Sensing Tablet Grasp + Micro-mobility for Active Reading
Dongwook Yoon1,2, Ken Hinckley1, Hrvoje Benko1, François Guimbretière1,2,
Pourang Irani1,3, Michel Pahud1, Marcel Gavriliu1
1
Microsoft Research, Redmond, WA, United States, {kenh, benko, mpahud}@microsoft.com
2
Cornell University, Ithaca, NY, United States, dy252@cornell.edu, francois@cs.cornell.edu
3
University of Manitoba, Winnipeg, MB, Canada, irani@cs.umanitoba.ca
ABSTRACT

offer a possible solution to these problems by sensing howtablets are grasped, fingered, oriented, passed around, or held
aside, thus opening up a new vocabulary of prehensile
behaviors with digital content on tablets (Figure 1).

The orientation and repositioning of physical artefacts (suchas paper documents) to afford shared viewing of content, orto steer the attention of others to specific details, is known as
micro-mobility. But the role of grasp in micro-mobility has
rarely been considered, much less sensed by devices.
We therefore employ capacitive grip sensing and inertial
motion to explore the design space of combined grasp +micro-mobility by considering three classes of technique inthe context of active reading. Single user, single device
techniques support grip-influenced behaviors such as
bookmarking a page with a finger, but combine this withphysical embodiment to allow flipping back to a previous
location. Multiple user, single device techniques, such as
passing a tablet to another user or working side-by-side on asingle device, add fresh nuances of expression to co-locatedcollaboration. And single user, multiple device techniques
afford facile cross-referencing of content across devices.
Founded on observations of grasp and micro-mobility, thesetechniques open up new possibilities for both individual andcollaborative interaction with electronic documents.
Author Keywords

Figure 1. (a) Tablet case with (b) grip and motion sensing
affords combined grasp + micro-mobility for (c) immersive
reading, (d) collaboration, and (e) multi-tablet interaction.

Grasp sensing; tablets; grip; micro-mobility; active reading.
ACM Classification Keywords

H.5.2 [Information Interfaces and Presentation]: Input

In particular, our research unites two traditionally isolated
perspectives: grasp sensing [19,36,38,41], and the micromobility of physical artefacts [13,23,25]. Micro-mobility
concerns how people employ subtle manipulations—such asa doctor orienting a medical record towards a patient to invite
comment—to afford the shifting demands of an activity.

INTRODUCTION

Active reading—the deep engagement with multiple
documents and work surfaces during challenging intellectualtasks—characterizes much of knowledge work [18,29,34].
With the rise of electronic reading comes a desire to bettersupport such activity on tablets and e-readers, for example
through stylus input for mark-up [1,29], or more embodied
interaction [5,10]. However, the user experience of suchdevices is still far from optimal for tasks such as nonlinear
navigation, working with multiple reading and writing
surfaces, or supporting nuanced co-located collaboration.
Sensor modalities that inform context and enrich interaction

We expand on this traditional view of micro-mobility in two
ways. First, rather than solely a manifestation of orientation,
we argue that the dynamics of grasp—how and where aperson grips an artefact—also influences the semantics of
micro-mobility. And likewise, micro-mobility affords
essential cues to interpret grasp. Second, we observe how
micro-mobility-like behaviors manifest at the level of theindividual—not just in joint activities—to optimize device
orientation for the task at hand, or to direct one’s own
attention across multiple mobile artefacts.

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Our techniques underscore this perspective. For example, we
recognize characteristic ways of holding the device during
immersive reading; we design gestures founded on natural
behaviors to support collaboration; and we employ finegrained grip sensing to afford facile cross-referencing of477

information across multiple tablets. As a whole, then, ourwork contributes the following: The fundamental insight that grasp and micro-mobility areinterrelated human behaviors that provide complementary
information (via capacitive grip sensing and inertial
motion) to lend meaning to dynamic grasp interactions.
 Behavioral observations that document grasp + micromobility behaviors for individual and collaborative tasks.
 Grip-sensing algorithms that segment multiple hands andrecognize grasp types (built upon existing hardware [16]).
 Interaction techniques that employ grip and micromobility to support active reading for 1) individual workon a single tablet, 2) dyadic collaboration over a single
tablet, and 3) cross-referencing between multiple devices.
 Preliminary user reactions to our techniques.

[12,16,17,33], and motion resulting from handling devices
reveals telling details of context [12,30].
Sensors also afford more embodied ways of interacting with
devices. For example, angling a device can evoke flipping
through rolodex contacts [10], turning over pages [5], or
sliding content across devices [25]. Our work probes thecontinuum between directly embodied interactions thatinterpret device grips and motions as overt gestures, all theway to subdued background-sensing techniques [4] that
adapt to a specific activity such as immersive reading.

Collectively, these contributions emphasize our perspectivethat grasp + micro-mobility are closely coupled humanbehaviors. And our techniques show ways one can leveragethese modalities during both individual and collaborative
activities associated with active reading.

Our system builds on existing hardware [16], but here wefocus on the theme of grasp + micro-mobility. While [16]does briefly touch on the idea of handing a tablet to anotheruser, here we explore this class of multi-user, single device
techniques much more deeply. Our work also differs
significantly from the micro-mobility interactions exploredby [25], which lacked grip sensing and relied on depth
cameras to support multi-user, multi-tablet interactions. In
the present paper we contribute techniques that demonstrate
how grasp enhances micro-mobility.

