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Using small screen space more efficiently http://www.acm.org/sigchi/chi96/proceedings/papers/Kamba/tk_txt.htm
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Using small screen space more efficiently
Tomonari Kamba
Shawn A. Elson
Terry Harpold
Tim Stamper
Piyawadee "Noi" Sukaviriya
ABSTRACT:
This paper describes techniques for maximizing the efficient use of small screen space by combining
delayed response with semi-transparency of control objects ("widgets") and on-screen text. Most
research on the limitations of small display screens has focused on methods for optimizing
concurrent display of text and widgets at the same level of transparency (that is, both are equally
opaque). Prior research which proposes that widgets may be made semi-transparent is promising,
but it does not, we feel, adequately address problems associated with user interaction with text that
is partially obscured by the widgets. In this paper, we will propose that a variable delay in the
response of overlapping widgets and text improves the effectiveness of the semi-transparent
widget/text model. Our conclusions are based on usability studies of a prototype of an online
newspaper that combined transparency and delayed-response techniques.
Keywords:
PDAs, icons, transparency, usability study
INTRODUCTION AND PROBLEM STATEMENT
The dramatic growth in recent years of the personal digital assistant (PDA) market demonstrates that
users are willing to put up with small, hard-to-read, displays, limited storage and battery life, slow CPU
speeds and cumbersome data transfer, in the hope of achieving truly portable access to electronic data. It
is probably safe to predict that devices available only a few years from now will be dramatically
improved. Future generations of PDAs will have higher-contrast, easier-to-read displays, they will have
greater storage capacities, they will be much faster and run for longer periods of time between charges,
and they will be more flexible in how they communicate with each other and with other computing
devices. These probable changes in PDAs are, however, limited by two constraining factors: the
dimensions of displayed text and of the screens on which the text is displayed are unlikely to change very
much. Reductions in the size of displayed text will be limited by the ability of users to discern small type
sizes on any display device, especially one of relatively low resolution. Our informal observations of
PDAs based on the Apple Newton and General Magic's Magic Cap operating systems suggests that a
practical threshold of legibility for most users lies somewhere between 9 and 12 points (72 points = 2.5
cm.) The small screen size of PDAs is not a technical limitation, but a key factor in their usefulness. Users
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clearly want devices that can be easily carried about and held in one hand. It is not difficult to imagine
even smaller PDAs, worn on the wrist or carried on a keychain.
The small physical size of a PDA limits the maximum size of its screen, which can be no larger than the
dimensions of the machine in which it is embedded. On the other hand, the need for displayed text to be
legible defines another, more subtle boundary: if the size of text cannot be reduced below a threshold of
legibility, then, as the screen shrinks in size, and less information may be shown on it, and the user will be
required to increase the level of interaction with the device in order to get to desired information.
The design of user interfaces for PDAs must balance two opposing forces: the need to shrink the screen
to a size that fits inside a very small box (we'll call this the "physical" limitation), and the need to keep the
screen sufficiently large to show enough information that the device is actually useful (we'll call this the
"functional" limitation.) This balance becomes particularly difficult in the case of navigational or functional
controls, the widgets that must somehow be made available to the user to allow interaction with the
information on-screen (switching between tasks, selecting information to be changed in some way, adding
new information, etc.). Most of the interface objects used in desktop computing environments -
pull-down or popup menus, multiple windows, icons - consume a great deal of valuable screen space on
the PDA screen. This accounts, we believe, for the efforts of the designers of the current crop of PDAs to
minimize the size of these interface objects, or, in some cases, to eschew them altogether. It explains also
the emphasis that many of these systems place on handwriting recognition technologies - the keyboard is
perhaps the most cumbersome of control widgets. (The document-oriented, pen- and gesture-driven
elements of the Newton interface are a good example of these approaches.)
Once again, however, the interface designer faces a version of the functional limitation: the user will expect
not only to passively read content displayed on the device, but also to do something with it (to select it,
modify it, or enter new content, etc.), and this means that he or she will need to interact with objects
(widgets) that are different from the text, but equally available. These widgets must be large enough to be
readily distinguished from the content and from each other, and to be practical targets for some kind of
interaction (by means of a finger or pen, for example.) But if they are displayed all or most of the time,
they will consume screen space that could otherwise be used to display content. Because the number of
these widgets will depend on the functions supported by the active application, and not on the size of the
screen, the portion of screen that must be surrendered to them will increase as the screen grows smaller,
as illustrated in Figure 1.
