Three's Company: Understanding Communication
Channels in Three-way Distributed Collaboration
Anthony Tang1, Michel Pahud2, Kori Inkpen2, Hrvoje Benko2, John C. Tang2, Bill Buxton2
2
Human Communication Technologies Lab
Microsoft Research
University of British Columbia, BC, Canada
Redmond, WA, USA
tonyt@ece.ubc.ca {mpahud | kori | benko | johntang | bibuxton}@microsoft.com
1

Figure 1. Three collaborators working around a shared tabletop (left: overhead schematic). In each physical space, remoteparticipants are embodied as surrogates (with display, camera, microphone and speaker). Note that the spatial relationships arepreserved in this setup. As B works in the space, her arm shadows are propagated to remote surfaces.
ABSTRACT

We explore the design of a system for three-way
collaboration over a shared visual workspace, specificallyin how to support three channels of communication: person,reference, and task-space. In two studies, we explore theimplications of extending designs intended for dyadic
collaboration to three-person groups, and the role of each
communication channel. Our studies illustrate the utility ofmultiple configurations of users around a distributed
workspace, and explore the subtleties of traditional notionsof identity, awareness, spatial metaphor, and corporeal
embodiments as they relate to three-way collaboration.
Author Keywords

Shared workspace, tabletop, media space, video-mediatedcommunication.
ACM Classification Keywords

H5.3. Group and Organization Interfaces: Computersupported cooperative work..
General Terms

Design, Experimentation, Human Factors
INTRODUCTION

In designing systems to support distributed collaborative
activity, researchers have frequently sought to merge theutility of video conferencing systems with collaborative
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workspaces (e.g., [32,16]). Video conferencing systems
support interpersonal interaction, allowing distributed users
to discuss ideas while also supporting non-verbal
interaction such as facial expression, body language andgesture (e.g., [3,7,25,29,22]). On the other hand,collaborative workspaces enable co-workers to shareartifacts and data that constitute or support the purpose of
meetings (e.g., [13,30]). These bodies of work enable richremote collaboration, each focusing on a specific aspect orchannel of interaction [5]: video-conferencing systems
enable person space for relationship and trust development[3], while shared collaborative workspaces are a task space,
where work is accomplished. In this work, we consider theintersection of these domains, articulating and exploring thecommunicative and coordinating role of reference space[5]—mechanisms allowing collaborators to reference,point, and relate with one another in task space.
Our interest is in supporting collaboration in distributed
teams. Industry attention has primarily focused on videoconferencing, from consumer-level webcams to dedicatedvideo conferencing installations (e.g., Cisco TelePresence).
Yet there is still a pressing need to better support
collaboration over digital artifacts associated with themeeting (e.g., documents, diagrams) [35]. The focus of thepresent work is to explore collaborative activity overconnected digital tabletops which support a shared sense ofpresence amongst people and artifacts associated with thetask. In particular, we focus on supporting users’ ability toemploy a rich gestural vocabulary to directly refer tovarious aspects of the artifacts. Through the iterative designand study of such a system, we bring attention to issues ofconfiguration, workspace awareness, and coordination. In
particular, our studies point to two conclusions: Person and reference space may be spatially disjoint, so
task characteristics can be used to determine the optimal
configuration; The design of mechanisms to support reference spaceshould consider issues of fidelity, identity, and saliency.
We arrive at these conclusions based on studies of threeperson, distributed collaboration. As illustrated in Figure 1,each workstation employs a multi-touch tabletop surface asa collaborative workspace, as well as two LCD monitors—
physical surrogates for the remote collaborators (e.g.,[29,20]). Each surrogate includes a camera, speaker andmicrophone which correspond to the remote person’s eyes,mouth and ears. Additional cameras above the tabletops
capture images of the users’ arms as they move over theworkspace, transmitting them to remote workstations.
Varying the spatial configuration of the surrogates andwhere the shadow of remote hand and arm gestures appear
on the tabletop workspace revealed participant behaviour
that underscored the utility of different configurations, aswell as aspects of the shared reference space.
The vast majority of explorations into distributed
collaboration employing video conferencing and sharedworkspaces has studied pairs (e.g., [16,17,18,21,26,32], cf.
[2,30,37]). It is unclear whether these designs scale beyond
two users, and indeed, it seems like many will not. The
number of collaborators working together exponentially
increases the complexity of possible interactions, increasing
the likelihood of misinterpretation and misunderstanding.
We chose to base our studies around 3-way meetings as ameans of extending beyond the dyad while still keeping
things practically manageable. While these findings may
not immediately scale beyond groups of three, they identify
directions for further study regarding group size.
This paper makes three contributions: first, we articulate therole of reference space in distributed collaboration (asdistinct from person and task space) in a way that builds on[5]; second, we report on studies of three-person
collaboration, identifying subtle issues in supporting
reference space that are less pertinent in dyadic scenarios;and finally, we show how spatial configurations of person
and reference space affect behaviour in task space.
In the next section, we outline prior work that provides thetheoretical foundation for this work, drawing on the same
literature to articulate a vocabulary that describes thecomponents of reference space. Next, we discuss the designof the system we employed to study distributed
collaboration. We then describe our iterative studies of thesystem, drawing on observations from those studies toillustrate the points outlined earlier. Finally, we discuss theimplications of this work and outline plans for future work.
BACKGROUND