RELATED WORKThe Micro-mobility of Physical Artefacts

Our research combines themes from human grasping
behavior, grip sensing, and the micro-mobility of physicalartefacts. Our emphasis on active reading—nonlinear
navigation, cross-referencing content, working together overa document—lends a fresh task-centric view to these topics.

Traditionally, micro-mobility is (1) a manifestation of device
orientation that is (2) specific to co-located collaboration
with shared physical artefacts. Yet to manipulate an object
for micro-mobility [23,25], clearly grip (and not justorientation) must come into play. Territoriality [21] andpassing prehension while handing objects to another person[28] have been studied, but how and where users grasp
artefacts remains a neglected aspect of micro-mobility.
Furthermore, regarding point (2), although “people smoothlyand easily shift their artefacts from personal to public and themany shades in-between” [13], the manifestation of micromobility at the personal end of this spectrum—toaccommodate shifting task demands at the level of theindividual—has not been articulated. In this view, how aperson grasps and orients objects to manipulate the attentionof others may follow from habits first developed to allocate
one’s own attention to multiple physical artefacts.

Human Grasping Behaviors and Grip Sensing

Human grasp [24] can reveal intentions [31] and as such
represents a potentially rich source of insight into user
activity [2,19,41]. The sub-conscious level at which people
shape their hands to manipulate objects [2] or pass objects toone another [28] makes grasp difficult to interpretunambiguously [41]. Nonetheless, insights into how users
grip objects such as tablets [40], pens [16,36], or mobiles [7]can inform designs and suggest relevant aspects of context tosense. Several systems use 3D orientation as a machinelearning feature to improve grip recognition [16,36]. Our
research further advances these ideas by showing how grasp
and device micro-mobility offer mutually reinforcing
perspectives.

A microanalysis of active reading [18] notes that people keep
gripping artefacts (such as a document, or a laptop) that they
intend to return to shortly. This observation bolsters point (1)because it shows how grip, and not just orientation, plays arole in directing attention. It further bolsters point (2)because this behavior occurs in the context of individual
work (as opposed to the traditional association of micromobility with collaboration). Likewise, Chen’s Conduit
technique [6] enables users to touch a control on a ‘source
tablet’ while employing a pen to transfer content to a ‘target
tablet.’ And Conductor [14] prominently illustrates the user
holding the specific tablets he is interacting with—but the
paper does not remark on this, nor does grip sensing comeinto play. We build on these examples by sensing whichtablet a user holds, and where the user grips it, to support
multi-device interactions.

Small devices have motivated research into bezel [19,20] andback-of-device interaction [30,43]. Grasp sensing can detecthandedness [10,42], avoid unintended screen rotations [7], or
call up a graphical keyboard [8]. Other work explores gripdependent functions [19,38,41] as well as combined front /back touch gestures [43] and bimanual tablet interactions[40]. Our work goes beyond these efforts by using finegrained grip sensing on the entire back surface and sides of atablet in combination with inertial motion sensing.
Several efforts have explored hybrids of touch and motion
sensing, but none consider the interplay between micromobility and grasp sensing. For example, touchscreen
contact combined with motion enables multimodal gestures478

Active Reading Behaviors in Knowledge Work

named the objects; this naturally led users to work face-toface [35]. In the presentation task, one user read text or an
infographic for 1 minute—a solo activity—and thenexplained key details to their peer using the content as avisual aid—a dyadic activity. The peer then repeated this taskwith different content. This activity afforded observation ofindividual reading behaviors as well as collaborative micromobility in relative body orientations that shaded between
side-by-side and corner-to-corner [25,35]. The cooperative
task asked participants to discuss an infographic on a single
prop while viewing it together. This let us observe how users
shared the device in a side-by-side setting.

Studies show that knowledge workers interleave episodes ofdeep, immersive reading [18] across multiple reading andwriting surfaces [34], while annotating, cross-referencing,and collecting encountered information [27]. Similar
patterns hold for both paper and electronic documents[29,34], but the ease with which paper supports suchactivities has motivated the design of many nonlinear
navigation techniques [6,15,37], including techniques thatclosely mimic paper-like interactions [5,20,44].
Occasionally, microanalyses delve into specific manual
actions, such as turning pages [26], multi-tasking [18], or
lifting documents for closer inspection [39], but detailed
observations at the level of hand grips and micro-mobility
remain uncommon. We therefore further inform andmotivate our work with the following observational study oftablet grips and movements in diverse active reading tasks.

Apparatus

The non-interactive acrylic props were 9mm thick, in tablet
and e-reader sizes of 176 × 250 mm vs. 135 × 200 mm,
weighing 464g vs. 284g, respectively. Since the larger prop
was only 5g lighter than the iPad Air, it represents a goodproxy for state-of-the-art tablets. Paper sheets, which the user
could mark-up as desired, were affixed to the acrylic to serveas the ‘on-screen’ content. Users employed the props with
content formatted in both portrait and landscape orientations,and while standing as well as while seated at a table. All of
the tasks allowed users to optionally employ a pen to point
or mark up the infographics, and hence our tasks sometimes
also elicited grasp behaviors with a pen in hand [16,32].

