SAN FRANCISCO JULY 22-26
`Volume 19, Number 3, 1985
`
`Issues and Techniques in
`Touch-Sensitive Tablet Input
`William Buxten
`Ralph Hill
`Peter Rowley
`
`Computer Systems Research Institute
`University of Toronto
`Toronto, Gntario
`Canada M55 144
`
`(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 distine-
`tions are reinforced, and possible uses of touch
`tablets are illustrated, in an example application.
`Potential enhancements to teuch 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: [.3.1 [Cor
`puter Graphies]: 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.
`
`Permission to copy without fee all or part of this material is granted
`provided that the copies are not made or distributed for direct
`commercial advantage, the ACM copyright notice and the title of the
`publication and its date appear, and notice is given that copying is by
`permission of the Association for Computing Machinery. To copy
`otherwise, or to republish, requires a fee and/or specific permission.
`
`@ 1985 ACM 0-89791 -166-0/85/007/0215 $00.75
`
`1. Introduction
`
`Increasingly, research im human-computer interac-
`tion is focusing on problemsof input [Foley, Wallace
`& Chan 1984; Buxton 1983; Buxton 1985], Much of
`this attention is directed towards input technole-
`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 fora
`particular application? And once a technology is
`chosen, how can it be used most effectively? These
`questions are important, lor as Buxton [1983] has
`argued, the ways in which the user physicaliy
`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 af dev-
`ices: touch-sensitive tablets. We will identify touch
`tablets, enumerate their important properties, and
`compare them to amore common input device, the
`Inouse. We then go on to give examples of transac-
`tions where touch tabiets 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 af 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 onit. That is, itis a tablet that can sense
`that it is being touched, and where it is being
`
`215
`Valve Exhibit 1058
`Valve Exhibit 1058
`Valve v. Immersion
`
`Valve v. Immersion
`
`

`

`
`
`™ SIGGRAPH ‘85
`
`touched. Touch tablets can vary greatlyin 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 stmple touch tablet. Gnly 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. Hawever, Lee, Buxtan 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 [1988] 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 makeit
`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 toa
`use itfor. 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, imvestigation.
`
`Touch tablets have a numberof properties that dis-
`tinguish them from other devices:
`
`* They have no mechanical intermediate device
`(such as stylus or puck). Hence theyare 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 gel bumped, the track-
`ing symbol “stays put” once placed, thus making
`them well suited for pointing tasks in environ-
`ments subject to vibratian or mation (e.g., fac-
`tories, cockpits).
`
`* They present no mechanical or kinesthetic res-
`trictions on our ability to indicate more than one
`point at atime. That is, we can use two hands or
`more than one finger simultaneously on a single
`tablet. (Remember, we can manually control at
`
`216
`
`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 avery
`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 {1980], reduces
`homing time from the keyboard to the pointing
`device.
`
`e 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 maving 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 inthe 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 Lo 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 inleraction without con-
`suming display real estate for visual feedback.
`With conventional tablets and touch tablets,
`graphical, physical or virtual Lemplates 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 Lhem
`in the context of a concrete example. We wish to
`stress Lhal we da not pretend that the program
`represents a Viable paint programm or an optimal
`interface. It is siroply a vehicle to illustrate a
`variety of transactions in an easily understandable
`context.
`
`

`

`SAN FRANCISCO JULY 22-26
`Volume 19, Number 3, 1985iEE)
`
`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!
`
`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 someuses for which touch
`tablets are not suitable, but other devices are, and
`vice versa. We make no quantitative claims vr com-
`parisons regarding performance.
`
`Signaling
`
`Consider a rubber-band line drawing task with a one
`button mouse. The user would first position the
`tracking symbolat 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.”
`
`Figure i 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}
`
`press button
`(start rubber-
`banding)
`
`move to select
`starting point
`state ] - button up
`state 2 - button down
`
`move to select
`end point
`
`Figure 1. 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 Z2isa
`state diagram representation of the capabilities of
`asimple touch tablet. In state 0, there is no contact
`with the tablet.5 In this state only one action is pos-
`
`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 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 diagramsfor
`
`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, as it could also
`mean that the user is pausing to think, or wishes ta
`initiate some other activity.
`release
`
` touch
`
`state 0 - no contact
`state 1 - 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.zg.,
`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 Direct Manipulation [Shneid-
`erman 1983].
`
`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 somelevel 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-bandline 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
`
`tablets,
`
`this corresponds to
`
`mice. With conventional
`“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).
`4One would conjecture that
`in the absence of button
`clicks or other feedback, pressure would be dificult to
`regulate accurately. We have found twe levels of pres-
`sure to be easily distinguished, but this is a ripe area for
`research. For example, Stu Card [private ecommunica-
`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
`
`

`

`© SIGGRAPH ‘85
`
`
`release
`
`{anchor end)
`
` state 0 ~ no contact
`
`state } - light contact
`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 numberof
`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 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 numberof 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 useris res-
`tricted 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 ‘'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 makeit easier to change
`menus, but still does not allow “touch typing”,
`and consumes display space,
`
`Touch tablets allow a new option:
`
`Multiple Position Sensing
`
`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
`With a traditional mouse or tablet, only one position
`options, there is an application for which it is best.
`can be reported per device. One can imagine using
`We have contributed a new option, which makes pos-
`two mice or possibly two transducers onatablet,
`sible new interfaces. The new possibilities include
`more elaborate virtual devices because the
`but this increases costs, and two is the practical
`limit on the numberof mice or tablets that can be
`improved kinesthetic feedback allows the user to
`operated by a single user (without using feet). How-
`concentrate on providing input, instead of staying
`ever, while we have only two hands, we have ten
`in the assigned region. We will also show (below)
`fingers. As playing the piano illustrates, there are
`that its main cost (time consuming menu changes)
`some contexts where we might want to use several,
`can be reduced in some applications by eliminating
`or even all of them,at once.
`the templates.
`
`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.
`
`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 systemisa
`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,
`
`218
`
`