FIGURE 1. Surrendering screen space to widgets
The price paid for showing control widgets can be substantial in the Magic Cap interface, for example,
nearly 25% of the screen is used to display icons along the bottom and right edge of the screen.
Our research focused on techniques for reducing the screen space that must be surrendered to widgets,
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thereby maximizing the available space for display of content. In the next section, we will review general
solutions to this problem suggested by the work of other researchers and designers working with
interfaces of both handheld devices and desktop computers. Among these solutions, we will single out
one approach that seems especially promising to us: the use of semi-transparent widgets, laid over text, so
that both are present on the screen at the same time, but the text is able to fill nearly the entire screen. We
will, however, point out a serious limitation with this solution, related to user interaction with the text. In
the remaining sections of the paper, we will propose a variation of the semi-transparent widget/text model
that improves its responsiveness to user interaction, and describe the results of a series of usability studies
of a prototype that applies this improved model.
RELATED WORK
Showing Information Structure More Efficiently
One approach to the problem of maximizing the display of content is to improve the efficiency by which
its underlying structure is represented on-screen. Several tools in Information Visualizer [4] take this
approach. Cone Tree [13] and Hyperbolic Geometry [8] display in lucid forms complicated data
hierarchies that might be otherwise invisible to the user, and Perspective Wall [10] does much the same
for complex linear data structures. Cone Tree represents data hierarchies in 3D cone-shaped graphics.
Hyperbolic Geometry displays a focused point within a data hierarchy in a large bounded space, and its
context in a smaller bounded space. Perspective Wall displays relations between different nodes within
the same document on two adjacent planes ("walls"), with semantic or structural differences between the
nodes represented by the relative positions of the nodes on these walls. These methods for representing
underlying data structure are arguably more precise and efficient than, for example, the very concrete
desktop paradigm of Magic Cap (in which, for example, the Datebook application is accessed by tapping
on a miniature appointment book), but they do not address the functional limitation of simultaneous
display of widgets and content on a small screen.
Showing Information Content More Efficiently
A second approach to the problem directly addresses the presentation of the displayed content. Magic
Lens [3] is a filtering tool that can change or modify the presentation strategy of objects on the screen
over which it is laid. For example, a portion of a geographical map can change into a weather map or a
population map when a special lens is applied to it. Starfield [6] displays a two-dimensional scatterplot of
a multidimensional database, applying a dynamic filtering mechanism which continuously controls the
density of information shown on the screen, and panning/zooming techniques to focus on the portion of
the content displayed. Table Lens [11] represents large databases in table format. The user can zoom in
on an arbitrary part of a table, and view it alongside a global view of the table.
Pad++ applies another kind of zooming/panning technique to displayed content [2]. The user can focus
on and zoom into any part of the content, and smooth animation during the zoom insures that the semantic
link between the zoomed information and its context are maintained. Galaxy of News [12] uses similar
zooming techniques combined with dynamic restructuring of data hierarchies during the zooming/panning
process.
We feel that these methods suggest several improvements on the simple, scrolling displays of long lists or
blocks of text that are commonly encountered when using PDAs. However, they suffer from a critical
limitation: filtering information content so that it may be displayed in smaller chunks that require less screen
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space does not obviate the need for widgets required to apply the filters, or to enable manipulation of the
filtered content.
Showing Widgets More Efficiently
We were surprised to find little research that directly addresses the efficient display of the widgets
themselves. We suspect that this may be because the physical/functional opposition outlined above is both
obvious and intractable - designers know that widgets can only be so small before they cease to be
useful, so they have concentrated their efforts on increasing the intelligence or functionality of the widgets,
rather than how much screen space must be sacrificed because of them.
Gestural interfaces, like that used in Apple's Newton operating system, mark an important departure from
the paradigm of on-screen control objects. Tying functionality to pen-based gestures frees up valuable
screen space, because the user can directly interact with displayed content by simply drawing over it with
the pen. This technique works well in many applications, but it also has limitations. First, the user must
learn the gestures, and be able to reproduce them accurately. (The list of gestures on the inside of the
screen covers of the Newton MessagePad 110 and 120 suggests that users are unlikely to be able to
recall all the possible gestures without a mnemonic aid.) Second, not all functions supported by current
PDAs can be mapped to intuitive gestures. There is a row of unchanging iconic buttons below the active
screen space of the MessagePads. These buttons are used to switch between applications, and to undo
previous tasks. By moving these buttons out of the active screen space, Apple has reclaimed area for
displaying information, but in so doing has also underscored the difficulty of the problem illustrated in
Figure 1.