FORMATIVE STUDY OF GRIP + MICRO-MOBILITY

We conducted a formative study with the goal of observing
solo and dyadic instances of grasp + micro-mobility during
document work. Our research ventures into areas lacking
clear design guidance (grasp + micro-mobility, passing
prehension [28], and the micro-territoriality of shared grips).
To address this gap, our study sought primarily to generate
design insights through qualitative observations that could
inform, inspire, and serve as points of departure for our
techniques.

Participants and Design

16 people (2 female, all right-handed) participated in 8 pairs.
The study lasted one hour, composed of 4 sessions, each withcompetitive, presentation, and cooperative tasks lasting
about 1½ –2 minutes each. The conditions and tasks were
fully counterbalanced to control for the order of presentation.

We gathered these observations using non-interactive acrylic
props, rather than fully interactive electronic tablets. This
was a carefully considered choice that best serves ourprincipal goal: to observe natural human behaviors related tograsp + micro-mobility. Non-interactive props have different
affordances than touchscreens. Users can grasp andmanipulate them in fully natural ways, without fear of‘accidentally’ touching something. Hence a broader
spectrum of natural behaviors can potentially be observed.

Grips and Behaviors Observed

All sessions were videotaped and analyzed by the lead
investigator via 2-step encoding—open coding followed byflat coding—with an emphasis on how people grasped,
passed, angled, or gestured at the acrylic props. A second
author independently sampled the study sessions and noted
consistent behaviors as well, and furthermore several of the
behaviors had been initially suggested in a separate pilot
study conducted by a third author. Our qualitative analysis of
the present study thus yielded the following noteworthy
behaviors (B1-B9):

This choice also extends and complements previous studies
that show how touchscreens can bias users’ grip on the tablet
or the posture of their hands – such as to prevent drops, or to
avoid Midas Touch [1,16,40]. The props enable us to largely
remove these biases and instead focus our findings andinsights around the underlying grasp + micro-mobility
behaviors themselves, naturally-occurring behaviors thatserve as the bedrock for our designs. Of course, the resulting
designs still must take into account the interactive propertiesof touchscreens; as such it would be interesting to repeat the
study that follows for fully interactive tablets in future work.
For our present purposes, however, we assert that conducting
the study with non-interactive props was the best choice.

B1. Thumb grips for temporary one-handed use. All 16
participants held the props with the nonpreferred hand while
the preferred hand gestured. We observed the following
principal grips:

Tasks

We explored competitive, presentation, and cooperative
tasks. By design these elicited different roles between users,
hence varying relative body orientations [25,35] that might
influence grasp and micro-mobility. In the competitive task,
users memorized a list, passed it to their partner, and thenFigure 2. Grips: a) Thumb Left, b) Thumb Left–Edge variant,c) Thumb Bottom, d) Tray, e) Fingers Top (wrap-around).

As in the Thumb Left and Thumb Bottom grips noted by
BiPad [40], participants held the props deep in the thumb479

3a,b). Bimanual support also encouraged greater use of theEdge variant (Figure 2b) of the Thumb Left grip.

cleft, with fingers extended for support, preferentially along
the left or bottom edges (Figure 2a,c). However, the lightnessof our props also allowed us to observe a new variation: theThumb Left–Edge grip (Figure 2b). Users employed this grip
to avoid occluding content, especially during bimanual use(see also B3, B5 below). We saw only a few instances of
wrap-around grips (Figure 2e), which emphasizes that people
rarely needed such grips for our lightweight devices.

B4. Flexibility and diversity of grips. Behaviors B1-B3 arenot static but part of a shifting allocation of grips. As suchthe assignment of grip to task is a somewhat flexible
construct. For example, at times all participants transferred
grips from hand to hand in response to the task context or a
movement initiated by their partner (in addition to likely
considerations of fatigue and comfort [40]). Furthermore, thedetails of how the user’s thumb, fingers, and palm contacted
the tablet exhibited much diversity, suggesting that any
particular grip does not unambiguously indicate the user’s
intent in the absence of additional cues from micro-mobility.

B2. Tray grip for longer-term support & writing. About halfof the participants exhibited Tray grips (Figure 2d) when
they expected to maintain possession for a longer period oftime, or if they needed a stronger opposition force, such as
when annotating or gesturing at content (Figure 3c).
However, users were not consistent about when they used the
Tray vs. Thumb grips; preference seemed to depend on
fatigue / comfort as well as the preceding grip on the device.

