Buxton, W., Hill, R. & Rowley, P. (1985). Issues and techniques
in touch-sensitive tablet input, Computer Graphics, 19(3), Proceedings
of SIGGRAPH'85, 215-223.
Issues and Techniques in
Touch-Sensitive Tablet Input
William Buxton
Ralph Hill
Peter Rowley
Computer Systema Research Institute
University of Toronto
Toronto, Ontario
Canada MSS 1A4
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 distinctions 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 recommendations for future work.
CR Categories and Subject Descriptors: I.3.1 [Computer Graphics]:
Hardware Architecture: Input Devices. 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.
1. Introduction
Increasingly, research in human-computer interaction is focusing on problems
of input [Foley, Wallace & Chan 1984; Buxton 1983; Buxton 1885]. Much
of this attention is directed towards input technologies. The ubiquitous
Sholes keyboard is being replaced and/or complemented by alternative technologies.
For example, a major focus of the marketing 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 expanding, how does one select
the best technology for a particular application? And once a technology
is chosen, how can it be used most effectively? These questions are important,
for as Buxton [1983] has argued, the ways in which the user physically
interacts with an input device have a marked effect on the type of user
interface that can be effectively 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 of devices. Our
approach is to investigate a specific class of devices: 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 transactions 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 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. Only 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 contact. In this case, the location (and possibly pressure)
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 communication] have developed prototypes
of multi-touch, multi-pressure sensing tablets.
We wish to stress that we will restrict our discussion 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 & Weinsapfel [1978], Nakatani & Rohrlich [1983] and Minsky
[1984]. We acknowledge that a flat 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.
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 limited
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 distinguish 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 devices can get lost, stolen,
or damaged
- Having no puck to slide or get bumped, the tracking symbol "stays
put" once placed, thus making them well suited for pointing tasks in
environments subject to vibration or motion (e.g., factories, cockpits).
- They present no mechanical or kinesthetic restrictions 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 simultaneously. 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 keyboards
(e.g., the Key Tronic Touch Pad, see Appendix A). This has potential benefits
in portable systems, and, according to the Keystroke model of Card, Newell
and Moran [1980], 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 disadvantages, 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 comparing to
simple touch tablets.
- Touch tablets can be made that can sense multiple points of contact.
There is no analogous property for mice.
- The surface of a tablet can be partitioned into regions representing
a collection of independent "virtual" devices. This is analogous
to the partitioning of a screen into "windows" or virtual displays.
Mice, and other devices that transmit "relative change" information,
do not lend themselves to this mode of interaction without consuming 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.
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[1]
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 comparisons regarding performance.
Signaling
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 button.[2]
Figure l 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 l and 2 both motion
and signaling (by pressing or releasing the button, as appropriate) are
possible.
![](touch1.gif)
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 tablet.[3] In this state only one action
is possible: the user may touch the tablet. This causes a change to state
l. In state l, 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,
as it could also mean that the user is pausing to think, or wishes to initiate
some other activity.
![](touch2.gif)
Figure 2. Diagram for showing states ofsimple 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
"flre" 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 selection); techniques that are especially characteristic of interfaces
based on Direct Manipulation [Shneiderman l983].
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 example, 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 drawing task with this form
of touch tablet is shown in Figure 3.[4]
![](touch3.gif)
Figure 3. State diagram for rubber-banding with pressure sensing
touch tablet.
As an aside, using this pressure sensing scheme would permit us to select
options from a menu, or activate light buttons by positioning the tracking
symbol over the Item and "pushing". This is consistent 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 duplicating the behaviour of a one-button mouse. This
is not to say that these devices are equivalent or interchangeable. 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 generate one pair of signals that the one-button
mouse cannot specifically, press and release (transition to and from state
O in the above diagrams). These signals (which are also available with many
conventional tablets) are very useful in implementing certain types of transactions,
such as those based on character recognition.
An obvious extension of the pressure sensing concept 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). However, 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 illustrated 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 [1990]) that similar benefits can be gained by developing an
input window manager that permits a single physical input device to be partitioned
into a number of virtual input devices. Furthermore, we claim that multitouch
tablets are well suited to supporting this approach.
![](touch4a.gif)
Figure 4a. Sample template
![](touch4b.gif)
Figure 4b. Sample template in use.
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 restricted to touching only certain parts of the tablet. More importantly,
the user can feel the parts that are touchable, and their shape.
Each of the "touchable" regions represents a separate virtual
device. The distinction between this template and traditional tablet mounted
menus (such as seen in many CAD systems) is important.
Traditionally, the options have been:
- 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.
- 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:
- 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 devices. 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 possible
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 applications 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
examples 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.
