User Performance in Relation to 3D Input Device Design
ShuminZhai
IBM Almaden Research Center
Abstract
Based mainly on a series of studies the author
conducted at the University of Toronto, this
article reviews the usability of various six
degrees of freedom (6 DOF) input devices for
3D user interfaces. The following issues are
covered in the article: the multiple aspects of
input device usability (performance measures),
mouse based 6 DOF interaction, mouse modifications for 3D interfaces, free-moving
isotonic 6 DOF devices, desktop isometric and
elastic 6 DOF devices, armature-based 6 DOF
devices, position vs. rate control and the form
factors of 6 DOF control handle.These issues
are treated at an introductory and practical
level, with pointers to more technical and
theoretical references.
Introduction
As three-dimensional (3D) graphics moves to
the core of many mainstream c o m p u t e r
systems and applications, the search for usable
input devices for 3D object manipulation
becomes both an academic inquiry and a practical concern. In the case of the 2D graphical
user interface (GUI), the computer mouse
established itself very early and quickly
replaced the light pen as the de facto standard
input device (see [13] for a review of mouse
history). In the case of 3D interfaces, however,
there is still not an obvious winner suitable for
all applications. Primarily based on the author's
own research, this article offers a few perspectives on the usability of various input devices
for 3D interface. This article does not intend
to present a comprehensive literature review
or a series of experimental studies in a
methodical manner. Rather, it intends to be
introductory and practical. Interested readers
are encouraged to examine more technical
details in the papers referenced.
In o r d e r to be able to manipulate 3D
objects, one generally needs at least six
degrees of freedom (6 DOF), three for X,Y
and Z translation and three for 3D rotation.
The difficulty in establishing a standard 6 DOF
device is twofold. First, there are engineering
challenges in terms of sensor technologies,
manufacturing cost and designer's creativity. It
is highly likely that the most elegant 6 DOF
device has still not been designed. Second, and
perhaps more importantly, even if we could
easily make any device we like to, there is only
a very limited knowledge about what properties a good 6 DOF controller should have.
Given the long history of human factors study
on input control devices, dating back to World
War II [I I],"One would expect the relation50
November 1998 Computer Graphics
ship of the hand to the controlled element,
being at the one time both an input and
output, to be a fruitful area for research" but
the reality is that little is well understood [3].
Burrows pointed out that the reluctance to
conduct research in this area is understandable
in view of the immensity of the possible interactions among the many dimensions of control
feel.
This is not to say that there isn't any intellectual guidance to 6 DOF input device design.
Motivated by the manual control problems in
vehicles, air crafts and other dynamic complex
machines, the topic of "manual control and
tracking" has been extensively studied (see
[14] for a summary). However, system
dynamics resulted from mass, spring, viscosity,
transmission delay, etc. in these systems soon
dominated the area.The study of input control
device properties (e.g. [ I ] ) quickly gave way to
mathematical control theory modeling of manmachine systems. The more general body of
knowledge on human m o t o r control and
learning (see [I 6] for example), while offering
many insights, rarely provides direct design
guidelines. One recent review of the scattered
literature related to input device design is
provided in [21 ].
Performance Measures of
6 DOF Input Devices
There are many choices in designing o r
selecting a 6 DOF input device.The choice on
every design dimension may have implications
on users' performance. Aside from application
specific requirements, there are at least six
aspects to the usability of a 6 DOF input
device, namely:
• Speed
• Accuracy
• Easeof learning
• Fatigue
• Coordination
• Device persistence and acquisition
The first four aspects are common to all
input devices and their meanings are obvious.
The fifth aspect, coordination, is unique to
multiple degrees of freedom input control.
There are many ways of measuring the degree
of coordination. One effective way of quantifying it is based on the ratio between the
length of actual trajectory and that of the most
efficient trajectory in the coordination spaces,
including translation space, rotation space and
the 2D space between translation and rotation
[23]. By such a measure, in order to produce
the most coordinated path, one has to simultaneously move all degrees of freedom involved
at the same pace towards their respective goal
states.
The sixth aspect of input device usability is
the ease of device acquisition. This is often an
overlooked aspect of input device usability.