Kurtenbach and Buxton's Marking Menus [7] offer an interesting variation on the gestural interface. In
their system, when the user holds a pen to the screen for a predefined period of time, a pie-shaped menu
is displayed below the pen, and the user can select an item in the menu by stroking toward it. Users who
are familiar with the structure of the menu can select menu items without waiting for the pie to be
displayed, by simply stroking in the direction of that slice of the pie. This approach appears to combine
strengths of a gestural interface and a menu- or icon-driven interface, as it hides widgets until they are
needed, and allows the content to fill the entire screen. For these reasons, Marking Menus may be a good
solution for reclaiming space on small screens. It does not, however, permit simultaneous display of
widgets and content. Because the pie menu is opaque, it will at least temporarily obscure underlying
content.
Semi-transparent Widgets And Their Limitations
An approach that begins to address this objection makes use of semi-transparent popup menus that do
not completely obscure underlying content [5]. (Lieberman, et al. have used a similar semi-transparency
method to show data context in a zooming interface [9].) Harrison, et al. proposed allowing users to
define the level of menu transparency. They showed that the transparency levels selected by users will
vary, depending on the tasks to be completed, and each user's level of expertise.
We believe that the use of control widgets of variable transparency is a promising method for maximizing
usable screen space. If the transparency of widgets is adjusted so that the content with which they
intersect on-screen is nonetheless legible, then it should be possible to display both the widgets and a full
screen of content without sacrificing any screen space reserved for the latter. There is, however, a serious
obstacle that must be overcome before this method will support user interaction with the content.
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For the purposes of our argument, let us assume that the widget layer is on top of the content layer. If
opaque widgets obscure otherwise selectable content, and no mechanism is provided for passing through
the widget layer to the content, then the user will be able to interact with only those parts of the content
which are not obscured. If, however, the widget layer is semi-transparent, that is, if the underlying content
and the widgets are legible at the same time, then it is not immediately clear which of the two layers is
selectable (for copying, editing, etc.), even if there is no formal mechanism for selecting the bottom
layer. This is illustrated in Figure 2.
Figure 2. Selecting content beneath semi-transparent widgets
One solution to this problem is to turn off the selectability of underlying objects, even when they are
visible. However, this is likely to confuse and frustrate users, who will reasonably expect that visible
objects will sustain some degree of interaction, simply because they are visible. It is, moreover, fairly easy
to build a scenario where this solution will effectively prevent underlying objects from ever being selected
- if, for example, the upper layer cannot be moved, and the lower layer cannot be scrolled so as to move
the partially-obscured content into a selectable region of the screen. The only course of action in this case
would be a hardware toggle of some kind to force selectability - an awkward exception to the normal
behavior of the interface and an unnecessary extra task for the user.
Instead, we propose a technique for determining the layer receiving user interaction that does not require
an additional modifying step, as it based upon variations in the duration of the interaction. In this variation
on the transparent widget/ content model, the length of time during which the user engages with a region
of the physical screen (wherein widgets and content overlap) would determine which virtual layer of the
screen receives the interaction.
THE EXPERIMENT
Overview
To evaluate the merits of this technique in a practical example, we decided upon a prototype emulating a
text-based online newspaper. PDAs are widely used to read text downloaded from digital news sources.
These applications involve moderate quantities of relatively unstructured text, and usually support limited
interactivity - simple navigation between stories, or between issues, searching for text strings, transferring
information from a story to another application (for example, a scrapbook), and hypertext linking between
stories or issues. The emulation of hypertext linking offered a focused example of object selection through
layers - how, we wondered, would users attempting to select a passage of "hot" text (that is, a hypertext
link) that was partially covered by a semi-transparent icon (or vice-versa) manage the transition between
layers?
The prototype was implemented on a Macintosh in Allegiant SuperCard. Although the mouse-driven
interface of the Macintosh differs from the pen-based interfaces of many PDAs, we believed that using a
desktop computer for initial evaluation would nonetheless generate valuable data. Though we have
emphasized in this paper the importance of the issue of screen size for PDAs, the balancing of window
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size with maximum content display in small windows is of value on desktop computers, where multiple
windows may be open at any one time, and the user may wish to reduce the size of any given window to
a practical minimum, while still being able to display as much content as possible. Interaction techniques
developed for small screens would in this case apply equally well to larger screens displaying many small
windows. Moreover, this initial run of the experiment was designed to test the merits of varying the
responsiveness of semi-transparent objects (widgets and text), and those kinds of objects need not exist
only on PDAs.
