GraspZoom: zooming and scrolling control model forsingle-handed mobile interaction
Takashi Miyaki ††Jun Rekimoto †‡‡Interfaculty Initiative in Information Studies,
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033 Japan

Interaction Laboratory,
Sony Computer Science Laboratories, Inc.
3-14-13 Higashigotanda, Shinagawa-ku, Tokyo
141-0022 Japan

miyaki@acm.org

rekimoto@acm.org

ABSTRACTA pressure sensing based single-handed interaction model is presented in this paper. Unlike traditional desktop GUI model, mobile
UI model has not been established yet. For example, Apple iPhone
proposed “Pinch” operation, which use two fingers to zoom-in andzoom-out objects. However, in a today’s hand-held situation, manipulation methods using two fingers are not always good solution
because they require two hands in order to hold the device itself inmost cases. We propose a single-handed UI scheme “GraspZoom”:
multi-state input model using pressure sensing. Force Sensitive Resistor (FSR) attached on backside of a mobile phone was employed
in order to evaluate effectiveness of pressure based control model.
We also describe example applications which enable intuitive andcontinuous zooming and scrolling. By using tiny thumb gesture
input along with this pressure sensing method, bi-directional operations (e.g., zoom-in and -out) are also achieved.

Categories and Subject Descriptors
H.5.2 [User Interfaces]: Input devices and strategies

General Terms
Designsmall size display. Thus effective zooming and scrolling control
methods are highly demanded.
Recently, a lot of interaction methods are proposed to make mobile hand-held interaction more intuitively and easily. Many industrial company make consumer products which employs multitouch interaction. Apple proposed UI scheme named “Pinch” and“Flick”, which is designed for multi-touch capable hand-held device like iPhone [1]. “Pinch” is two fingered manipulation technique. When applying zooming operation, users simply touch two
points and enlarge or small displayed images by changing the distance between those two fingers. This operation basically requires
both hands because the user have to hold the mobile device itself
by non-operation hand. Thus this method cannot be suitable forthe device which is normally operated by single hand such as mobile phones. “Flick” is mainly for scrolling large image or long
list objects. When users touch objects and move toward one direction, the objects go on moving as if by its inertial force. If the user
wants to move the object quickly, scrolling speed is adjustable byspeed at the moment of releasing a finger. However, if the list has
huge number of items, users have to do “Flick” operation so manytimes. It seems so stressful operation when the device is used bysingle hand. Actually Apple provides A-to-Z index along with list
for second option in order to solve this problem. Thus consensus
of standard single-handed mobile interaction method has not been
established yet.

Keywords
Input devices, interaction techniques, single-handed, pressure sensing, mobile device

1.

INTRODUCTION

Traditional desktop GUI operations with pointing device (e.g.
mouse, touch pad, etc.) are basically designed to be used for controlling two dimensional graphics by using a device with two degreesof-freedom input for x-y position input and one or multiple clickable buttons for selection. In addition, it is common that additional
one dimensional input mechanisms (e.g. scroll wheel) is used for
operation of zooming or scrolling.
In today’s typical mobile hand-held interaction models, it is often
difficult to achieve simultaneous input of position and additional
scale value because of limitation of input devices. However it will
frequently occur that changing its scale and position because of its’
Copyright is held by the author/owner(s).
MobileHCI09 , September 15-18, 2009, Bonn, Germany.
ACM 978-1-60558-281-8.

Figure 1: Usage example of “GraspZoom” experimental implementation device.
If continuous and easily controllable parameter input device isavailable, the problem above will be solved. A lot of previous re-          

Figure 2: “GraspZoom” sensor configuration.
search investigate pressure sensing based UI, which use mouse surface [4] or stylus pressure of tablet [7] as continuous pressure sensitive input device, and users are able to adjust its pressure as well as
they do in the real world. However for the mobile GUI inputs purposes, pressure sensing is not widely used. This is because sensing
mechanisms such as pressure sensitive LCDs or tablets tends to becomplex and require higher costs for mobile devices.
In this paper, we present a new interaction method, “GraspZoom”,
which enables a pressure sensing based manipulation for mobile
hand-held user interaction with minimal hardware costs. Our experimental implementation uses commonly available PDA alongwith Force Sensitive Resistor (FSR) attached on backside of it.
With this minimal modification of hardware implementation, we
have investigated combination with tiny gesture input while a fingertip touches the screen of mobile devices for bi-directional parameter input.
The main contribution of this paper is single-handed interaction strategies for a pressure sensing based zooming and scrolling
method. An experimental implementation for exploring pressure
sensing oriented interactions for mobile environment is also provided.

2.

PROPOSED METHOD

2.1

Pressure Input for Single-handed Operation

Figure 2 shows the sensor configuration setup of GraspZoom. It
consists of a FSR (Interlink Electronics [2]) as pressure sensor andan acrylic cover which is attached backside of a mobile device andpropagates pressure force.
FSR is in the form of polymer film, and has a variable resistanceas a function of applied pressure. Capturing dynamically changing
pressure values can be implemented in low cost using this sensor.
This FSR and serial port of the mobile device are connected via
AVR 8 bit micro-controller.
Figure 4 shows the state transition diagram of proposed method.
GraspZoom has tri-state model: “Out of range” mode as idle state,“Touched” mode same as traditional touch screen input state and“Pressed” mode which is distinguished from “Touched” by threshold of pressure sensor output value. The threshold value is different
depends on direction of changing state in order to prevent unstable“chattering” conditions. This hysteresis threshold value makes operation stable even in mobile environment.
GraspZoom UI uses combination of conventional position input
from touch screen and this additional continuous parameter control

Figure 3: GraspZoom sensor device. (FSR and serial port ofApple iPhone are connected via AVR 8 bit micro)
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Figure 4: Interaction state diagram of GraspZoom.
from pressure sensing. “Pressed” state and its actual values fromsensors are mainly used for changing continuous parameter such aszoom scale or scroll operation.

