CHI 97 Electronic Publications: Late-Breaking/Short Talks
A Two-Ball Mouse Affords Three Degrees of Freedom
I. Scott MacKenzie1, R. William Soukoreff1, & Chris Pal3
1,2Dept. of Computing & Information Science
3Dept. of Physics
University of Guelph
Guelph, Ontario, Canada N1G 2W1
1mac@snowhite.cis.uoguelph.ca
2will@snowhite.cis.uoguelph.ca
3cpal@uoguleph.ca
ABSTRACT
We describe a prototype two-ball mouse containing the electronics and mechanics
of two mice in a single chassis. Unlike a conventional mouse, which senses
x-axis and y-axis displacement only, our mouse also senses
z-axis angular motion. This is accomplished through simple calculations
on the two sets of x-y displacement data. Our mouse looks and feels
like a standard mouse, however certain primitive operations are performed with
much greater ease. The rotate tool - common in most drawing programs - becomes
redundant as objects are easily moved with three degrees of freedom. Mechanisms
to engage the added degree of freedom and different interaction techniques are
discussed.
Keywords
pointing devices, multi-degree-of-freedom input, rotation
© 1997 Copyright on this material is held by the authors.
INTRODUCTION
Since the introduction of the Apple Macintosh in 1984, the form of
desktop systems has not changed substantially. Manipulating complex graphic
objects usually combines mouse movement with special tools or modes. These
permit simple displacement data from the mouse to map to and control the
displacement and/or angular location of objects or scenes. These mappings are
often unnatural.
DIMENSIONS AND DEGREES OF FREEDOM
Before we describe our mouse, it is important to distinguish between dimensions
(D) and degrees of freedom (df). In three dimensions, there are six degrees
of freedom: three for position along the x, y, and z axes,
and three for angular orientation (THETA-x, THETA-y, and
THETA-z). In 2D, there are three degrees of freedom. If we consider
a 2D surface such as a mouse pad (see Figure 1), then there is an x and
y positional degree of freedom and a z-axis rotational degree of
freedom.
Figure 2 illustrates the dimensions and degrees of freedom for several
"mouse-type" devices. As a standard mouse is manoeuvred on a mouse pad, only
its x and y displacement are sensed, as indicated in the "Mouse"
column. This is sufficient for most tasks using, for example, word processors
or spreadsheets. However, within drawing packages and other graphics programs,
a common task is to move an object to a new location and with a neworientation.
This is a full two dimensional task and it requires three degrees of freedom.
Since mouse angular motion is not sensed, a rotate tool or a manipulator handle
is usually required. Although other schemes can be devised to control the
orientation of objects [3], they are unnatural and violate the basic perceptual
structure of interaction [2].
Figure 1. A mouse pad is a two-dimensional surface
with three degrees of freedom: x, y, and THETA-z.
Figure 2. Dimensions vs. degrees of freedom. Grey dots indicate degrees of
freedom sensed for several types of input devices. (See text for discussion)
A TWO-BALL MOUSE
We have built a prototype of a 2D/3df mouse (see Figure 3) that makes a rotate
tool redundant. The mouse was built using the mechanical parts from two
Microsoft 2.0 mice and the electronics from two Fellowes MousePens. The
prototype weighs 156 g, compared to 104 g for the standard Microsoft 2.0
mouse. It interfaces to a host computer through two serial ports. This is
sufficient for a prototype and it allowed a variety of simple demonstrations to
be implemented. As a product, a complete bottom-up redesign would be
required.
With some simple arithmetic in the interface software, the z-axis or
rotational component of the mouse's motion is easily computed from the two
streams of x-y positional data. The result is a mouse that senses all
three degrees of freedom in a two-dimensional surface. ("2-ball Mouse" column
in Figure 2).
(a) 
(b) 
Figure 3. Two-ball mouse (a) bottom view. (b) top view with cover and balls removed.