B5. Bimanual symmetric grip for immersive reading. About
three-quarters of the users consistently elevated the props
with both hands while reading, often at bilaterally symmetriclocations (Figure 3a,b). They also angled the prop at an
optimum angle for reading [29] and pulled it closer, to fill
their field of view and mask out peripheral distractions. This
reinforces and brings together scattered findings [18,29,39].
It also shows how micro-mobility, in the form of an orientto-self behavior, reinforces the interpretation of a particulargrip—in this case, a symmetric bimanual grip with the
thumbs held along the left and right edges of the device.
B6. Grip indicates locus of attention. While sitting, several
participants placed their left hand near the line they werereading (Figure 3b), echoing prior observations of the hands
anchoring attention [18,39]. The thumb tended to track thereading position as the user progressed down the page. We
also noted that most users made fine-grained optimizationsof the tablet’s tilt to adjust the reading angle to suit thecurrent reading position on the page. At present we do not
employ this in our proposed techniques, but it does suggest
another orient-to-self manifestation of micro-mobility.
B7. Directing the attention of others. Of course, as reported
by Luff [23], during collaborative activity we observed allparticipants adjust tablet slant or lateral translation to steer
attention (Figure 3c,d). This confirms our study elicited
micro-mobility as traditionally conceived—yet we werestruck by how users performed similar changes in tablet
orientation during their individual work also (Figure 3a,b).
B8. Lateral swing for face-to-face handoff. During ourcompetitive task, passing a tablet while standing in the faceto-face relative body orientation entailed a “lateral swing” –a Frisbee-toss motion, as in Figure 3e-h. All but one
participant employed this lateral swing to accommodate therecipient’s viewpoint. The thumb at the top keeps a firm grip
while also allowing the giver to pass and rotate the prop witha single deft flip of the wrist. Here again, we see the interplayof a particular grip (preferentially at the top) with micromobility (reorienting the tablet to face the partner).

Figure 3. Grip + micro-mobility behaviors observed.

B3. Bimanual grips for a stable grasp. Bimanual grips
enhanced stability and minimized fatigue; although perhaps
only essential for extended use in landscape format, weobserved bimanual grips in all settings (e.g. Figure 3a,b). All
but one participant exhibited bimanual grips when thepreferred hand was not immediately needed to gesture or
write. We also observed many instances of users holding theprops with the pen tucked near the edge (also seen in Figure

B9. Micro-territoriality. When performing our side-by-side
cooperative task while standing, the giver often extended thetablet towards the recipient with two hands: one hand on the480far edge, and one hand near the middle but still on the“giver’s half” of the tablet. The recipient responded by
supporting the opposite side of the tablet with one or both
hands (Figure 3d). In this way, three or four hands gripped
the tablet simultaneously, and the collaborators jointly
adjusted the device’s orientation. This accommodation cedes
part of the tablet’s territory to the partner; yet users clearly
respected each other’s grasping territory, rarely touching
their peer’s portion of the tablet. While territoriality on large
tabletops has been noted [21], this micro-territoriality within
a single mobile artefact (observed for 10/16 participants)
appears to be a novel result of our study.

we squelch minor capacitance noise by thresholding (20%intensity cutoff), and smooth temporal signal fluctuations via
low-pass filtering (fc = 19.89Hz) with bilinear interpolation.

Closing Remarks on the Formative Study

One theme, evident for B5-B9, is the many examples of theinterplay between grasp and micro-mobility. As such—just
as is the case with micro-mobility itself—the way people
employ grasp, orientation, and movement of physicalartefacts is specialized to the task, the context of use, andspecific situational aspects—such as whether the users areworking face-to-face or side-by-side [25,35].
The diversity of grips observed (B4) might seemdiscouraging if one fixates on recognizing only naturallyoccurring behaviors. However, we instead take theseobservations as points of departure—as provocations forpossible gestures, contexts, and designed interactions
founded on common patterns in how people employ grasp +micro-mobility. With this perspective and goal in mind, we
set off to explore interaction techniques as follows.

Figure 4. Tablet grip sensor data and recognition results.
Capacitive touches on the side-edges were mapped outside thedotted blue area, with 3 pixel thickness in the sensor map.

Segment. From the denoised capacitance map, oursegmentation module (using OpenCV) extracts isolated
images of each hand via geometric criteria, such as edge
connectedness or contact size, of the observed grip types (perBiPad [40], B1-B3, and Figure 2; with Thumb and Finger
grips on the edge, and Tray grip in the middle). By
segmenting grasp images, we can handle the shape of eachgrasp as a mutually independent entity; otherwise training
and classifying the entire capacitance map results in an
intractable explosion of grasp types and hand positions.

GRIP + MICRO-MOBILITY SENSING AND RECOGNITION

We built a system to sense tablet grasp + micro-mobility. We
use the hardware from [16], which consists of inertial sensors
plus a capacitive array on the back and edges of the tablet.
We improved the firmware to bring the grip sampling rate to
50Hz. The capacitive sensor array (44×26) covers the backand sides of a 337×197×16 mm case which weighs 1.4 kg,
including the enclosed 11.6” Samsung ATIV tablet.
Grip Recognition Software

We devised algorithms to classify the grip type of multiple
hands grasping the tablet. The recognition software consistsof two modules that are independent of one another: a grip
recognizer and a motion-gesture recognizer. The motiongesture recognizer is built on top of the grip recognizer, at
the application level, by using the inertial motion data in
addition to the sensed grips.

The recognizer first segments images of grips on the edges
by regarding an edge blob as the seed of a hand’s contact
region. A labeling algorithm iteratively dilates the windowof each hand region, which stems from the seed, to expand
the hand’s blob-set until no more adjecent blobs can be foundin the window’s adjacency set, or the integrated intensity
sum of the window exceeds a predefined threshold (9 fullintensity grids). Our algorithm then separately segments nonedge grasp images if any remaining blobs exceed a minimum
intensity threshold sum for a hand. The grip regions ofdifferent hands rarely intersect, since holding the tablet with
two overlapping hands is unnatural and was not observed inour study even when a pair of users held the same tablet (B9).