Figure 3. Typical screens in the prototype
The overall design of the prototype strongly resembled that of AT&T's PersonaLink news reader for
Magic Cap devices (see Figure 3). The active screen region in the prototype was 320 x 240 pixels. The
top portion of most screens in the prototype contained labels identifying the current story and icons used
to move between stories or to return to an index for that issue of the newspaper. The text of the story
filled the center of the screen. Hypertext links in the story were underlined. Along the bottom of the
screen were seven icons (shown left to right in Figure 3): Desk (to quit the newspaper and return to a
hypothetical desktop), Search, Archive (to display previous issues of the newspaper), Append (to copy
the current story to a Scrapbook), Scrapbook (to open a hypothetical Scrapbook application), and
Trash. Along the right edge of the screen were icons for moving "up" or "down", to the next or previous
page of the current story.
Most of the icons, and all of the hypertext links were only partially functional - when the user successfully
highlighted a link or icon, a status message was briefly displayed in a floating palette on the screen. (This
palette is not shown in Figure 3.)
The layered icons and text at the bottom of the screen - comprising nearly 20% of the total screen space
- were of variable translucency. As the icon layer was made more translucent, the text layer became more
opaque, and vice-versa. After a small series of pilot tests, we decided to fix the translucency settings at
80% opacity for the text, and 20% for the icons, to insure good legibility of both icons and text. As we
were principally interested in recording the test subjects' reactions to changes in the responsiveness of
links and icons, we did not allow the subjects to change the transparency settings, nor did we run the test
at other transparency levels.
Sixteen volunteer subjects were selected from among the faculty, graduate students and staff of the
Graphics, Visualization and Usability Center, and the School of Literature, Communication and Culture of
the Georgia Institute of Technology. Most of the subjects were expert users of the Macintosh or another
mouse-driven graphical user interface. Most were familiar with hypertext concepts. Fewer than half had
any experience with PDAs.
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After a brief training session with the prototype to demonstrate the functions of the semi-transparent icons
and the delayed response behavior of the software, each subject was asked to perform eight tasks:
From the story index (not shown in Figure 3), select a story title overlapping an icon. (Selecting a
story title jumps the user to the first page of that story.)
1.
From the story index, select a story title that does not overlap an icon, scroll two pages into the
story, and select a link overlapping an icon.
2.
From the story index, select a story title that does not overlap an icon, and select an icon that does
not overlap with any linked text.
3.
On the same page, select a link that does not overlap with any linked text.4.
From the story index, select a story title that does not overlap with an icon, scroll three pages into
the story, and select a link overlapping an icon.
5.
From the story index, select a story title that does not overlap with an icon, and select a link
overlapping an icon. (The "Greenpeace" link shown in Figure 3.)
6.
From the story index, select a story title that does not overlap with an icon, And select an icon that
does not overlap with any linked text.
7.
On the same page, select an icon that does not overlap with any linked text.8.
(The actual instructions given to subjects were more specific than those listed above.)
Each subject repeated the eight tasks six times: three times with the prototype configured to favor link
selection (that is, to initially highlight the link when the mouse was clicked where a link and an icon
overlapped, as if the link were on top of the icon), and three times with the prototype configured to favor
icon selection. In both selection methods, if the mousebutton were held down for longer than a predefined
period of time, the click would appear to pass through the object on top, highlighting the object under it.
The association between response delays and layers was rigorously enforced: if selection of links was
favored, the subject was required to hold down the mousebutton for the stipulated period of time before
any icon would be selected, even if the icon did not overlap with any links.
We further divided the link-first and icon-first selection groups according to the response delay that
determined the switching between layers. Each subject attempted all eight tasks for each method of
selection with preset delays of 2/5 second and 4/5 second. In the third set of tasks for each selection
method, the subjects were allowed to adjust the response delay, between a range of 1/5 of a second and
a full second. The sequence in which the subjects performed the series of tasks was shuffled to
compensate for learning effects.
At the conclusion of the experiment, each subject was given a brief questionnaire that reviewed the test,
and included questions asking which of the selection methods was preferred (links first or icons first), and
which switching delay (between 1/5 and 1 second) was preferred for each method.
The Results
As the subjects attempted the eight tasks, the prototype recorded every mouseclick, tabulating successful
and unsuccessful hits on targets (links or icons). Gross errors (unsuccessful hits) for all subjects for the
task series during which we controlled the delay times are summarized in Table 1.