We situate our work within a long lineage of research that
explores the intersection of video media spaces and sharedinteractive workspaces [1,7,14,22].

Video Media Spaces

Originating primarily in the seminal work of Xerox
PARC/EuroPARC, BellCore, US West and the Universityof Toronto, video media spaces (VMSs) were borne out of
the need to connect distributed collaborators (e.g.,[1,7,14,22]). VMSs provided collaborators with always-on
audio and video connections, which were as much anaugmentation of physical architecture as telephony. Having
established a shared space, to speak with a collaborator, one
only needed to glance over and begin talking. As
exemplified in [16, 29], the spatial configuration of thesespaces was important if collaborators were to be able toexploit everyday social skills, such as those based on gazeawareness. Because time and spatially-multiplexedmonitors inhibit gaze awareness [6], the evolution of thesespaces has begun to transition to more tangible surrogatesof remote collaborators (e.g., [29,20]), which we mimic.
Shared Visual Workspaces

A considerable body of research has explored how to
support collaboration over artifacts in a shared workspace.
For instance, [13] explore how augmenting conventional
desktop applications with shared workspace awarenesstools (awareness of others’ interactions in the workspace)facilitate smoother workspace coordination. The concept ofworkspace awareness encompasses notions of presence (isJanet present), communicative gesture (pointing and deictic
reference), consequential communication (backgroundawareness of others’ interactions), and feedthrough(equivalent of feedback, but sent to remote parties).
These aspects of workspace awareness are somewhat
implicit in the design of several video-based sharedworkspaces [32,16,37,21,36] where videos of remote andlocal spaces are fused into a single workspace. In manysystems, video cameras also capture remote collaborators’arms and bodies, providing a rich sense of embodiment andpresence, as well as communicative context.
Of interest here, the systems employed fairly diverse spatial
metaphors with regard to placement of collaborators aroundthe virtual workspace. ClearBoard [16] employed an
elegant ―separated by glass‖ metaphor, while Agora [37],and its successor [21] employed physical table metaphors,placing collaborators around or on opposite sides of a table.
VideoDraw [32], VideoDesk [19] and C-Slate [17] departfrom these physical metaphors, placing collaborators’ video
surrogate in a face-to-face configuration, but configuring
the workspaces so that users shared the same perspective(i.e., they effectively sat in one another’s laps). Our firststudy explores the consequences of these spatial metaphors.
These shared spaces, of course, are distributed analogs to
our everyday collocated workspaces (i.e., tabletops).
Researchers have also studied collocated collaboration, andfor example, how the design of digital tabletop software can
support or inhibit coordination [24,15,28]. For instance,[31,28] observed that in non-digital tabletop collaboration,
users partition work artifacts both semantically andspatially to help coordinate activity. Our first study alsoIn particular, he suggests three distinct types of spaces for
distributed collaboration:

Figure 2. Arm shadows are displayed locally as feedback, aswell as at remote sites. Finger contacts are transmitted toremote sites, and conveyed via trace pearls: the contact pointis represented by a small disc, and the trail fades over time.

explores this partitioning practice in the context of a
distributed tabletop system.
Embodiment in Distributed Tabletops

The video-based embodiments of collaborators’ arms fromworks like [16,19,32] have received renewed interest (e.g.,[18,26,30,33,36]), in part due to a growing literature on
distributed tabletop workspaces. Motivated by Tang’s
original study of collaborators in collocated, paper-andpencil design activities [31], these embodiments were
postulated to aid communication and coordination insimilar distributed workspaces. In principle, users’ handand arm gestures, learned over a lifetime of day-to-day
interaction, would be supported by such embodiments.
As described by [18] and [30], these embodiments provide
many such communicative functions in distributed
workspaces: drawing attention, supporting mimicry, ameans for deictic reference, and so forth. Suchembodiments also provide an important coordinating
resource when multiple users occupy a shared space (e.g.,[30,33,37]), though their use to coordinate territorial
behavior has been questioned [33]. We address this idea inour second study.
An equally pressing concern is whether the arm
embodiments (being in the reference space) need to be―connected‖ somehow to the video-based embodiment of aremote collaborator in person space. Luff et al. for example,
suggest that the interpretation of a gesture is not only of its
final manifestation, but of the entire production of a gesture[21]. The immediate implication of this work raises thequestion of how connected the person space need to be withreference space. Several designs have taken painstaking
efforts to mimic and reproduce real-life spatial metaphors,
keeping reference and person space connected (e.g.,[21,37]). Other designs have clearly placed priority on the
document or workspace (e.g., [32,17]), breaking the spatial
relationship between the workspace embodiment of acollaborator and his video-based embodiment. In
continuing this line of inquiry [26], we explore the benefits
and drawbacks of different configurations in our first study.
REFERENCE SPACE AND SYSTEM DESIGN

Buxton [5] outlines a technology-independent vocabulary todescribe video conferencing and shared workspace systems.



person space: where verbal and facial cues are used forexpression, trust and gaze—typically realized as video
and audio connections;task space: where the work appears—typically realized
through a shared workspace application, andreference space: where remote parties can use bodylanguage to refer to the work—often realized as mousepointers, though also as video embodiments of arms.

This conception separates distributed collaboration into
three (relatively) independent components, allowing
researchers to focus on one or more of these spaces. In thissection, we outline the design of our own system, paying
particular attention to our designs for supporting reference
space. The discussion underscores several design
requirements we derived from prior work.
Design Requirements for Reference Space

The following design requirements were derived fromprevious research (e.g., [30]) and our exploration withseveral prototype systems.
Support foreground “use” of reference space. The
embodiment needs to enable users to perform deictic
gestures in the workspace to support meaningfulcommunication with other collaborators.
Support background “use” of reference space. At the sametime, the embodiment should be easily ignored, allowing
remote parties to maintain an awareness of others’ activities
in the workspace while performing their own activities.
Support both coarse and fine-grained activity. The
embodiment should enable the interpretation of both coarse
activity (e.g., presence or approach of remote collaborators)
and fine-grained activities (e.g., artifact manipulation).
Support local feedback. The system should ―reflect back‖what is being transmitted to remote stations. This enables
collaborators to modify their gestures and behaviours in situ
so they will be ―correctly‖ interpreted by remote parties.
Implementation

Reference Space. Figure 2 illustrates the two mechanismsthat we employed to facilitate reference space: arm shadowsand trace pearls. Eight-bit grayscale images of acollaborator’s arms are captured at 320×240 at 15 fps using
an infrared camera mounted above the tabletop (Point Gray
Research FireFly MV). These images are alpha-blended
atop the workspace at both the remote (60% opacity) andlocal (20% opacity) workstations. The trace pearls are akinto telepointer traces [12], tracking each point of contact foreach user with a trail that fades after 2s.
System. Our system connects three workstations through thelocal network (Figure 1). The Surface tabletops ran custombuilt WPF shared workspace applications, communicating
via GT# [11]. Each surrogate employed a 17‖ display, awebcam/microphone placed at the top and centre of theFigure 4. On the left, examples of tiles participants were given
to construct approximations of the logos on the right.