Our grip recognizer processes the raw capacitance map into
multiple hand datasets (Figure 4). These consist of handregions labeled with respective grip types through a 4-step
denoise-segment-classify-stabilize process. This approachcontributes a practical and fairly robust way to recognize thegrip types noted in Figure 2 (and B1-B3) even when multiple
users' hands (per B9) grasp a single device.

Classify. From the set of grasp images, we then classify the
low-level individual hand grips using features including
hand position, size, and intensity pattern of the contact area
window. Edge grasp images can be classified as Thumb,Thumb Left, or Finger grips, while the non-edge contacting

Denoise. This step suppresses temporal noise in the raw
capacitance signal. We first perform per-cell min-max
calibration of capacitance intensity for normalization. Then,481

multiple device techniques with grip + micro-mobility
remains a potentially fertile area to explore in future work.

grasp image becomes a Tray grip (B3—Figure 2d) without
running the classifier. A one-versus-all multiclass Support
Vector Machine (SVM) classifier takes the grip boundingbox (x, y, width, and height) and the normalized grip image(16 x 16) as the feature vector. Dynamic motions and theinertial sensors were not employed at this low level ofprocessing, because here we are just identifying individual
hand grips as building-blocks for subsequent interactions.

SINGLE DEVICE

Grip

Stabilize. Finally, we recognize stable grips using a voting
scheme with a state transition model where each state
represents a grasp type. We queue the grasp types detectedfrom a dataframe. The model moves from the current griptype to a new one, and triggers a corresponding grasp event,
only when one grip type dominates the queue. Our prototype
employs a queue size of 10 and voting threshold of 7.

SINGLE USER
Micro-mobility Grip

Grip sensing[19,38,41]
Dual-display eFlexAura [22]book [5]iGrasp [8]BiPad [40]Systems which exhibit some elementsof combined grip & orientation sensing:

MULTIPLE DEVICE

We collected training data with 4 participants (2 female, allright-handed). We led users to exhibit each grip type forvarious positions, with 50 × 6 (positions and types) × 4 users=1200 samples total. All grip recognition was performed
from this training data.

Territoriality on
tabletops [21]Passing prehension [28]iRotate Grasp [7], Embodied UI’s [10]Single User, Single Device techniques: Multiple User, Single Device techniques:
Sensing Immersive Reading
Thumb Bookmark with ‘Tip-to-Flip’

Training Data

MULTIPLE USER
Micro-mobility

Multi-slate reading Terrenghi(conduit) [6]observations [39]Single User, Multi-Device Techniques:
Fine-Grained Reference
Hold to Refer Back
Tablet+Stylus Sensing [16,36]

Face-to-Face Handoff
Side-by-Side Micro-Territoriality
GroupTogether [25]
PDA’s & Shared
Public Displays [13]Multiple User, Multi-Device
grasp + micro-mobility techniques:(Left for exploration in future work)

Figure 5. Design space of grip and micro-mobility techniques.
Techniques highlighted in green are explored in this paper.
SINGLE-USER, SINGLE-DEVICE INTERACTIONS

The first category of grasp interactions for active reading
supports individual reading and navigation tasks.

GRIP + MICRO-MOBILITY INTERACTION TECHNIQUES

Building on behaviors B1-B9 above, we set out to explore
interactions that leverage the interplay between grip andmicro-mobility in active reading. Active reading representsa cognitively demanding task with ingrained habits
stemming from physical pen and paper. It therefore seems
ripe for fluid interaction techniques that keep users in theflow of deep concentration while also affording moreembodied ways to interact with electronic books anddocuments. By the same token, our intent was not to follow
a strictly literal interpretation of B1-B9: interfaces can be
evocative of familiar interactions without being strictly
beholden to the limitations of documents and physicalartefacts. Our application also supports ink annotation with astylus because mark-up is central to active reading [29,34].

Sensing Immersive Reading for Deep Engagement

Following from our observation B5, we sense immersive
reading when the user exhibits a bimanual symmetric grip on
the left and right sides of the tablet. This grip conveys aheightened level of engagement in a reading activity.
When we sense immersive reading, the application fades
away peripheral distractions (such as the menus at theborders of the screen, as well as any overlaid ink mark-up).
The application simultaneously performs a slow zoom by afactor of 1.15 such that the content expands into the margins
and the space freed up by fading away the menus. Thisfocuses closely on the book content itself (Figure 1c) andproduces a visual effect which is the equivalent of pulling abook about 5cm closer (from a typical reading distance of40cm). This technique thereby supports deeper engagementwith the content in a refined and non-demanding manner.

We deliberately set out to explore techniques covering thethree major categories of behaviors that we observed, as
illustrated in the design space of Figure 5 – in addition to afourth possible area of muti-user, multi-device techniquesthat we leave to future work. The design space highlights
how our work fully combines grip and micro-mobility
sensing, while most related techniques either focus on grip
or micro-mobility, but not both. However, as noted in thetable, a few previous works do hint at combined grip andorientation sensing.