Although a mouse is less dexterous than a
pen-like input device (a stylus), the fact that a
mouse can be more easily acquired is one
important reason that made it the dominant 2
DOF input device. Many factors, such as the
distance to the computer keyboard home row
(ASDFGHJKL keys), contribute to the ease of
device acquisition. One of them is the device
location persistence when released. With a
mouse or a trackball, when released by the
hand (in order to type something, for example)
it stays in position. This is not true with a
stylus.
W i t h these measures in mind, the
remainder of the paper examines a few
common classes of 6 DOF input devices.
Mouse Based 6 D O F
Input
Mouse Mapping
The simplest implementation of 6 DOF manipulation can be six graphical sliders on the
computer screen. Each can be dragged with a
standard 2 DOF input device, such as a mouse.
There are two fundamental usability problems
to such a 6 DOF interaction technique. First,
people cannot mentally decompose orientation into separate rotational axes [12]. Second,
since one has to time-multiplex between the
separate degrees of freedom, it is not possible
to form a coordinated movement in the 6
DOF space.
Researchers soon moved away from the six
slider implementation to more complex
mapping techniques. One such technique is
enclosing the manipulated 3D object with a
virtual sphere [4]. Some of these techniques in
fact have been widely used in 3D graphics software such as VRML browsers and CAD packages. A recent study [7] found the mouse
mapping techniques still inferior to integrated
6 DOF devices.
Mice Modified for 3D Operation
Efforts have been made to add more physical
degrees of freedom to the mouse for 3D
interfaces. One example is the roller mouse
that had an additional degree of freedom by
means of a roller ([18], see Figure I).A user
could rotate the roller to move a 3D cursor in
the depth dimension (translation only).
Another example is the rockin' mouse with
two additional degrees of freedom by means
of rocking motions on a tablet [2] (see Figure
2).
Since dedicated physical degrees of
freedom are provided for the third dimension,
these modified mice should outperform a
% inefficiency
il
~
0 BasicRateControl
II. Free PositionControl
o
..~=¢.m
213]
113],
"n
Figure I:A mousewith a miler for 3D input [18].
T2
"l-J T*~ "1~ "PI "1"2 "lI-J T** "1~
BII'I 1
.B(p'l2
Rgure 4:Trial completion time in a 6 DOF d0ck/ngtask.
The Fee mawngdewcewas faster,pa~culorly m early
stage [23].
Rgure 2cA mouse with two additionaldegreesof
freedom [2].
,o)
Figure 3: A sample of Fee moving 6 DOF devices- 'T~ing mice."(a) The "Bc¢" deJignedby C.Ware [I 9]; (b)
The CricketTM, manufacturedby Digital Image Design
Inc., NewYork NY, USA;(c) The MITS Glove,designedby
the author,consistsof a BirdTM tracker and a clutch.See
pale 102 for color Image.
conventional mouse that operates by means of
simple mode switching. (One common mouse
switching technique, f o r example, is that
pressing and holding a mouse button down
switches vertical mouse cursor m o t i o n t o
motion in the depth dimension). It was shown
that in a 3D positioning task, the rockin'
mouse was 30 percent Paster in comparison to
a standard mouse tablet [2]. On the other
hand, since the depth dimension is operated by
a behavior and muscle groups different from
those of the x-y mouse motion, it can be difficult t o produce simultaneous, coordinated
motion with either the roller mouse or the
rockin' mouse.
"Flying" Mice
Given the success of the mouse in 2D interfaces, it is natural to attempt to make a flying
mouse - - a mouse that can be moved and
rotated in the air For 3D object manipulation.
Indeed many such devices have been made.
Theoretically, a mouse is a free-moving, i.e.
isotonic device.VVhen using such a device, the
displacement of the device is typically mapped
to a cursor displacement.This type of mapping
(transfer function) is also called position
control. Figure 3 shows a few examples of 6
DOF isotonic position control devices. Host of
these devices are i n s t r u m e n t e d w i t h a
magnetic tracker for 6 DOF sensing.
The advantages of these "flying mice"
devices are:
• Easy t o learn, because of the natural,
direct mapping.
• Relatively fast speed. Studies have shown
that 6 DOF isotonic position control
devices tend to outperform other type of
devices in speed. This was particularly
true for novice users [21, 23, 25] (see
Figure 4).