Figure 3. The two configurations investigated in Study 1.
Around-the-table mimics real-life, while same-side allows
users to share the same perspective on the task space.

display, and a speaker. These surrogates enabled spatiallycorrect gaze awareness and spatialized audio.
STUDY
1:
CONFIGURATIONSCOLLABORATION

FOR

3-PERSON

There are many different ways to structure a distributed,
three-way collaboration using a tabletop display. Of theconfigurations explored by prior work, only one hasaddressed the case of more than two distributed
collaborators [37]. Our first study considers the two
configurations illustrated in Figure 3. The first, around-thetable, mimics the real-life metaphor of people sitting
around a table (from [37]), where each user has a uniqueposition and perspective, and any hand and arm gestures areseen by others to emanate from that position. The second,same-side, has all three collaborators see the table contentsfrom the same perspective. Since hand and arm gestures
emanate from the viewing side, the perception in terms of
the reference space was that all three participants sit in eachothers’ laps. However, each participant saw the two remotepeople—by way of their video surrogates—sitting side-byside across the table. Thus, person and reference spaces of
the two remote participants were spatially disjoint.
Each configuration has potential strengths and weaknesses.
The around-the-table configuration enables users to rely on
their intuitions of space; however, each user’s uniqueperspective may be problematic for oriented tasks (e.g.,
reading text). Nevertheless, this configuration does provide
a spatial connection between person space and reference
space. On the other hand, the same-side configuration
overcomes the problems of oriented tasks since allcollaborators share the same view of the workspace;however, the configuration does not map to a physicalanalog—thus, micro-coordination activities such as
territoriality may be impacted negatively. Furthermore, thisdisjoint configuration may also introduce problems, such asgaze awareness, and identifying which remote person ismaking a particular gesture (cf. [21]).
MethodConfigurations. Users completed activities using eachcondition: 1) around-the-table and 2) same-side.
Tasks. We used two different types of tasks: an orientationfree task, and a single-orientation task. Both tasks involved
moving and arranging a set of tiles, initially piled in thecentre of a shared workspace. In the orientation-free task(―logo‖ task), participants were asked to recreate two offour possible logos (Figure 4, right) using a set of tiles
containing various shapes (Figure 4, left). Participants wererequired to use at least eight tiles and the tiles could be
rotated and translated. This task mimics the photographsorting/organization of many tabletop studies (e.g., [15]),
where the content of the tiles and logos are less strongly
oriented than the text-based task..
In the single-orientation task (―text‖ task), tiles contained
two sentences of text (17-37 words), and the tiles were alloriented in a single direction. Participants were asked to
select and order a subset of these tiles according to aprescribed set of conditions. As an example, one version ofthis task was derived from Gottman’s Desert Island
Survival Task [9]. Users had to discuss and negotiate tocome to a consensus about which tiles to select, and decide
on an ordering in terms of importance. This task mimics thecore activities in shared document editing such as gross andfine references to paragraphs and text, editing (moving oftiles), and discussion. The tiles in this task could betranslated, but not rotated. In the around-the-tablecondition, tiles were oriented toward the short side of thetable, while in the same-side condition tiles were oriented
toward the participants (long side of the table).
Measures. We were primarily interested in qualitative
measures of activity: examining users’ interactions in eachconfiguration, tracking their problems, and identifying
interesting patterns of use. Each session was videotaped andwe logged participants’ interactions with the workspace.
Finally, post-study questionnaires assessed preferences.
Participants. Four groups of three (12 participants – one
female) were recruited for this study. Groups knew eachother beforehand, but were not from the same familial unit.
Participants’ ages varied (18-42). Participants had little orno prior experience with touch-interfaces to computers, andwere provided a gratuity for their participation.
Design. We employed a counter-balanced 2×2 withinsubjects design, with two configurations (around-the-table,same-side) and two task types (text, logo). Each groupcompleted both configurations with one task type before
moving on to the second task type.
Procedure. Participants were given a brief introduction to
the study, the equipment being used, and were given an
opportunity to interact with tiles on the Surface. They werethen led to their individual workstations (in separate rooms)
and provided with paper-based instructions. Participants
were given 15 minutes to complete each task, thencompleted a paper-based survey. After each task, depending