The user can exit immersive reading in a straightforwardmanner simply by relinquishing his grip on the tablet.
However, the user may remove one hand to perform othertasks (e.g. pick up a pen) without exiting the slightly
zoomed-in view. Thus, immersive reading is a “mode,” but
one implemented in a very lightweight manner that largely
retreats to the background of user attention [4].
We experimented with sensing B5 literally, i.e. with abimanual symmetric grip while angling the tablet. However,given the bulk of our prototype, and since our slow-zoom
animation produces a similar visual effect as lifting thetablet, we found the technique more satisfactory when webased sensing immersive reading solely on the grip rather
than requiring users to also lift the tablet. This shows one
example where a design may be inspired by a natural

Single user, single device interactions start from the coretasks of immersive reading and single-document navigation[26]. Multiple user, single device interactions explore gripdependent micro-mobility behaviors for co-locatedcollaboration between a pair of users working on a tablet.
And single user, multiple device interactions explore how
grasp + micro-mobility enriches activities spanning multiple
surfaces [6,14]. The intriguing class of multiple user,

482

behavior (B5), yet interprets the user’s intent in a non-literalfashion to take advantage of the digital affordances ofelectronic documents. It would be interesting to see if users
would prefer this with a much more lightweight version of
our prototype, but at present our grip sensors add too muchbulk and weight to the tablet to investigate this.

We explored two different techniques, a Face-to-FaceHandoff based on the lateral swing observed in B8, as well
as a Side-by-Side technique based on the micro-territoriality
observed in B9 during cooperative work. However, we
decided to support these behaviors as designed gestures thatusers would intentionally trigger, rather than attempting to
respond to any plausible activity that looked like it might bedevice sharing. This leaves users in control of theseinteractions in a way that respects the spirit of micromobility, which users employ to share or not share according
to the specific context, task, or mood demanded by theoccasion of an encounter. Employing a specific, designed
gesture also acknowledges the practical constraints imposedby the flexibility and diversity of grips we observed (B4).

Thumb Bookmark with ‘Tip to Flip’

In physical books, users naturally interleave a thumb or
finger between pages to easily flip back to content of interest.
Note that this involves both placing a finger as well as
physically orienting the document so that the document flips
back to the desired page [26]. Our system supports thislightweight bookmarking behavior by sensing when the user
rests a thumb along the edge of the tablet (Figure 6a). Thisdraws on the prevalence of the thumb in reading grips (B5)and in the role of grip in tracking the locus of attention (B6).

Face-to-Face Handoff

Our Face-to-Face Handoff technique (Figure 1d) provides alightweight way for a user to give a collaborator temporary
guest access to the content on his or her screen. Furthermore,
we also employ this as a lightweight way to distinguishwhether the owner or the guest has made annotations without
requiring authentication (log on, swiping a badge, etc.). Notealso that this form of sharing content differs markedly fromsending the content itself to another device. The owner
maintains control of the digital content and it is only
accessible to a collaborator while he is in physical possessionof the tablet itself. This semantic of sharing is more akin tophysical sharing, yet it represents a shade of meaning thathas been largely absent from electronic documents.

Figure 6. Holding the edge creates a Thumb Bookmark. The
user can then ‘Tip to Flip’ back to the page held by the thumb.

Based on the hand-off behavior we observed in B8, we look
for a hand grip at the top of the device followed by a lateralswing of the device. We sensed this as a quick yawing
motion that covered more than 80 degrees of rotation.
Pursuant to our stated goal to support designed, intentional
gestures for this class of interaction, we only recognize
handoff when the lateral swing occurs with the hand grip at
the top. This is consistent with the grasp + micro-mobility
behaviors we observed, yet also keeps the user in full controlof the interaction and the degree of sharing (or not sharing).

Resting a thumb on the edge of the tablet accordingly reveals
a small dog-ear that shows the current page is held by a
thumb bookmark. This is intentionally subtle so that any
incidental activation of these lightweight bookmarks does
not distract the user, yet the feedback is clear when the user
intentionally rests a thumb to bookmark a page.
We use the ‘Tip to Flip’ interaction to ensure that flippingback to a bookmark is always an intentional gesture. To
return to a thumb bookmark, the user must make an overt
gesture of tipping up the bookmarked edge of the tablet by atleast 10 degrees. This “peeks back” to reveal more of thebookmarked page (Figure 6) in a physically embodied
manner, but does not actually perform page navigation. To
confirm the action, the user taps his thumb against the edgeof the tablet while it is tilted. The page then flips back to thepage indicated by the thumb bookmark. The user canalternatively cancel the flip-back by withdrawing his grip.

When we recognize the Face-to-Face handoff gesture, our ereading application shows clear feedback of the Guest profile“swinging” onto the screen in an animation that mimics thelateral swing motion itself. Note that the current holder is notsensed per se, but rather assumed based on the order of
appearance of the grips. The application then shows thedevice holder’s mark-up with higher opacity, while the otheruser’s annotations fade away. Furthermore, contrary to
Hinckley et al.’s hand-off gesture [16], our interaction moreclosely follows natural hand-off movements by taking thelateral swing motion into account. It also thereby prevents
false-positives that could be triggered by the user’s otherunintentional grip changes or device movements.

MULTI-USER, SINGLE-DEVICE INTERACTIONS

Our second class of interactions support collaboration with ashared device [21,23,25] in both face-to-face and side-byside settings, as observed in B7, B8, and B9. Our e-reading
application color codes ink mark-up as information elementsthat are bound to each user, with blue annotations for one
user (the owner) and red for the other (the guest). Our
techniques fade in and fade out these annotations to indicate
dynamically changing ownership and access for each user,
as appropriate to the specific technique.