However, there are many disadvantages to
this class of devices:
• Limited movement range. Since it is posit i o n c o n t r o l , hand m o v e m e n t can be
mapped to only a limited range of the
display space. In the case of standard 2
DOF mouse, this problem can be solved
by making i¢ a "relative" device. One can
lift the mouse up and put it down at a
new location of the mouse pad. Similarly,
one can also use a clutch (Figure 3(::) in a
6 DOF isotonic position control device.
• Lack of coordination. In position control,
object movement is directly proportional
t o hand/finger m o v e m e n t and hence
;, 4
~ ~
B~'l 1
~;
;,,;
;= ;, ;,
B~'l Z
Rgure5: Coordinationas measuredby inef~c/encyin free
moving position contro/vs,in elas',icrate control.
Signi~can~longermovementwas "~wasted"with the free
mov/ngdevice.See [23] far details.
constrained t o anatomical limitations:
joints can only rotate to a certain angle.
These limitations prohibit well-coordinated movement, particularly when the
user has to reclutch o r reposition the
hand (finger) relative to the input device
surface (see Figure 6). See [23] f o r
detailed discussion on coordination (see
Figure 5).
• Fatigue.This is a significant problem with
free moving 6 DOF devices because the
user's arm has to be suspended in the air
without support.The mouse did not have
such a problem because it stays on the
deskrop. One can rest the arm and hand
on the desktop surface when operating a
mouse, but not with the flying mice.
• Difficulty of device acquisition.The flying
mice lack persistence in position when
released. W i t h a mouse o r trackball,
when released by the hand, it stays in
position. This is nor true with a stylus,
nor with any of the flying mice.This property of not "staying put" inhibits some
classes of interaction. Such devices may
have to be put on desk and picked up for
the next transaction. In the case of a
glove, this is even more problematic,
Form Factor Matters
O f course, there are many variations within
each class of devices. One such variation is the
form factor of the control handle (shape and
size). As shown in Figure 3, there has been a
variety of shape and size used in constructing
free moving 6 DOF input devices. Some of
these devices, such as the glove implementation (Figure 3c) require manipulation w i t h
wrist and arms, but exclude the fingers. Figure
6 shows an alternative design, based on the
very same sensor with a different shape and
size that do allow the participation of the
fingers that have higher dexterity. It was shown
that significant performance advantage could
be gained with such a design [22]. See figure 7.
Computer Graphics November1998 51
(se¢i
• Glove
O FingerBall
,3
8
'~
Test1
Test2
Test3
Test4
Test5
Figure 7: Performance difference between the Fingerball
and the Glove in a 6 DOF docking task [22]
Figure 6:The FingerbalI.This device usesthe same sensor
as in the glove in Figure 3c. However, the different form
factor made it possible to manipulate it with fingers, in
addition to wrist and arm, leading to better performance
Transfer functiol
Y~
[22] See page 102 for color image.
"Desktop" Devices
The alternative to free moving input devices,
are devices that are mounted on a stationary
surface. We may call these desktop devices.
Figure 8 shows some examples of 6 D O F
" j o y s t i c k " devices that fall into this class.
C o m m o n to such devices is a self-centering
mechanism. They are either isometric devices
that do not move by a significantly perceptible
magnitude o r elastic devices that are springloaded. When tension is released from the
handle, the handle returns to a null position.
Typically these devices w o r k in rate control
mode, i.e. the input variable, either force o r
displacement, is mapped onto the velocity of
the cursor. In other words, the cursor position
is the integration of input variable over time.
Hence the t e r m f i r s t o r d e r c o n t r o l . In
c o n t r a s t , in the case of the free m o v i n g
( i s o t o n i c ) devices, i n p u t variable (device
displacement) is scaled to position (location
and orientation) itself. Hence the term zeroorder control.
Compatibility Between Resistance and
Transfer Function
W i t h the p r o p e r clutching mechanism, it is
conceivable to implement an isometric device
in position control mode or an isotonic device
in r a t e c o n t r o l m o d e (see Figure 9)
Interestingly, these t w o combinations tend to
produce p o o r user performance (see Figure
10).The reason is q u i t e simple: the selfcentering mechanism in an isometric device
facilitates the "start, speed-up, maintain speed,
slow-down and stop" cycle in rate c o n t r o l . T h e
later half of the cycle is somewhat automatic
with the self-centering mechanism in isometric
devices.With a free moving device, one has to
deliberately return to the null position.