surrogates would be problematic in the same-side
configuration. Users did not appear to have any trouble withthe fact that the reference space was disjoint from person
space, and none expressed any concern over thisdisembodiment. We return to this idea in Study 2.
Clutter. Although previous work shows that providing arm
shadows for distributed colleagues benefits collaboration, it
was unclear whether this approach would scale effectively
given the additional clutter of shadows resulting fromadditional users. Despite the fact that we regularly saw
three, and occasionally saw six arm shadows (two perperson) in the task space, users did not seem to be distracted
by this additional clutter. In tasks where colour is relevant,this design approach would need to be revisited.

Figure 5. Contact traces for one group in three separate trials.
Each cell represents a user, and each row is a trial. (a) aroundconfiguration, logo task; (b) same configuration, logo task; (c)around configuration, word task. Orange streaks represent
instances of where two users attempted to move the same tile.

Readability. Similar to Pauchet et al. [26], the singleorientation tasks (i.e., text tasks) caused problems forparticipants in the around-the-table configuration. Whensitting with a compromised view of the text (i.e., notoriented with the text), participants frequently contorted
their bodies and heads to read the tiles. As such,participants preferred the same-side configuration:on the condition, the space was reconfigured. Once all fourtasks were complete, they answered a final survey.

“Sitting together allows us to see everything from the same
perspective, which was helpful.”Analyses. We developed a custom visualization application
to analyze the log data. During each trial, we also collected
field notes, and corroborated these notes with observations
from the video data. We were particularly interested inproblems arising from three-way collaboration that would
have been unique from dyadic scenarios.

Territoriality. We observed some spatial partitioning
consistent with [28]. Figure 5 illustrates representative
contact traces from three trials by the same group, showingthat for the logo task, in the around-the-table configuration(Figure 5a), participants partitioned their activities indifferent regions of the workspace, whereas in the sameside configuration (Figure 5b), they did not. Figure 5cillustrates the text task in an around-the-table configuration.
The spatial partitioning behaviour that we observed in theorientation-free task (Figure 5a) is noticeably absent here,
showing that the semantics of the tiles in the text taskchanges the way in which users interact with the space.

Findings

Configuration Preference. With regard to overall
preference, 9 out of 12 participants preferred the same-side
configuration. When asked for preference on a per-task
basis, for text tasks, 8 of 12 participants preferred the sameside configuration. For the logo tasks, 8 of 12 participantspreferred the same-side configuration for the logo task.
Identity. Attributing workspace activity to a remotecollaborator is difficult when there is more than one remotecollaborator. Problems with identity attribution were notobserved in the around-the-table condition, suggesting thatthe spatial distribution of users provided a sufficient spatialcue to know who was doing what. However, in the sameside configuration (when all of the hands came out from thesame side of the table) participants were sometimes
confused about whose hands were whose. The following
excerpt provides an example:
Jack studies the workspace, and suddenly sees a tile being
moved into the wrong place.
Jack: No, wait. Larry, Larry…
Larry: I’m not touching anything, dude!
Roper retracts his hands quickly upon realizing that Jackwas referring to him.
Disembodiment. We were interested in whether thedisembodiment of users’ arm shadows from their video