Side-by-Side: Micro-Territoriality during Joint Use

Since our Face-to-Face Handoff technique accommodates
collaborative tasks of a more competitive nature [35], wesought to explore different behaviors for more cooperative483

interactions typical of side-by-side interaction. In particular
we thought it would be interesting to sense the microterritoriality of grips on a shared artefact as observed in B9.

location where they were reading (B6). This pattern of
behavior is particularly informative for capturing user
intentions in multi-device interaction, since it provides a clue
about not only which device a user is referring to, but also
what part of the screen is likely the focus of their attention.

Figure 7. Micro-Territoriality during Side-by-Side use.

Our system detects the Side-by-Side interaction by sensing
multiple (more than two) hands simultaneously gripping thetablet while it is close to being held level (Figure 7). When
our e-reading application recognizes such a side-by-side
interaction, we again show distinct animated feedback thatshows the screen sliding sideways towards the collaborator
before coming back into place. This indicates side-by-side
use with the users on equal footing as collaborators. We
furthermore show each user’s ink mark-up in different
colors, but with the same opacity level, so that they canmutually view and discuss one others’ ideas.

Figure 8. Fine-grained reference to part of a document.

With this in mind we employed the location of isolated
touches along the screen edge to enable fine-grainedreference to a portion of the screen. As shown in Figure 8,
this echoes the corresponding horizontal strip of the screen
to the user’s most recently used remote tablet. The user canthen clip out part of the echoed content by performing amarquee (rectangular) selection with the pen. This simple
touch-and-clip interaction is quick to perform, and leverages
the natural behavior of keeping the hand grip close to thepassage being read. This makes it easy for the user to gather
encountered content in a lightweight manner that is
minimally disruptive to the reading task itself [27]. At
present, we only support this when the tablet is flat, because
using multiple tablets in this manner only makes sense on a
supporting surface.

We maintain this state until the user who initiated side-byside sharing releases his grip on the bottom. This embodied
way of holding the mode clearly indicates who initiates—
and in a sense, ‘owns’—the exchange, while also offering alightweight way to back out of the shared status if the userchanges his mind, or happens to trigger a false positive.
However, while this design choice minimizes the users’
collective physical effort, and leaves their other hands free tointeract or gesture as desired, it also means that the user who
initiated the interaction cannot fully relinquish his grip (e.g.
to pick up a pen). We discuss this further in our evaluation.

Hold to Refer Back Technique

SINGLE-USER, MULTI-DEVICE INTERACTIONSWhen a user picks up an artefact and orients it to himself (B5,
B7), this indicates increased focus of attention on the object[18,39]. Inspired by these observations we devised a “hold torefer back” technique for revisiting pages.

While we did not explicitly probe multi-tablet behaviors in
our formative study (B1-B9), reading and writing on
multiple working surfaces is a pervasive theme in active
reading tasks [34], and several systems have already
explored multiple-device approaches to support such activity[6,14]. Furthermore, some of our observations, such as thetendency for grip to indicate the locus of attention (B6),resonate with behaviors noted elsewhere, such as users’
prevalence to hold and orient an artefact to themselves (B5,
B7) if they intend to devote their attention to it soon [18,39].
We therefore explored two techniques for multiple-device
interactions. At present, we only have one grip sensing case,so we prototyped these techniques by employing the second
device’s tilt to infer its state (resting flat on a surface, or heldin midair). We also employed the pen and touchscreen inputs
to keep track of the most recently used device.

Figure 9. Hold to Refer Back Technique.

The clippings captured by the Fine-Grained Referencetechnique automatically include hyperlinks to the content.
When the user subsequently taps on the clipping, our ereading application triggers an animated page flip back to theFine-Grained Reference to Parts of a Document by Grip

Participants in our formative study often put their thumb orother fingers on the outer edge of the device, close to the484source content on the other device – i.e. the device that theuser is currently holding (Figure 9). In this way the user caneasily cross-reference content from other documents andarrange a set of working documents on multiple tablets tofacilitate viewing scattered pieces of desired information.

sensing grip might obviate the need for micro-mobility
manipulations in certain cases.
Thumb Bookmark. People enjoyed the direct intuition ofholding a finger in place to anchor a page, tipping up thescreen to reveal the held page, or withdrawing the hand to let
go of a bookmark. However, one minor issue we noted wasthat letting go of a bookmark currently requires completely
letting go; this transfers the entire weight of the tablet solely
onto the remaining hand, which users did not like. We plan
to explore a more subdued notion of withdrawing the thumb,but this of course must be balanced against the possibility ofletting go by accident which would also be undesirable.

Although we currently employ orientation sensing to support
additional tablets without our grip-sensing hardware, thecapacitive sensor offers a more robust and physicallyintuitive solution for detecting which device a user grasps. In
the future this may enable techniques that leverage richer
grasp-sensing capabilities on both tablets to be more fully
explored.
PRELIMINARY EVALUATION

Face-to-Face Handoff. Participants appreciated that oursystem responded when they “tried to do [the handoff]
naturally, and it worked almost every time.” Users also feltthat switching the tablet to a guest profile, with thecorresponding annotations emphasized, made good sense.
While most users employed Face-to-Face Handoff as weenvisioned it, a few pairs of users co-opted the sensing to be
triggered by the recipient, such that the giver would rotate
the device halfway through the lateral swing without
grasping the top, and the recipient would then grasp the topof the device and complete the rotation, triggering hand-off.
This illustrates that our designed gesture for handoff actually
supports one of our intended goals effectively, namely toleave the users in control of the micro-mobility of the device.

