SAN FRANCISCO JULY 22-26 Volume 19, Number 3, 1985
Issues and Techniques in
Touch-Sensitive Tablet Input
William Buxton
Ralph Hill
Peter Rowley
Computer Systems Research Institute
University of Toronto
Toronto, Ontario
Canada M5S 1A4
(416) 978-6320
Abstract
Touch-sensitive tablets and their use in human-
computer interaction are discussed, It is shown
that such devices have some important properties
that differentiate them from other input devices
(such as mice and joysticks). The analysis serves
two purposes: (1) it sheds light on touch tablets,
and (2) it demonstrates how other devices might be
approached. Three specific distinctions between
touch tablets
and
one button mice are drawn. These
concern the
signaling
of events, multiple point
sensing and the use of templates. These distinc-
tions are reinforced, and possible uses of touch
tablets are illustrated, in an example application.
Potential enhancements to touch tablets and other
input devices are discussed, as are some inherent
problems. The paper concludes with recommenda-
tions for future work.
CR Categories and Subject Descriptors: 1.3.1 [Com-
puter Graphics]: Hardware Architecture: Input Dev-
ices. 1.3.6 [Computer Graphics]: Methodology and
Techniques: Device Independence, Ergonomics,
Interaction Techniques.
General Terms: Design, Human Factors.
Additional Keywords and Phrases: touch sensitive
input devices.
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1.
Introduction
Increasingly, research in human-computer interac-
tion is focusing on problems of input [Foley, Wallace
&Chan 1984; Buxton 1983; Buxton £985]. Much of
this attention is directed towards input technolo-
gies. The ubiquitous Sholes keyboard is being
replaced and/or complemented by alternative tech-
nologies. For example, a major focus of the market-
ing strategy for two recent personal computers, the
Apple Macintosh and Hewlett-Packard 150, has been
on the input devices that they employ (the mouse
and touch-screen, respectively).
Now that the range of available devices is expand-
ing, how does one select the best technology for a
particular application? And once a technology is
chosen, hove can it be used most effectively? These
questions are important, for as Buxton [ 1983] has
argued, the ways in which the user
physiccdty
interacts with an input device have a marked effect
on the type of user interface that can be effectiveIy
supported.
In the general sense, the objective of this paper is
to help in the selection process and assist in
effective use of a specific class o:F devices. Our
approach is to investigate a specific class of dev-
ices: touch-sensitive tablets. We will identify touch
tablets, enumerate their important properties, and
compare them to a more common input device, the
mouse. We then go on to give examples of transac-
tions where touch tablets can be used effectively.
There are two intended benefits for this approach.
First, the reader will acquire an understanding of
touch tablet issues. Second, the reader will have a
concrete example of how the technology can be
investigated, and can utilize the approach as a
model for investigating other classes of devices.
2.
Touch-Sensitive Tablets
A touch-sensitive tablet (touch tablet for short) is a
flat surface, usually mounted horizontally or nearly
horizontally, that can sense the location of a finger
pressing on it. That is, it is a tablet that can sense
that it is being touched, and where it is being
215
@ S I G G R A P H '85
touched. Touch tablets can vary greatly in size,
from a few inches on a side to several feet on a side.
The most critical requirement is that the user is
not required point with some manually held device
such as a stylus or puck.
What we have described in the previous paragraph
is a
simple
touch tablet. 0nly one point of contact
is sensed, and then only in a binary, touch/no touch,
mode. One way to extend the potential of a simple
touch tablet is to sense the degree, or pressure, of
contact. Another is to sense multiple points of con-
tact. In this case, the location (and possibly pres-
sure) of several points of contact would be
reported. Most tablets currently on the market are
of the "simple" variety. However, Lee, Buxton and
Smith [ 1985], and Nakatani [private communica-
tion] have developed prototypes of multi-touch,
multi-pressure sensing tablets.
We wish to stress that we will restrict our discus-
sion of touch technologies to touch tablets, which
can and should be used in ways that are different
from touch screens. Readers interested in touch-
screen technology are referred to Herot & Weinsap-
fel [1978], Nakatani & Rohrlich [1983] and Minsky
[1984]. We acknowledge that a fiat touch screen
mounted horizontally is a touch tablet as defined
above. This is not a contradiction, as a touch screen
has exactly the properties of touch tablets we
describe below, as long as there is no attempt to
mount a display below (or behind) it or to make it
the center of the user's visual focus.
Some sources of touch tablets are listed in Appen-
dix
A.