`

`SAN FRANCISCO JULY 22-26
`Volume 19, Number 3, 1985
`
`
`Figure 4a. Sample template.
`
`
`
`Figure 4b. Sample template in use.
`
`Figure 6. Touch tablet used in demonstrations.
`
`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 8cm x 8cm touch tablet is used (Figure 6),
`but the pressure sensing requirements vary. These
`are noted in each demonstration.
`
`5.2. 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 bypressing
`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).
`release
`
`(stop painting
`
`
`press
`(start painting)
`
`rove while
`painting
`
`Figure 7. State diagram for drawing portion
`of simple paint program.
`
`219
`
`

`

`—& SIGGRAPH ‘85
`
`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
`untilit appears. This is likely to be too late. Some
`form of feedback, that shows the user where the
`brush is, without painting, is needed. Unfor-
`tunately, this cannot be done with this input device,
`as it is not possible to signal the change from track-
`ing to painting and vice versa.
`
`The simpiest solution to this problem is to use a
`button (e.g., a function key on the keyboard) to sig-
`nal state changes. The problem withthis solution is
`the need to use two hands on two different devices
`to do one task. This is awkward and requires prac-
`tice to develop the co-ordination 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 pressurelevel (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 paint-
`ing with this program). The two levels of contact
`pressure allow us to make a simple but practical
`one finger paint program.
`
` Trove (to
`
`Starting point)
`
`move while
`painting
`
`Figure 8. State diagram for painting portion of
`simple paint program using pressure
`sensing touch tablet.
`
`This version is very muchlike using the one button
`mouse on the Apple Macintosh with MacPaint[Willi-
`ams, 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.
`
`5.3. Painting with Contmuous Pressure Sensing
`
`In the previous demonstrations, we have only imple-
`mented interaction techniques that are common
`using existing technology. We now introduce a tech-
`nique that provides functionality beyond that
`obtainable using most conventional input technolo-
`
`5 Also, there is the problem of friction, to be discussed
`below under ‘‘Inherent Problems"’.
`
`220
`
`gies.
`
`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 contro}
`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
`
`TABS (the GTCO Digipad with pressure pen [GTCO
`
`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 doo-
`dling 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 “win-
`dows” onto virtual input devices. We use the same
`basic techniques as discussed under templates
`(above), but show how to use them without tem-
`plates. We do this in the context of a colour selec-
`tion module for our paint program. This module
`
`introduces a new display, shownin Figure 9.
`
`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 pri-
`mary function of the bar graphsis to provide feed-
`back, 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
`
`

`

`SAN FRANCISCO JULY 22-26
`Volume 19, Number 3, 1985
`
`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.
`3 _valuators
`
`tablet surface
`
`SS 8cem x 3 cm
`
`2 push buttons
`
`Figure 10. Layout of virtual devices on touch tablet.
`
`The single most important point to be madein this
`example is that a single physical device is being
`used to implement 5 virtual devices (S valuators
`and 2 buttons). This is analogous to the use of a
`display window system, in its goals, and its imple-
`mentation.
`
`The second main pointis that there is nothing on
`the tablet to delimit the regions. This differs from
`the use of physical templates as previously dis-
`cussed, and showshow, in the absenceof 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 com-
`plex, 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 impor-
`tantly, 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 {0
`em square tablet that is divided into 3 sliders witha
`single button across the top).
`Because no template is needed, there is no need for
`the user to pause to change a template when enter-
`ing 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
`dragging 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 abso-
`lute position), and are manipulated like one dimen-
`sional 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 (muchlike strok-
`ing a mouse). Feedback is provided by changing the
`level in the bar graph on the screen and the colour
`
`of the patch.
`
`¥¢
`
`>thea
`yas= N
`
`“
`
`Figure i1. 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 acuity
`and concentration that distracts attention from the
`primary task ~ monitoring the colour of the patch.
`Furthermore, note that the touch tablet implemen-
`tation 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 mani-
`pulated 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 nof be supported bya
`mouse, nor any single commercially available input
`device we know of. Controlling several valuators
`with one hand is common in many operating con-
`soles, for example: studio light control, audio
`mixers, and throttles for multi-engine vehicles (e.g.,
`aircraft and boats). Hence, this example demon-
`strates a cost effective method for providing func-
`tionality that is currently unavailable (or available
`only at great cost, in the form of a custom fabri-
`cated console), but has wide applicability.
`
`9.5. Summary of Examples
`
`Through these simple examples, we have demon-
`strated several things:
`
`* The ability to sense at least two levels of pres-
`sure is a virtual necessity for touch tablets, as
`without il, auxiliary devices must be used for
`signaling, and ‘‘direct manipulation” interfaces
`cannot be effectively supporied.
`
`e The extension to continuous pressure sensing
`opens up new possibilities in human-computer
`interaction.
`
`221
`
`

`

`& SIGGRAPH ’85
`
`
`e 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 consumevaluable 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 multi-
`ple points of contact does not exist for, nor is it
`applicable to, current commercially available
`mice and tablets.
`
`6. Inherent Problems with Touch Tablets
`
`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 calibra-
`tion of the tablet.® Also, the user interface can be
`designed to avoid extended periods of high pres-
`sure.
`
`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 feed-
`back (illuminated buttons or different button
`heights) cannot be us