2.2

Combination of Pressure Input and Tiny
Gesture on Screen

Pressure based interaction, for example zooming and scrolling,
requires control of scale parameters in two directions (zoom-in, out or scrolling-up, -down). Thus intuitive mode switching operation is important for applications of GraspZoom. We employedcombination of pressure input and tiny thumb gesture input on thescreen to achieve this bi-directional zooming and scrolling operation. The gesture motion can be sensed by touch screen of themobile device. Although there is another possibility to sense tiny
gesture on backside of the device, from our preliminary observation, gesture input on backside is too difficult to manipulate by single hand.

2.3

Usage Examples

GraspZoom could be applied to various finger-tip operation formobile hand-held interaction, such as zooming photos, scrollinglists, drag-and-drop emulation, visual assistance of software keyboard. Some examples of how GraspZoom works are described inthis section. Figure 5 shows interaction style notation used in thissection in order to describe state of manipulation.

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Figure 6: Operational state of zooming interaction. ( a: zoom
in , b: zoom out)

2.3.1

Zooming

Users wants to temporarily zoom-in or zoom-out more often in
mobile hand-held situation, because physical size of screen is limited and cannot see entire figures and details at the same time. Thus
intuitive and smooth manipulation method of changing scale parameter is important for mobile devices.
Using a conventional interface, zoom-in and zoom-out are separate commands, and users have to change modes (e.g. “loupe magnifier” from tool palette). Pinch-in and -out operations of iPhone
which needs two hands are not always suitable for single-handed
operation under mobile situation.
Our technique enables users to controlling zooming parametersmoothly with continuous visual feedback while using single hand.
Figure 6 describes state diagram of zooming interaction. On zoomin operation, the users simply touch and press where they wants tomagnify as shown in the Figure 7. Asymmetric hysteresis thresholdof state transition (“pressing threshold” and “releasing threshold”)
enables stable operation. On zoom-out operation, we have employed combination of tiny sliding gesture on screen before pressing the screen. If the sliding operation on touch screen is sensed
in the state of “(b). Touch”, zooming direction in “(c). Press” ischanged to zoom-out. In this way, bi-directional parameter input ispossible with GraspZoom.

2.3.2

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Scrolling

Scrolling is second most important operation for today’s mobile
devices. Because of huge storage size in mobile devices, there area lot of situations to select an item from long scrolling list (e.g.,

Figure 7: State diagram and pressure intensity of zooming interaction.
address list or music playlist).
Figure 8 shows state diagram of scrolling interaction. In thiscase, position of pushing determine the direction of scrolling. Moreover, the users can change direction the other way using tiny thumbgestures sliding before pressing the screen similar to zoom-out operation in previous example.
Our proposed method enables a user to scroll continuously using single long pushing operation while conventional method (e.g.,
flicking or button pushing) requires multiple time operation relative
to demanded amount of scrolling. This is a significant difference inusability for single-handed user operation.

3.

DISCUSSION

Our implementation design provides the opportunity to explore
example applications and to observe people using it. From this observation, we were able to find several other design spaces to possibly accomplish the task. In this section, a set of possible options
and its feasibility are described.
Sensor configuration In our implementation, pressure sensor was
attached to back-side of mobile devices. However this could
be second-best solution in order to use commonly available
mobile device for our exploring. Force sensitive touch screen
device is also possible of course in the future. However thisis a subtle implementation problem, because it is enough tobe able to sense position of a fingertip and its pressure value
simultaneously.
Interaction state model There are other design spaces for triggering desired zooming or scrolling interaction, for example anumber of tapping, variation of gestures before pressing or
tactile feedback.



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mobile devices, and can effectively perform pressure sensitive singlehanded interactions.



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Figure 8: Operational state of scrolling interaction.

Sensing modality Pressure sensing is one of feasible solutions formaking additional continuous parameter of course. However,stability of pressure intensity can be a problem for consumer
products. Another possible option is utilizing contact areaof a fingertip as an pseudo pressure values in place of using
actual pressure sensors.

5.

CONCLUSION

In this paper, we introduced the concept of GraspZoom, a pressure sensing based interaction techniques for single-handed operation with mobile devices. We have presented operational models for pressure based input devices that aims to support mobile
small screen interaction. The prototype system which composed of
consumer PDA and FSR was used for exploring its design spaceof interaction technique. Usage examples of continuous zooming and scrolling operations are also shown with operational statemodel. Asymmetric hysteresis threshold of state transition was effective to stabilize mode change operation. In order to achieve bidirectional zooming and scrolling, combination of pressure sensing
and tiny gesture of sliding a fingertip before start pressing. Thus it
is confirmed that intuitive direct operation of zooming and scrolling
can be realized only minimal hardware modification of commercial
mobile devices. Future work must include application design andquantitative evaluation in order to judge its effectiveness of performance.

6.

REFERENCES[1] Apple Knowledge Base - iPhone: Using the multi-touch
display. http://support.apple.com/kb/HT1636.
[2] Interlink electronics: Force Sensing Resistors.
http://www.interlinkelectronics.com/
4. RELATED WORK
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Augmenting the mouse with pressure sensitive input. In
Although these are effective for desktop environment, it is difficult
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In our approach, we combined these multi-level pressure sensing’07 (2007), pp. 667–676.
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configuration requires minimal hardware modification to today’s