3. Properties of Touch-Sensitive Tablets
Asking "Which input device is best?" is much like
asking "How long should a piece of string be?" The
answer to both is: it depends on what you want to
use it for. With input devices, however, we are lim-
ited in our understanding of the relationship
between device properties and the demands of a
specific application. We will investigate touch
tablets from the perspective of improving our
understanding of this relationship. Our claim is
that other technologies warrant similar, or even
more detailed, investigation.
Touch tablets have a number of properties that dis-
tinguish them from other devices:
• They have no mechanical intermediate device
(such as stylus or puck). Hence they are useful
in hostile environments (e.g., classrooms, public
access terminals) where such intermediate dev-
ices can get lost, stolen, or damaged.
• Having no puck to slide or get bumped, the track-
ing symbol "stays put" once placed, thus making
them well suited for pointing tasks in environ-
ments subject to vibration or motion (e.g., fac-
tories, cockpits).
. They present no mechanical or kinesthetic res-
trictions on our ability to indicate more than one
point at a time. That is, we can use two hands or
more than one finger simultaneously on a single
tablet. (Remember, we can manually control at
most two mice at a time: one in each hand. Given
that we have ten fingers, it is conceivable that we
may wish to indicate more than two points simul-
taneously. An example of such an application
appears below).
• Unlike joysticks and trackballs, they have a very
low profile and can be integrated into other
equipment such as desks and low-profile key-
boards (e.g., the Key Tronic Touch Pad, see
Appendix A). This has potential benefits in port-
able systems, and, according to the Keystroke
model of Card, Newell and Moran [19S0], reduces
homing time from the keyboard to the pointing
device.
• They can be molded into one-piece constructions
thus eliminating cracks and grooves where dirt
can collect. This makes them well suited for very
clean environments (eg. hospitals) or very dirty
ones (eg., factories).
• Their simple construction, with no moving parts,
leads to reliable and long-lived operation, making
them suitable for environments where they will
be subjected to intense use or where reliability
is critical.
They do, of course, have some inherent disadvan-
tages, which will be discussed at the close of the
paper.
In the next section we will make three important
distinctions between touch tablets and mice. These
are:
• Mice and touch tablets vary in the number and
types of events that they can transmit. The
difference is especially pronounced when com-
paring to simple touch tablets.
• Touch tablets can be made that can sense multi-
ple points of contact. There is no analogous pro-
perty for mice.
• The surface of a tablet can be partitioned into
regions representing a collection of independent
"virtual" devices. This is analogous to the parti-
tioning of a screen into "windows" or virtual
displays. Mice, and other devices that transmit
"'relative change" information, do not lend them-
selves to this mode of interaction without con-
suming display real estate for visual feedback.
With conventional tablets and touch tablets,
graphical, physical or virtual templates can be
placed over the input device to delimit regions.
This allows valuable screen real estate to be
preserved. Physical templates, when combined
with touch sensing, permit the operator to sense
the regions without diverting the eyes from the
primary display during visually demanding tasks.
After these properties are discussed, a simple
finger painting program is used to illustrate them
in the context of a concrete example. We wish to
stress that we do not pretend that the program
represents a viable paint program or an optimal
interface. It is simply a vehicle to illustrate a
variety of transactions in an easily understandable
context.
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SAN FRANCISCO JULY 22-26 Volume 19, Number 3,1985
Finally, we discuss improvements that must be
made
to
current touch tablet technology, many of
which we have demonstrated in prototype form.
Also, we suggest potential improvements to other
devices, motivated by our experience with touch
technology.
4. Three Distinctions Between Touch Tablets and
Mice t
The distinctions we make in this section have to do
with suitability of devices for certain tasks or use
in certain configurations. We
are
only interested in
showing that there are some uses for which touch
tablets are not suitable, but other devices are, and
vice versa. We make no quantitative claims or com-
parisons regarding performance.
Signals
Consider a rubber-band line drawing task with a one
button mouse. The user would first position the
tracking symbol at the desired starting point of the
line by moving the mouse with the button released.
The button would then be depressed, to signal the
start of the line, and the user would manipulate the
line by moving the mouse until the desired length
and orientation was achieved. The completion of the
line could then be signaled by releasing the buttonfi
Figure 1 is a state diagram that represents this
interface. Notice that the button press and release
are used to signal the beginning and end of the
rubber-band drawing task. Also note that in states
1 and 2 both motion and signaling (by pressing or
releasing the button, as appropriate) are possible.
release
{anchor, end}
sta.rt.tag point ~ point
state
1 - button up
state
2 - button
down
Figure l. State diagram for rubber-banding with
a one-button mouse.