Many participants asked for space where they could try out
ideas without being observed by others. Consistent with
Tang’s observations [31], the workspace functions as a typeof ―stage‖ for behaviour, and the inability to rehearse, or tryout ideas in a personal space was a concern for users. Whilepersonal territories may still form when using a larger
workspace, the character of these may be somewhat
different with a same-side configuration.
Conflict. Instances where participants attempted to touchand manipulate the same tile simultaneously were coded as―conflicts‖. A t-test of log data showed that for theorientation-free tasks (logo tasks), there were significantly
fewer conflicts with the around-the-table configuration(mean=22, σ=11) compared to the same-side configuration(mean=36, σ=17), t3=2.8, p<.05. For the single-orientation
tasks (text tasks), this was not the case: groups in thearound-the-table configuration experienced a similar
number of conflicts (mean=8.3, σ=4.9) compared to thesame-side configurations (mean=13, σ=8.3), t3=1.4, p=0.31

(although statistical power is low given the small samplesize). In general, there were fewer conflicts overall for the
text tasks, which may suggest that the task structure
reduced conflict.
In the around-the-table configuration, participants remarked
that it was easier to see what everyone else was doing. This
likely contributed to the fewer number of conflicts since theusers could see remote collaborators’ arms as they
approached a tile. However, in the same-side configuration,
the user’s own hand often occluded the shadow of theremote collaborator’s hand, making it difficult to be awareof concurrent activity.
Discussion of Study 1

Both configurations we explored had utility and our results
indicate that the advantages of arm shadows (in terms ofreference space) do scale to three-way collaborations. The
around-the-table configuration demonstrated two chief
benefits: enabling spatial partitioning, and identification(therefore awareness) of others’ activities. The same-side
configuration facilitated reading and shared perspective.
The same-side configuration was also preferred by many
participants. Separation of person space and reference space
did not seem to cause any difficulties; however, occlusionof arms shadows may be a concern in same-side
configurations, especially if the number of participants isincreased even further.
Although multiple configurations are possible and likely
useful—the appropriate configuration likely depends ontask characteristics. Some relevant issues include:orientedness of the task artifacts (e.g., document editing vs.
photo sorting) and parallelism of task (demand for
simultaneous vs. serial interaction). For shared orientation
tasks, such as text documents and spreadsheets, the benefitsof the same-side configuration may well out-weigh the
problems, whereas for orientation-free tasks, around-thetable may well be the better choice.
An interesting observation from this study was thatparticipants did not make extensive visual use of the video
surrogates. While they continually spoke to discuss andcoordinate activities, participants rarely looked up from thetask space. To better understand the role of arm shadows in
the reference space, as well as the video surrogates of
person space, we designed a second study.
STUDY 2: AFFORDANCES OF REFERENCE SPACE

Although previous research has demonstrated the efficacy
of arm-based embodiments in theworkspace[32,17,18,30,26,33], we were interested in further exploring
the relationship between our proposed communicative―spaces‖. In particular, given that the participants in Study
1 did not seem to use the video surrogates a great deal, wewere interested in the relationship between person space(i.e., our video and audio channels), and reference space(i.e., the video arm shadows and trace pearls). We wanted
to better understand the importance of these channels andthe degree to which a collaborative activity would behindered if these channels were not present.
MethodConditions. We varied the presence and absence of thecommunication channels, resulting in three conditions: (1)audio + arm shadows (A+S), (2) audio + video (A+V), and(3) audio + video + arm shadows (A+V+S). Given that
individual and group variability can be high forcollaborative activities, we also wanted to gather data froma baseline condition. As such, every group also completed
the task in a co-present, face-to-face configuration.
Tasks. We used the same orientation-free ―logo‖ task from
Study 1, and created four equivalent versions of the task.
Measures. We collected log data from interactions with theworkspace and videos of each session. We repositioned thevideo cameras so that we could see where participants werelooking, though we were only able to collect this data fromtwo of the three participants.
Apparatus. We used the around-the-table workstation
configuration from Study 1 since the task for this study did
not have an orientation. The hardware and softwareremained the same except that we slightly tinted the arm
shadows (red, blue and green) to provide stronger
identification cues.
Participants. We recruited six new groups of three (18participants – four females) from the local population.
Design. We employed a three-way within subjects design
with three main conditions: (A+V, A+S, A+V+S), fully
counterbalanced. In addition, we ran a control, face-to-facecondition for each group. The face-to-face condition wascompleted first for half of the groups, while the remaining
groups completed the face-to-face condition last. Eachgroup completed one task for each condition.
Procedure. We followed the same procedure as Study 1(introduction to study and equipment, 15 minutes per task,
paper-based surveys following each condition). For theface-to-face condition, participants were brought into thesame room.
Analyses. We coded video from three of the six sessions,specifically looking for: (1) instances of interaction
problems (between collaborators), and (2) any glances,
looks or long stares from one collaborator to another (be it
through the surrogates, or in face-to-face). Finally, we usedfield notes collected from each trial to group observations.
Findings