MULTI-DEGREE-OF-FREEDOM INPUT DEVICES
There are numerous input devices supporting more than two degrees of freedom.
The ProAgio by Mouse Systems and the Intellimouse by Microsoft
are 3df mice however they are fundamentally different from our device because
rotation is sensed through a roller manipulated by a finger. We characterise
these as having 2+1 degrees of freedom - two positional degrees of freedom,
controlled as usual, and a third, but separate, rotational degree of freedom
controlled by a finger. Venolia describes a similar device with two rollers
[3]. With the frequent occurrence of carpal tunnel syndrome in today's
workplace, it is debatable whether increasing the load on the mouse-controlling
hand is wise. Our mouse has three integral degrees of freedom making it a
natural choice for 2D/3df interaction.
The Owl by Pegasus Technologies and the AeroDuet by Creative Labs
are 3df pointing devices that are held in the air. They sense x,
y, and z position so, again, they are distinctly different from
our device ("A" column in Figure 2). There are also a variety of 6df devices,
such as the InsideTrak by Polhemus ("B" column in Figure 2). Devices
that sense z-axis position are operated in the air and are suited to
games, virtual reality and other non-mainstream applications. In our view,
such devices are not likely to garner the support of users of desktop systems,
even if the application leans toward 3D interactive graphics. Current systems
that combine a mouse with rotate tools or manipulation handles are familiar and
there will be resistance to change unless the benefits are substantial and
immediate. Since our device is a mouse, it does not impose a major new
technique on the user. And it will work with existing tools as new techniques
gain acceptance.
An interesting new device is the Rockin'Mouse [1]. It looks like a
mouse except it has a curved bottom allowing it to "rock" on the desktop. The
rocking motion yields THETA-x an THETA-y data ("C" column
in Figure 2). It is operated without lifting from the desktop, so it also has
that special mouse-like appeal. Finally, column "D" in Figure 2 identifies a
hypothetical 5df device. It would sense rocking motion about the x and
y axes (like the Rockin'Mouse) as well as z-axis angular
motion (like our mouse). And it would not require lifting from the desktop.
Such a device could be very appealing to users who are reluctant to change
their ways.
OPERATION
That our mouse looks and acts like a regular mouse "most of the time" is
important for user acceptance. A problem we faced early on was how to engage
the third degree of freedom. In an earlier wooden prototype, this was
accomplished using a thumb-operated momentary switch on the side of the
chassis. Although this worked, it was awkward and error prone. We finally
settled on the following two-handed approach: An object is acquired in the
usual way, by positioning the cursor over it and pressing the primary mouse
button. If the CTRL or ALT key is also depressed (presumably using the other
hand), movement is in 3df mode; otherwise it is in 2df mode. This is a simple
and natural interaction technique. Using the CTRL key, the hand-to-object
angular mapping is 1-to-1. Using the ALT key, the mapping is 5-to-1. The
latter helps extend the range of rotation beyond that afforded by the
biomechanics of the lower arm. We are currently implementing a set of
representative tasks for empirical testing and comparison with current practice.
ACKNOWLEDGEMENTS
We thank NSERC of Canada for supporting this research. Thanks as well to the
members of the Input Research Group at the University's of Guelph and Toronto
for many helpful suggestions during the development of this research.
REFERENCES
1. Balakrishnan, R., Baudel, T., Kurtenbach, G., and Fitzmaurice, G.
The
Rockin'Mouse: Integral 3D manipulation on a plane, In Proc. of CHI
'97. New York: ACM, 1997.
2. Jacob, R. J. K., Sibert, L. E., McFarlane, D. C., and Mullen Jr., M. P.
Integrality and separability of input devices, ACM Trans. Computer-Human
Interaction 1 (1994), 3-26.
3. Venolia, D. Facile 3D manipulation, In Proc. of INTERCHI
'93. New York: ACM, 1993, pp. 31-36.
CHI 97 Electronic Publications: Late-Breaking/Short Talks