Now consider a simple touch tablet. It can be used
to position the tracking symbol at the starting
point of the line, but it cannot generate the signal
needed to initiate rubber-banding. Figure 2 is a
state diagram representation of the capabilities of
a simple touch tablet. In state 0, there is no contact
with the tabletfl In this state only one action is pos-
* Although
we are
comparing touch tablets
to
one
but-
ton
mice
throughout this section, most of the comments
apply equally
to tablets with
one-button pucks or (with
some caveats) tablets with
styli.
2 This assumes that the interface is
designed so
that
the button is held down
during drawing. Alternatively,
the
button can be released during drawing, and
pressed
again, to signal
the completion of the
line.
S We use state 0 to represent a state in which no loca-
tion.
information
is transmitted. There no analogous
state for mice, and hence no
state 0 in the diagrams for
sible: the user may touch the tablet. This causes a
change to state 1. In state 1, the user is pressing on
the tablet, and as a consequence position reports
are sent to the host. There is no way to signal a
change to some other state, other than to release
(assuming the exclusion of temporal or spatial cues,
which tend to be clumsy and difficult to learn). This
returns the system to state 0. This signal could not
be used to initiate rubber-banding, ~s it could also
mean that the user is pausing to think, or wishes to
initiate some other activity.
release
state
I
- contact
move
Figure 2. Diagram for showing states of
simple touch-tablet.
This inability to signal while pointing is a severe
limitation with current touch tablets, that is,
tablets that do not report pressure in addition to
location. (It is also a property of trackballs, and
joysticks without "fire" buttons). It renders them
unsuitable for use in many common interaction
techniques for which mice are well adapted (e.g.,
selecting and dragging objects into position,
rubber-band line drawing, and pop-up menu selec-
tion); techniques that are especially characteristic
of interfaces based on L~reet
Mc~r~ipulat£o~.
[Shneid-
erman
198~].
One solution to the problem is to use a separate
function button on the keyboard. However, this
usually means two-handed input where one could
do, or, awkward co-ordination in controlling the
button and pointing device with a single hand. An
alternative solution when using a touch tablet is to
provide some level of pressure sensing. For exam-
ple, if the tablet could report two levels of contact
pressure (i.e., hard and soft), then the transition
from soft to hard pressure, and vice versa, could be
used for signaling. In effect, pressing hard is
equivalent to pressing the button on the mouse. The
state diagram showing the rubber-band line draw-
ing task with this form of touch tablet is shown in
Figure 3. 4
As an aside, using this pressure sensing scheme
would permit us to select options from a menu, or
mice. With conventional tablets, this corresponds to
"out of range" state.
At this point the alert reader will wonder about difficulty
in distinguishing between hard and soft pressure,
and
friction (especially when pressing hard). Taking the last
first, hard is a relative term. in practice friction need
not
be a problem (see Inherent Problems, below).
40ne would conjecture that in the absence of button
clicks or other feedback, pressure would be difficult to
regulate accurately. We have found two levels of pres-
sure to be easily distinguished, but this is a ripe area for
research. For example, Stu Card [private communica-
tion] has suggested that the threshold between soft and
hard should be reduced (become "'softer") while hard
pressure is being maintained. This suggestion, and oth-
ers, warrant formal experimentation.
217
S I G G R A P H
'85
~igh~
release
{anchor- end)
state
0 - no contact move to select ~
to select-
state
I - light
oontact startln 9 Font ~ point
state
2 - 'hard' contact
Figure 3. State diagram for rubber-banding with
pressure sensing touch tablet.
activate light buttons by positioning the tracking
symbol over the item and "pushing". This is con-
sistent with the gesture used with a mouse, and the
model of "pushing" buttons. With current simple
touch tablets, one does just the opposite: position
over the item and then lift off, or "pull" the button.
From the perspective of the signals sent to the host
computer, this touch tablet is capable of duplicat-
ing the behaviour of a one-button mouse. This is not
to say that these devices are equivalent or inter-
changeable. They are not. They are physically and
kinesthetically very different, and should be used in
ways that make use of the unique properties of
each. Furthermore, such a touch tablet can gen-
erate one pair of signals that the one-button mouse
cannot -- specifically, press and release (transition
to and from state 0 in the above diagrams). These
signals (which are also available with many conven-
tional tablets) are very useful in implementing cer-
tain types of transactions, such as those based on
character recognition.
An obvious extension of the pressure sensing con-
cept is to allow continuous pressure sensing. That
is, pressure sensing where some large number of
different levels of pressure may be reported. This
extends the capability of the touch tablet beyond
that of a traditional one button mouse. An example
of the use of this feature is presented below.