The goal for our evaluation was to assess preliminary user
reactions to our proposed grasp + micro-mobility techniques.
Hence our approach was an informal one, intended to solicit
qualitative feedback of users’ experience with the techniquesin action. Since this type of evaluation lacks ground truth
data, we did not tabulate false positive rates, but ourrecognition techniques worked well enough for users toexperience the techniques as we intended.
We recruited 16 users (4 female, all right-handed, 25-48
years old, mean 28.5) in 8 pairs for 90 minute sessions. Noneof these users participated in the formative study. The
experimenter briefly demonstrated each technique, and theparticipants then practiced until they felt comfortable toproceed. Each user performed the single-user interactions
alone, and the multi-user interactions in collaborative pairs.

However, our detection mechanism did miss some passing
events. For example, one pair of users attempted to pass thetablet back and forth while standing side-by-side. Since thisform of handoff is not naturally accompanied by a lateralswing, it was not detected as such by our system. Thissuggests a design choice, of detecting a greater variety ofsuch handoffs and using them for augmented interactions, orof intentionally deciding not to recognize them so that theyremain available for natural micro-mobility motions. Whilethe correct choice remains unclear, our work has served its
purpose to the extent that it surfaces such considerations.

Overall, we found that we could reliably sense hand position
and grip size. However, a few participants initially held thetablet case too lightly, which caused it to sense unstable grip
data (we believe this was caused by air gaps forming under aprotective layer over our capacitance sensor). We addressed
this issue by simply asking these users to grip the tablet a bit
more firmly, which successfully resolved the problem.
Finally, although a pen was employed in our formative study,
our prototype tablet was too bulky to afford tucking a pen
while also holding the device [16,32]. We therefore did nothave users hold the pen while trying out our techniques.

Side-by-Side Micro-Territoriality. People understood that 3
hands on the tablet indicates a joint grip. And they could use
the supported interaction technique easily. But users
sometimes found our current implementation too inflexible
to trigger, because they sometimes tried to share the tablet
using a different pattern of shared grips, such as one casewhere each user attempted to support the tablet with a single
hand: “It limited my behavior too much. There can be many
ways to share the device.” This again reinforces the need tosupport flexibility in grasp behaviors (B4). However, wemust also recognize that attempting to recognize all possible
ways of sharing a device may not be feasible or evendesirable, if we are to leave open-ended micro-mobility
manipulations available to users who are collaborating.

Immersive Reading. Users readily picked up on thistechnique, with several commenting on its real-life
familiarity (per B5). Some users disliked the immersive
reading technique’s requirement to constantly hold thetablet’s outer edge, which leads us to contemplate morerelaxed ways of maintaining the immersive reading mode
once initiated. Beyond the overt behavioral level, some users
also remarked on how the technique afforded their
engagement with the content, such as one user whocommented “This [action] means a focusing for me.” Thissuggests that the semantics encoded in the physical action of
B5 were well supported by the digital counterpart that wedesigned. Even though a literal interpretation of B5 would
require lifting and angling the tablet, our technique clearly
captured the user’s intention effectively. It also shows how

Fine-Grained Reference and Hold to Refer Back. Users
easily understood the concept of touching and holding atablet to designate a source device. Participants found the485regard, it also seems likely that new techniques particularly
tailored to grasp and motion gestures operating in tandem
may be helpful, and hence offer a rich topic for future work.

cross-referencing feature useful, especially if they personallyowned multiple tablets. However, a couple of users reported
confusion that they had to perform the clipping action on thedestination tablet, rather than on the source tablet: “I would
accidentally clip the wrong tablet, because I focused on thefirst tablet—because I was using my finger there.” Thissuggests we could improve the technique by allowing users
to clip content on either tablet (rather than solely from theechoed content on the destination device).

Finally, while we have focused here on tablets, we fully
expect that grasp + micro-mobility could apply to diverse
form-factors such as smartphones, tangibles, or other “smart
objects”—as well as for heterogeneous combinations ofdifferent device types as well. As such, a society of mobile
appliances with rich grasp and micro-mobility sensing
capabilities could help lead us to far more natural, more
expressive, and more creative ways of engaging in theindividual, collaborative, and cross-device knowledge workof the future.

CONCLUSION AND FUTURE WORK

Our work establishes the interplay of grasp + micro-mobility
in physically-embodied human activity during individual
work, dyadic collaboration, and multiple-device interactions.
Sensing both grip and micro-mobility then supports a broad
design space of interactions or contextually-dependent
techniques, including a number of promising designs that we
explored in the context of active reading. Yet, since
relatively little is known about this space, we encountered anumber of challenges in sensing the diversity of grips anddevice motions that users might exhibit “in the wild,”
particularly for the multiple-user interactions we explored.
We believe our formative study, hardware-software
platform, and interaction techniques represent meaningful
contributions towards establishing the design space of grasp+ micro-mobility, and understanding it more deeply.

RIGHTS FOR FIGURES
Figures 1-4 and 6-9 © Ken Hinckley, 2015.

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