Use of Reference Space. Participants made consistent use of
the arm shadows—both for explicit communication and formaintaining awareness of collaborators’ activities. For
example, we observed three types of deictic reference: (1)pointing at or waving their hands over a set of tiles while
speaking (―over here‖, or ―these ones‖); (2) using a
combination of task space feedthrough along with theshadows (e.g., wiggling a tile to draw attention to it), andTable 1. Coded number of glances per group/condition.
Group IDxyz

Figure 6. This sequence illustrates how the arm shadows
helped users avoid conflict. Lasting 2s, we see (a) the local user
manipulating a tile; (b) a remote user’s arm shadow
approaches from the left; (c) the remote user arrives, and the
local user begins to withdraw his hand; (d) finally, only theremote user is manipulating the tile. For print legibility, we
have traced the arm shadow and made it opaque.

(3) drawing transient outlines or circles on the workspace
itself (where tiles were not) with the trace pearls.
Consequential communication provided by the reference
space was also of significant utility to participants. Many
participants expressed that the shadows could be used tomaintain an awareness of the remote collaborators:“I liked the hand shadows because you could tell who wasdoing what and what piece people were talking about”
Awareness: Confusion and Assists. The presence of thearm shadows reduced confusion, promoting awarenessamong collaborators of each others’ activities. The
questionnaire asked participants to rate their level ofconfusion over who was doing what on a 7-point scale. The
average ratings for each condition were: A+V (mean=4.2,
σ=1.9), A+S (mean=2.2, σ=1.3), and A+S+V (mean=2.2,
σ=1.4) A Friedman test indicated that the conditions weresignificantly different from one another, Χ2(2)=14.45,
p<0.001. Post-hoc pairwise comparison using the Wilcoxon
signed-ranks test, with a Bonferonni correction, revealed
that participants were significantly more confused in the
A+V condition (without shadows) than either of theconditions with shadows (A+S and A+S+V), p<.01. No
significant difference was found between the two
conditions with shadows (A+S and A+S+V).
Accordingly, the video data revealed clear instances where
the arm shadows aided micro-coordination, preventing
potential conflicts (a relevant indicator of coordination
between the partners). Figure 6 illustrates such an example
where the local collaborator withdraws his hands as he sees
the approach of a remote collaborator, and understands the
remote collaborator’s intention of ―fixing‖ the tile.
The arm shadows also enabled a fluidity of collaborativeinteraction that was characteristically absent from the A+V
conditions: when arm shadows were present, groups wereable to aid each other in retrieving tiles. These types ofassists were comparatively absent in the A+V condition.