Multiple Position
Sensing
With a traditional mouse or tablet, only one position
can be reported per device. One can imagine using
two mice or possibly two transducers on a tablet,
but this increases costs, and two is the practical
limit on the number of mice or tablets that can be
operated by a single user (without using feet). How-
ever, while we have only two hands, we have ten
fingers. As playing the piano illustrates, there are
some contexts where we might want to use several,
or even all of them, at once.
Touch tablets need not restrict us in this regard.
Given a large enough surface of the appropriate
technology, one could use all fingers of both hands
simultaneously, thus providing ten separate units
of input. Clearly, this is well beyond the demands of
many applications and the capacity
of
many people,
however, there are exceptions. Examples include
chording on buttons or switches, operating a set of
slide potentiometers, and simple key roll-over when
touch typing. One example (using a set of slide
potentiometers) will be i11ustrated below.
Multiple Virtual Devices and Templates
The power of modern graphics displays has been
enhanced by partitioning one physical display into a
number of virtual displays. To support this, display
window managers have been developed. We claim
(see Brown, Buxton and Murtagh [ 1985]) that similar
benefits can be gained by developing an input win-
dow manager that permits a single physical input
device to be partitioned into a number of virtual
input devices, Furthermore, we claim that multi-
touch tablets are well suited to supporting this
approach.
Figure 4a shows a thick cardboard sheet that has
holes cut in specific places. When it is placed over a
touch tablet as shown in Figure 4b, the user is res-
tricted to touching only certain parts of the tablet.
More importantly, the user can fee/the parts that
are touchable, and their shape. Each of the "touch-
able" regions represents a separate virtual device.
The distinction between this template and tradi-
tional tablet mounted menus (such as seen in many
CAD systems) is important.
Traditionally, the options have
been:
a) Save display real estate by mounting the menu
on the tablet surface. The cost of this option is
eye diversion from the display to the tablet, the
inability to "touch type", and time consuming
menu changes.
b) Avoid eye diversion by placing the menus on the
display. This also make it easier to change
menus, but still does not allow "touch typing",
and consumes display space.
Touch tablets allow a new option:
c) Save display space and avoid eye diversion by
using templates that can be felt, and hence, allow
"touch typing" on a variety of virtual input dev-
ices. The cost of this option is time consuming
menu (template) changes.
It must be remembered that for each of these
options, there is an application for which it is best.
We have contributed a new option, which makes pos-
sible new interfaces. The new possibilities include
more elaborate virtual devices because the
improved kinesthetic feedback allows the user to
concentrate on providing input, instead of staying
in the assigned region. We will also show (below)
that its main cost (time consuming menu changes)
can be reduced in some applicatio~ts by eliminating
the templates.
5. Examples of Transactions Where Touch Tablets
Can Be Used Effectively
In order to reinforce the distinctions discussed in
the previous section, and to demonstrate the use of
touch tablets, we will now work through some exam-
ples based on a toy paint system. We wish to stress
again that we make no claims about the quality of
the example as a paint system. A paint system is a
common and easily understood application, and
thus, we have chosen to use it simply as a vehicle
for discussing interaction techniques that
use
touch tablets.
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SAN FRANCISCO JULY 22"26 Volume 19, Number 3, 1985
Figure 4a. Sample template.
Figure 5. Main display for paint program.
Figure 4b. Sample template in use.
The example paint program allows the creation oI
simple finger paintings. The layout of the main
display for the program is shown in Figure 5. On the
left is a large drawing area where the user can draw
simple tree-hand figures. On the right is a set of
menu items. When the lowest item is selected, the
user enters a eolaur mixing mode. In switching to
this mode, the user is presented with a different
display that is discussed below. The remaining
menu items are "paint pots". They are used to
select the colour that the user will be painting with.
In each of the following versions of the program, the
input requirements are slightly different. In all
cases an 8 cmx 8 cm touch tablet is used (Figure 6),
but the pressure sensing requirements vary. These
are noted in each demonstration.
5.1. Painting Without Pressure Sensing
This version of the paint program illustrates the
limitation of having no pressure sensing. Consider
Figure 6. Touch tablet used in demonstrations.
the paint program described above, where the only
input device is a touch tablet without pressure
sensing. Menu selections could be made by pressing
down somewhere in the menu area, moving the
tracking symbo] to the desired menu item and then
selecting by releasing. To paint, the user would
simply press down in the drawing area and move
(see Figure 7 for a reprqsentation of the signals
used for painting with this program).
release
painti~
Figure 7. State diagram for drawing portion
of simple paint program.
219