The example paint program allows the creation of 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 free-hand
figures. On the right is a set of menu items. When the lowest item is selected,
the user enters a colour 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 cm x 8 cm touch tablet is used
(Figure 6), 1but the pressure sensing requirements vary. These are noted
in each demonstration.
![](touch5.gif)
Figure 5. Main display for paint program.
![](touch6.gif)
Figure 6. Touch tablet used in demonstrations.
5.1. Painting Without Pressure Sensing
This version of the paint program illustrates the limitation of having no
pressure sensing. Consider 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
symbol 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 representation of the signals used for painting with this program).
There are several problems with this program. The most obvious is in trying
to do detailed drawings. The user does not know where the paint will appear
until it appears. This is likely to be too late. Some form of feedback,
that shows the user where the brush is, without painting, is needed. Unfortunately,
this cannot be done with this input device, as it is not possible to signal
the change from tracking to painting and vice versa.
![](touch7.gif)
Figure 7. State diagram for drawing portion of simple paint program.
The simplest solution to this problem is to use a button (e.g., a function
key on the keyboard) to signal state changes. The problem with this solution
is the need to use two hands on two different devices to do one task. This
is awkward and requires practice to develop the coordination needed to make
small rapid strokes in the painting. It is also inefficient in its use of
two hands where one could (and normally should) do.
Alternatively, approaches using multiple taps or timing cues for signalling
could be tried; however, we have found that these invariably lead to other
problems. It is better to find a direct solution using the properties of
the device itself.
5.2. Painting with Two levels of Pressure
This version of the program uses a tablet that reports two levels of contact
pressure to provide a satisfactory solution to the signaling problem. A
low pressure level (a light touch by the user) is used for general tracking.
A heavier touch is used to make menu selections, or to enable painting (see
Figure 8 for the tablet states used to control painting with this program).
The two levels of contact pressure allow us to make a simple but practical
one finger paint program.
![](touch8.gif)
Figure 8. State diagram for painting portion of simple paint program
using pressure sensing touch tablet.
This version is very much like using the one button mouse on the Apple
Macintosh with MacPaint [Williams, 1984]. Thus, a simple touch tablet is
not very useful, but one that reports two levels of pressure is similar
in power (but not feel or applicability) to a one button mouse.[5]
5.3. Painting with Continuous Pressure Sensing
In the previous demonstrations, we have only implemented interaction techniques
that are common using existing technology. We now introduce a technique
that provides functionality beyond that obtainable using most conventional
input technologies.
In this technique, we utilize a tablet'capable of sensing a continuous range
of touch pressure. With this additional signal, the user can control both
the width of the paint trail and its path, using only one finger. The new
signal, pressure, is used to control width. This is a technique that cannot
be used with any mouse that we are aware of, and to our knowledge, is available
on only one conventional tablet (the GTCO Digipad with pressure pen [GTCC
1982]).
We have found that using current pressure sensing tablets, the user can
accurately supply two to three bits of pressure information, after about
15 minutes practice. This is sufficient for simple doodling and many other
applications, but improved pressure resolution is required for high quality
painting.
5.4. "Windows" on the Tablet: Colour Selection
We now demonstrate how the surface of the touch tablet can be dynamically
partitioned into "windows" onto virtual input devices. We use
the same basic techniques as discussed under templates (above), but show
how to use them without templates. We do this in the context of a colour
selection module for our paint program. This module introduces a new display,
shown in Figure 9.
![](touch9.gif)
Figure 9. Colour mixing display.
In this display, the large left side consists of a colour patch surrounded
by a neutral grey border. This is the patch of colour the user is working
on. The right side of the display contains three bar graphs with two light
buttons underneath. The primary function of the bar graphs is to provide
feedback, representing relative proportions of red, green and blue in the
colour patch. Along with the light buttons below, they also serve to remind
the user of the current layout of the touch tablet.
In this module, the touch tablet is used as a "virtual operating console".
Its layout is shown (to scale) in Figure 10. There are 3 valuators (corresponding
to the bar graphs on the screen) used to control colour, and two buttons:
one, on the right, to bring up a pop-up menu used to select the colour to
be modified, and another, on the left, to exit.
![](touch10.gif)
Figure 10. Layout of virtual devices on 8 cm x 8 cm touch tablet.
The single most important point to be made in this example is that a
single physical device is being used to implement 5 virtual
devices (3 valuators and 2 buttons). This is analogous to the use of a display
window system, in its goals, and its implementation.
The second main point is that there is nothing on the tablet to delimit
the regions. This differs from the use of physical templates as previously
discussed, and shows how, in the absence of the need for a physical template,
we can instantly change the "windows" on the tablet, without sacrificing
the ability to touch type.