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TABLE 1. Raw error counts
("Links-first 2/5 sec" = Links selected first, with a 2/5 second delay between icon and link layers)
Table 2. Normalized error counts
("Links-first, 2/5 sec" = Links selected first, with a 2/5 second delay between icon and link layers)
The error counts shown in Table 2 have been normalized so that each subject's contribution to the total
errors is (Standard deviations were calculated for each subject to insure that there were no outliers
included in the data.) Figure 4 shows a simple line graph of the data in Table 2.
Figure 4. Line graph of data from Table 2
DISCUSSION
After analyzing the results of the experiment, we arrived at the following conclusions:
Despite an overwhelming preference expressed for the links-first method of selection (15 of the 16
subjects), the overall error rate for the two methods of selection (links-first and icons-first) were
very similar.
The subjects' error rates decreased as they became more familiar with a method of interaction.
Subjects had greater difficulty selecting links in icons-first mode, and vice versa. This accounts for
the alternating peaks and troughs of Figure 4.
In the questionnaire, we asked subjects how they would change the behavior of the delayed
response. Many expressed the same desire: that all objects in one layer that did not overlap with
objects in the other layer should be immediately responsive - in other words, that the subject
should not have to wait for the mouseclick to pass through to the lower level if there is nothing on
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top of it in the upper level. This result surprised us, as we expected that a rigorous and consistent
distinction between the layers would make it easier for subjects to conceptualize the two layers.
The request for immediate responsiveness was given added weight by the subjects' answer to
another question. When asked which delay time they preferred, most (12 of 16) responded that
they preferred a delay time of 1/5 second - the minimum allowed by the prototype, regardless of
which selection method they used. What the subjects appeared to be asking for was a way to
simulate immediate responsiveness of non-intersecting objects, within the constraints of the
prototype.
The request was further supported by the difficulties that all subjects had with selection of
non-obscured text while using the icons-first selection method. This may account for the high error
rate in Task 6, when performed in icons-first, 2/5 second mode (see Figure 4). Users performing
the same task in icons-first, 4/5 second mode had less trouble, possibly because they were by
Task 6 accustomed to holding down the mousebutton for the extended duration required to select
a text link.
DIRECTIONS FOR FUTURE WORK
This expressed preference for immediate responsiveness of non-intersecting objects merits additional
research. How, for example, would this change to the delayed-response model affect complex text
selection, such as the drag-select gesture supported by many PDAs? How would it affect selection of
objects in PDA-based drawing applications? Would novices find this inconsistency in the method of
object selection confusing? Would further experiments support the subjectsÕ intuition that immediate
responsiveness is a more efficient way to select non-intersecting objects, and might the data from those
experiments reveal differences between the links-first and icons-first methods?
CONCLUSION
The very small screens of PDAs severely limit the space available for the display of both text and the
widgets required to navigate within or modify that text. A promising method for maximizing the usable
screen space of PDAs is to vary the transparency of overlapping objects on the screen, so that objects
beneath other objects are still visible, and valuable screen space is not sacrificed in order to display both
widgets and information at the same time. In this paper, we have proposed that a variable delay in the
response of overlapping widget and text improves the effectiveness of the semi-transparent widget/text
model. We assumed that varying the delay by which objects in different virtual layers of the screen
responded to users' attempts to select those objects would make it possible for users to select
partially-obscured objects without having to resort to a toggle to switch between layers.
Our experiment with a prototype for an online newspaper that uses this selection technique bears out this
assumption, and raises additional questions. After an initial learning period, the test subjects were able to
select underlying screen objects, regardless of whether those objects were text or icons. Subjects did,
however, have greater difficulty successfully selecting objects of one kind when the other kind of object
was on the top layer. Our results suggest that it may be easier for subjects to use a variant of the
semi-transparency/delayed response model, in which the delay in responsiveness does not apply to
objects in one layer which do not appear to intersect with objects in the other layer.
Nothing in our experiment calls into question the merits of techniques for maximizing usable screen space
that rely on alternative views of information content. Indeed, it should be possible to combine this
semi-transparency/delayed response model with content-based methods for reclaiming space on small
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screens.
ACKNOWLEDGEMENTS
We show great thanks to the News and Observers who allowed us to user their articles for our internal
experimental newspaper.
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Using small screen space more efficiently / tomo@cc.gatech.edu