52
November 1998 Computer Graphics
Rate
Control
Isotonic
Rate
Isometric
Rate
Position
Control
ISotonic
Position
Isometric
Position
x
Isotonic
Isometric
Controller Resislaq ce (spring slJffress)
Figure 8:A sample of desktop 6 DOF input devices.A
sample of input devices for 6 DOF manipulation: (a) The
SpaceballTM is an isometric device manufactured by
Spaceball Technologies Inc., Boston, MA, U.S.A.(b) The
SpaceMasterTM is an elastic device with a small range of
movement (5 mm in translation and 15°in
rotcrdon),manufactured by BASYS GmbH, Erlangen,
Germany. (c) The Space Mouse TM is an elastic device
with slight movement (5 mm in translation and 4° in
rotation). It was initially designed by DLR, the German
aerospace research establishment, manufactured by
Space Control Company, Malching, Germany and
marketed by Logitech, Fremont, CA, U.SJ~.See page
102 for color image.
Figure 9:Two input device design dimensions:transfer
function vs. controller resistance.
MeanCompletionTimeWith StandardErrors
161 PosJtionlS°m°t: rc'
14.
~
---o-- Position
• Rate
12
11'1
8-I
~
ISotonic
Rate
Isotonic
Position
S-~ ~O~......
i
Isometric
4"
T h e Pros and Cons of Desktop 6 D O F
Devices
When used in rate control, an isometric device
offers the following advantages:
• Reduced fatigue, since the user's arm can
be rested on the desktop.
• Increased c o o r d i n a t i o n . The integral
transformation in rate control makes the
actual cursor movement a step removed
f r o m the hand anatomy, resulting in
greater coordination (see Figure I I) [23].
• Smoother and more steady cursor move°
menr. The rate control mechanism (integration) is a low pass filter, reducing high
frequency noises.
• Device persistence and faster acquisition.
Since these devices stay stationary on the
desktop, they can be acquired m o r e
easily.
On the other hand, isometric rate control
devices may have the following disadvantages:
• Rate control is an acquired skilI. A user
typically takes tens of minutes, a signifi-
~.
Isotonic
Resistance
Figure I O:Isotonic rate control and isometric position
control tend to produce poor performance [24, 2 I].
O []asicRateControl
~ m FreePositionControl
\
11111
Sll
[]
I
I"I
I
T2
I
"r'J
E,cp'lI
I
T*
i
"I~
C
~
I
12
I
I
[
Ta
T~.
"15
E~I2
Figure I h Coordination as quantified by movement
inefficiency.The free moving position control device
"wasted"more movement.Adapted from Figure I0 in
[23].
Figure 15:A single arm mu/t/-DOFarmature input device
[9]. See page 102 for color image.
Figure 12:A prototype of a 6 DOF Elastic Genera/purp0se
Gr/p (EGG)[2 I, 20]. See page 102 for color image.
i
lu
~
s
j
.
J
.
~e
T~o
Three
Fora
Leaflnii~F:h~ t
Figure 13:Tirne performance difference between an
ela.~ic and an isometricrate controllerin a 6 DOF
docking task [2 I, 20].
P.
centering mechanism needed for rate control.
Figure 12 shows a prototype of a 6 DOF
elastic rate control input device, dubbed EGG
(elastic general-purpose grip).A similar suspension structure had been used for different
purposes [6]. Since a user can feel both the
pressure and the displacement with an elastic
device, a stronger feedback is provided (see
[21] Chapter 3 for derailed neuro-physiological
analysis). However, the use of spring loading is
not without tradeolf. The more displacement
for the same force is provided, the stronger
the kinesthetic feedback is provided, but the
weaker the self-centering mechanism is for
rate control compatibility (See [21] Chapter 3
for this two-factor analysis of elastic devices).
When optimized, the appropriate amount
of elasticity does improve a user's performance. This is particularly true at the early
learning stages (see Figures 13 & 14).