We have found that when the tablet surface is small, and the partioning
of the surfaces is not too complex, the users very quickly (typically in
one or two minutes) learn the positions of the virtual devices relative
to the edges of the tablet. More importantly, they can use the virtual devices,
practically error free, without diverting attention from the display. (We
have repeatedly observed this behaviour in the use of an application that
uses a 10 cm square tablet that is divided into 3 sliders with a single
button across the top).
Because no template is needed, there is no need for the user to pause to
change a template when entering the colour mixing module. Also, at no point
is the user's attention diverted from the display. These advantages cannot
be achieved with any other device we know of, without consuming display
real estate.
The colour of the colour patch is manipulated by draggng the red,
green and blue values up and down with the valuators on the touch tablet.
The valuators are implemented in relative mode (i.e., they are sensitive
to changes in position, not absolute position), and are manipulated like
one dimensional mice. For example, to make the patch more red, the user
presses near the left side of the tablet, about half way to the top, and
slides the finger up (see Figure 11). For larger changes, the device can
be repeatedly stroked (much like stroking a mouse). Feedback is provided
by changing the level in the bar graph on the screen and the colour
![](touch11.gif)
Figure 11. Increasing red content, by pressing on red valuator and
sliding up.
Using a mouse, the above interaction could be approximated by placing
the tracking symbol over the bars of colour, and dragging them up or down.
However, if the bars are narrow, this takes visual acuity and concentration
that distracts attention from the primary task - monitoring the colour of
the patch. Furthermore, note that the touch tablet implementation does not
need the bars to be displayed at all. They are only a convenience to the
user. There are interfaces where, in the interests of maximizing available
display area, there will be no items on the display analogous to these bars.
That is, there would be nothing on the display to support an interaction
technique that allows values to be manipulated by a mouse.
Finally, we can take the example one step further by introducing the use
of a touch tablet that can sense multiple points of contact (e.g., [Lee,
et al. 1985)). With this technology, all three colour values could be changed
at the same time (for example, fading to black by drawing all three sliders
down together with three fingers of one hand). This simultaneous adjustment
of colours could not be supported by a mouse, nor any single commercially
available input device we know of. Controlling several valuators with one
hand is common in many operating consoles, for example: studio light control,
audio mixers, and throttles for multi-engine vehicles (e.g., aircraft and
boats). Hence, this example demonstrates a cost effective method for providing
functionality that is currently unavailable (or available only at great
cost, in the form of a custom fabricated console), but has wide applicability.
5.5. Summary of Examples
Through these simple examples, we have demonstrated several things:
- The ability to sense at least two levels of pressure is a virtual necessity
for touch tablets, as without it, auxiliary devices must be used for signaling,
and "direct manipulation" interfaces cannot be effectively supported.
- The extension to continuous pressure sensing opens up new possibilities
in human-computer interaction.
- Touch tablets are superior to mice and tablets when many simple devices
are to be simulated. This is because: (a) there is no need for a mechanical
intermediary between the fingers and the tablet surface, (b) they allow
the use of templates (including the edges of the tablet, which is a trivial
but useful template), and (c) there is no need for positional feedback that
would consume valuable display space.
- The ability to sense multiple points of contact radically changes the
way in which users may interact with the system. The concept of multiple
points of contact does not exist for, nor is it applicable to, current commercially
available mice and tablets.
6. Inherent Problem with Touch Tablet:
A problem with touch tablets that is annoying in the long term is friction
between the user's finger and the tablet surface. This can be a particularly
severe problem if a pressure sensitive tablet is used, and the user must
make long motions at high pressure. This problem can be alleviated by careful
selection of materials and care in the fabrication and calibration of the
tablet.6 Also, the user interface can be designed to avoid extended periods
of high pressure.
Perhaps the most difficult problem is providing good feedback to the user
when using touch tablets. For example, if a set of push-on/push-off buttons
are being simulated, the traditional forms of feedback (illuminated buttons
or different button heights) cannot be used. Also, buttons and other controls
implemented on touch tablets lack the kinesthetic feel associated with real
switches and knobs. As a result, users must be more attentive to visual
and audio feedback, and interface designers must be freer in providing this
feedback. (As an example of how this might be encouraged, the input "window
manager" could automatically provide audible clicks as feedback for
button presses).
7. Potential Enhancements to Touch Tablets (and other devices)
The first problem that one notices when using touch tablets is "jitter"
when the finger is removed from the tablet. That is, the last few locations
reported by the tablet, before it senses loss of contact, tend to be very
unreliable.
