lower area. The differentiation of two areas should be more obvious. The look and feel of each area could be different. Or the items in each area could be different.

6.3.3 Summary of Discussion

Based on the comments gathered from user evaluation, we understand that relieving the mental load and supporting single-handed interaction should be concerned when designers develop a bimanual interaction in mobile device.

The modification of three design patters is summarized as follows:

(1) Pattern 1 3D Manipulation with Jig

For the Type I jig, mobile device itself, the additional recommendation is when the task is complicated, users tilt the device and turn back once, and then the cube only rotates one time.

For the Type 2 jig, graphic jig, the recommendation is the jig shouldn’t be the lower of the screen because it is hard to operate.

60 (2) Pattern 2 Bimanual Relocation of Object

For the concurrent relocation, the recommendation is the graphic jig might be the appropriate solution for bimanual interaction. For the sequent relocation, there are two recommendations.

One is the visual effect of the floated item should be obvious. The other is the item should be movable

(3) Pattern 3 Bimanual Object-Placing

The recommendation is the differentiation of the two areas should be obvious.