12
Multi-DOF A r m a t u r e s
10
"6
O-I
,r,
g4
,~
a4
4 i
,
phanO
,
PhBel
,
phBe2
,
Phase3
,
Phau4
Figure 14:Trackingerror with an elas~Jcvs. isometric 6
DOF controller task [2 I, 24].
cant duration for learning c o m p u t e r
interaction tasks, to gain controllability of
isometric rate control devices. It may
take hours of practice to approach the
level of isotonic posiUon speed [21,25].
• Lack of control feel. Since an isometric
device feels completely rigid, insufficient
feedback is provided to the user at the
k i n e s t h e t i c channel. Kinesthetic (or
proprioceptive) feedback can be critical
to user's conrrol performance.
Elastic Devices
An elastic device moves proportionally to rhe
force applied to it and yet maintains the self-
Another class of multi DOF input devices are
mechanical armatures. Some of these are
specialized and geometrically compatible with
the graphical object being controlled so as to
enable the posing of computer graphics characr.ers more-or-less using puppeteering techniques.This is a technique that has been used
by ILM, f o r e x a m p l e [10]. Digital Image
Design Incorporated's "Monkey" [5] as seen
in the cover of this special issue and the
armatures f r o m Puppet W o r k s [ I S ] are
commercially available examples of this class
of technology.
While strong in their intended, specialized
purpose, their use is limited. They can do an
excellent job for some types of character
posing, but their applicability to interaction in
general is limited, at least in this "puppet" type
configuration.
One option that can bring armatures much
closer to the generality seen with the other
devices discussed occurs if they are configured
as a single arm. This can be done with either
rhe DID or Puppet Works devices, and is the
standard configuration for other armatures,
such as Immersion Corp's MicroScribe illustrated in Figure 15 [9].
In this configuration, the armature is actually a hybrid between a flying-mouse type of
device and a desktop device. It is like a desktop
device in that it is typically mounted on the
desktop, and consumes a small footprint. On
the other hand, it is like a flying mouse in that
manipulating the end point of the arm in space
provides the desired 6 DOF position data.
Conceptually, these are near isotonic - - with
exceptional singularity positions - - position
control device like a flying mouse and thus
share many of the pros and cons of isotonic
position control discussed earlier. In addition,
this approach has the following particular
advantages:
• N o t susceptible to interference; most
flying mice, based on magnetic tracker,
have susceptibility to electromagnetic
interference as well as to metal objects,
• Less delay:,response is usually better than
most flying mouse technology, whose
built-in electromagnetic noise reduction
filters introduce significant time delay.
• Can be configured to "stay put," when
friction on joints is adjusted and theref o r better for device acquisition.
• Some, like the MicroScribe, have potential
to serve double duty as 3D digitizer
(which is what it was designed for) as
well as general input devices
On the other hand, they also have some
particular drawbacks:
• Device acquisition (especially if they do
not "stay put").
• Fatigue: as with flying mouse.
• Constrained operation.The user has to
carry the mechanical arm to operate,
which is more cumbersome than most
flying mice tethered by a cable.At certain
singular points, position/orientation is
awkward.
This class of devices can also be equipped
w i t h force feedback, as w i t h SensAble
Technology's Phantom Device [ 1 7 ] . W h i l e
showing some promise and with novel user
experiences, force feedback devices have yet
to demonsa-are their real benefit in the lab or
in practice for mainstream applications. Ir is, of
course, very interesting as a research area.
Conclusions
From this brief and incomplete guided tour,
one can conclude thor the complexity of 6
DOF input is far from being solved. None of
the existing devices fulfills all aspects of
usability requiremenr for 3D manipulation. A
six degree of freedom device that suits all, or
the majority of, 3D applications has yec to he
developed. However, the research conducted
so far has offered implications for potential
improvement - - with many insights into the
characteristics, pros and cons of various
designs. These insights should help the selection of various types of devices for different
Computer
Graphics
November
1998
S "a
tasks. For example, when speed and short
learning is a primary concern (such as in video
games), free moving devices are most suitable.
When fatigue, control trajectory quality and
coordination are more important, such as in
teleoperation, isometric or elastic rate control
devices should be selected.This article has only
covered a few aspects of 6 DOF input device
design. Many other areas, such as the cooperation of two hands in 3D input [8] and those
covered by other articles in this issue, are
equally important. Further research, both
developmental and experimental, is certainly
needed.