This problem can be eliminated by modifying the firmware of the touch tablet
controller so that it keeps a short FIFO queue of the samples that have
most recently be sent to the host. When the user releases pressure, the
oldest sample is retransmitted, and the queue is emptied. The length of
the queue depends on the properties of the touch tablet (e.g., sensitivity,
sampling rate). We have found that determining a suitable value requires
only a few minutes of experimentation.
A related problem with most current tablet controllers (not just touch tablets)
is that they do not inform the host computer when the user has ceased pressing
on the tablet (or moved the puck out of range). This information is essential
to the development of certain types of interfaces. (As already mentioned,
this signal is not available from mice). Currently, one is reduced to deducing
this event by timing the interval between samples sent by the tablet. Since
the tablet controller can easily determine when pressure is removed (and
must if it is to apply a de-jittering algorithm as above), it should share
this information with the host.
Clearly, pressure sensing is an area open to development. Two pressure sensitive
tablets have been developed at the University of Toronto [Sasaki, et al.
1981; Lee, et al. 1905]. One has been used to develop several experimental
interfaces and was found to be a very powerful tool. They have recently
become available from Elographics and Big Briar. Pressure sensing is not
only for touch tablets. Mice, tablet pucks and styli could all benefit by
augmenting switches with strain gauges, or other pressure sensing instruments.
GTCO, for example, manufactures a stylus with a pressure sensing tip [GTCO
1982], and this, like our pressure sensing touch tablets, has proven very
useful.
8. Conclusions
We have shown that there are environments for which some devices are better
adapted than others. In particular, touch tablets have advantages in many
hostile environments. For this reason, we suggest that there are environments
and applications where touch tablets may be the most appropriate input technology.
This being the case, we have enumerated three major distinctions between
touch tablets and one button mice (although similar distinctions exist for
multi-button mice and conventional tablets). These assist in identifying
environments and applications where touch tablets would be most appropriate.
These distinctions concern:
- limitation in the ability to signal events,
- suitability for multiple point sensing, and
- the applicability of tactile templates.
These distinctions have been reinforced, and some suggestions on how touch
tablets may be used have been given, by discussing a simple user interface.
From this example, and the discussion of the distinctions, we have identified
some enhancements that can be made to touch tablets and other input devices.
The most important of these are pressure sensing and the ability to sense
multiple points of contact.
We hope that this paper motivates interface designers to consider the use
of touch tablets and shows some ways to use them effectively. Also, we hope
it encourages designers and manufacturers of input devices to develop and
market input devices with the enhancements that we have discussed.
The challenge for the future is to develop touch tablets that sense continuous
pressure at multiple points of contact and incorporate them in practical
interfaces. We believe that we have shown that this is worthwhile and have
shown some practical ways to use touch tablets. However, interface designers
must still do a great deal of work to determine where a mouse is better
than a touch tablet and vice versa.
Finally, we have illustrated, by example, an approach to the study of input
devices, summarized by the credo: "Know the interactions a device is
intended to participate in, and the strengths and weaknesses of the device."
This approach stresses that there is no such thing as a "good input
device," only good interaction task/device combinations.
9. Acknowledgements:
The support of this research by the Natural Sciences and Engineering Research
Council of Canada is gratefully acknowledged. We are indebted to Kevin Murtagh
and Ed Brown for their work on virtual input devices and windowing on input.
Also, we are indebted to Elographics Corporation for having supplied us
with the hardware on which some of the underlying studies are based.
We would like to thank the referees who provided many useful comments that
have helped us with the presentation.
10. References
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Notes:
[1 ]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.
[3] We use state O to represent a state in which
no location, information is transmitted. There no analogous state for mice,
and hence no state O In the diagrams for mice. With conventional tablets,
this corresponds to "out of range" state.
At this point the alert reader will wonder about diimculty in distinguishing
between hard and soft pressure. and friction (especially when pressing hard).
Taking tha last first, hard is a relative term. In practice friction need
not be a problem (ice Inherent Problems, below).
[4] 0ne 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 pressure to be easily distinguished,
but this is a ripe area for research. For example, Stu Card [private communication]
has suggested that the threshold between soft and hard should be reduced
(become "softer") while hard pressure is being maintained. This
suggestion, and others, warrant formal experimentation.
[5] Also, there is the problem of frictIon, to
be discussed below under "Inherent Problems".
6 As a bad axanople, one commercial "touch" tablet requires so
much pressure for relIable sensIng that the nnger cannot be smoothly dragged
across the surface. Instead, a wooden or plastic stylus must be used, thus
loosing many of the advantages of touch sensing.