Acknowledgments
I thank Paul Milgram and Bill Buxton for their
longstanding mentorship. Some of the studies
cited in this paper were conducted at the
University of Toronto with funding support
from ITRC, NSERC and DCIEM. Bill Buxton
reviewed the article and co-authored the
Multi-DOF Armatures section.
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Shumin Zhai is a Research Staff Member at
the IBM Almaden Research Center where he
conducts research and innovative development
in input devices and interaction techniques,
theoretical modeling of human computer
interaction (HCI), advanced graphical user
interfaces and computer vision-based next
generation multi-modal interaction techniques.
He received his Ph.D. degree from the
University of Toronto where he worked on 3D
interfaces and six degrees of freedom input
control.
Shumin Zhai
IBM Almaden Research Center
650 Harry Road
San Jose, CA 95123
Tel: + 1-408-927- I I 12
Email: [email protected]
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FROM
THE
EDITOR
Explore the World of Computer Gaming and Computer Graphics
Gordon Cameron
SOFTIMAGE, Inc.
The February 1997 issue of Computer Graphics
contained a focus (expertly guest edited by
Mike Milne) on the entertainment industry,
but we chose to save an important area of
this industry for later investigation. It's with
great pleasure that I present that focus on the
computer games industry in this May 1998
issue of Computer Graphics.
Back in the early 'B0s when I was still
in school, I was enthusiastically coding away
on a variety of early machines such as the
Sinclair ZX8 I, Oric- I, Atari 800XL and Atari
ST. At the same time, I spent a great deal of
my hard-earned paper-round cash on games
for these machines, so it was with great
excitement that I recently discovered an
on-line "shrine" to the games and their progremmers. James Hague had painstakingly put
together a list of"classic game programmers"
and in addition had interviewed several of the
more revered game designers for a fascinating
electronic publication entitled Halcyon Days.
Around the same rime, I was trying to put
together an issue on computer graphics and
the games industry, and so contacted James
to see if he might be interested in guest editing such an endeavour. Luckily, he accepted,
and ~ e issue in your hands now contains the
resulting focus.
Over the past decades, computer games
have evolved at a remarkable pace. Many of
the early titles pushed the platform capabilities, but more recently the games industry is
proving one of the major factors in pushing
computer graphics in feneral forward at a
breakneck pace -- many of the new titles are
generating groundbreaking research of their
own, and forcing the hardware (and standards) to evolve co keep up,You can pick up a
consumer PC with graphics comparable (or
superior) to the workstations of a short time
ago, at a fraction of the cost today, and this
trend is really shaking up our industry and
forcing innovations at a startling rate.
At the same time, it is worthwhile to
Jook back at the amazing things people were
doing in the earlier days of computer gaming,
with far more limited resources (both technical and human). These early pioneers were
performing minor miracles to achieve effects
that today may look somewhat dated bur in
their time were bleeding edge, whilst still
managing to keep in mind that most important, yet too-oft neglected, aspect --- gameplay.
James has done a superb job in gathering
together a collection of thoughtful and personalarticles from both past and present
which together form a snapshot of the world
of computer gaming and computer graphics.
My thanks go out to all those who contributed, and especially to James for working
under extremely tight deadlines.
Also, once again we have a tremendous
series of columns. If you have any comments, I
encourage you to drop a note to the columnisl3. For any general questions, ideas, commerits, etc, please feel free to contact me at
one of the addresses listed below and I'll do
my best to answer -- thank you s o much for
your letters over the last few months and,
please, keep them cominl! The majority of
notes from the last issue complimented the
content, for which I'm extremely grateful on
behalf of the contributors. However, rather
than print only these, I've decided to wait
until we have a broad cross section of letters
to use in the next Letters column.
Until next issue, all the very besT,,and I
look forward to seeing some of you at the
upcoming SIGGRAPH 98 25th anniversary
conference.
Gordon Cameron
Software Development
SOFTIMAGE,Inc.
3510 boul.St-~urent
Suite 400
Montn~, Quebec,H2X 2V2
Canada
Tel:+ I-51A,aA,5-1636 ~ 3445
Fmc+ I-514-845-5676
Email:Eordon_cameron~sll~q-aph-ori
Computer Graphics ~
1998 3