A+V
29
33
38

A+V+S
25
33
22

F2F
14
14
7

Glances in Person Space. Beyond the arm shadows, wewere also interested in what role person space (i.e., theaudio/visual surrogates) had in the task. Table 1 reports thenumber of times participants glanced or looked at one
another. Interestingly, that total number of glances isrelatively low, and most of the glances were extremely brief(i.e., less than one second). By way of comparison, we also
recorded the number of glances that occurred during theF2F sessions, and again, these numbers are surprising low.
Given that the task the participants were completing wasembedded in the task space (i.e., the Surface), we speculatethat there may have been little to be gained from looking up
at the video surrogates, whereas a great deal could be
gained from studying the task space and the embedded
reference space (i.e., watching other users’ activity). Given
participants’ ability to communicate through the task space(e.g., wiggling tiles), reference space (e.g., pointing or
waving), and through person space (i.e., speech), it isperhaps understandable why groups did not make extensive
use of the video surrogates.
We performed a secondary analysis trying to understandwhen glances and use of the video space were occurring.
Although not our initial focus, we observed that participants
made consistent and extended use of the video space forconversation during setup between trials (e.g., whenexperimenters were setting up the next condition, or once
participants had completed the task and were filling out thequestionnaire) rather than during the trials. This ―inbetween‖ use was often social in nature, with participants
joking and laughing with one another. During the trial
itself, we noted that glances occurred primarily at the
beginning (to discuss strategy or coordinate on ideas) andend (to confirm when all participants thought the logo
designs were complete), or when someone was uncertain ofwhat another had said or needed clarification (consistentwith[34]).
Participants’questionnaire
feedback
corroborated this analysis:“Audio: Extremely necessary, w/o it we would be lost.
Hand: helped see what others were doing. Video: notextremely useful, but it made the experience more
personal”“I never looked at the video screen because it was not
helpful to look at another person. Audio was mostimportant because of directions, and shadows helped to notgrab the same objects”“While audio and hand shadows were important the video
made the exercise feel more group like and not sitting alone
in a room.”Figure 7. Participants freely overlapped their arms in thespace in ways that would not have been physically possible inface-to-face. Note that arm shadows and pearl traces havebeen artificially recoloured for print legibility.

When asked to choose two of the three channels (audio,
video, hand shadows), 13 participants out of 18 selectedaudio and arm shadows.
Corporeal Embodiments. We found interesting differences in
the way groups worked when face-to-face compared to
distributed, particularly in how corporeal embodiments
mandated serial access to shared space, and how they enabledadditional ways to communicate and gesture.
While the Surface itself supported multiple simultaneouscontacts, when the group sat together at the same tabletop,
each participant’s physical access to the Surface appeared tobe considerably reduced. For example, if a participant
reached into the ―shared‖ region of the Surface, she would
not only claim a tile, her arm would also block both visualand physical access to the same space. The only space aparticipant would typically have uncontested access was thespace immediately in front of herself. The physicality ofparticipants’ bodies thus afforded only serial access to large
portions of the shared space. During distributed trials,
participants appeared comparatively unfettered, and took
advantage of the absence of others’ bodies, freely crossing
over one another’s arm shadows without hesitation. Figure 7,
from a participant session, illustrates how the absence ofothers’ bodies allows simultaneous, parallel activity.
Thus, note that the mere presence of another’s arm shadow
did not necessarily inhibit interaction with a workspaceartifact—instead, the arm shadow, combined with the trace
pearls mediated a user’s understanding of the state of theworkspace. The arm shadow’s presence indicates that aremote user is present, but the absence/presence of trace
pearls helps users distinguish whether an artifact is actively
being used by the remote user. Because arm shadows
frequently cover items that people are not actively using, thefine-grain awareness provided by the trace pearls is of benefit
to this micro-coordination.
The saliency of users’ physical bodies also played a role in
how users formed and used gestures compared to their use of
the arm shadows. Arm shadow gestures are projected ontoand into the tabletop workspace, thereby implying aworkspace location—even if it is not intended. Thus, users

employing arm shadow gestures tended to be fairly precise
when referring to tiles—often going so far as to tap the tile inquestion, since gestures made above Surface could be
misinterpreted if the arm shadow appeared in the wrong
place. In contrast, during the face-to-face condition,
participants used more gestures that were comparatively
vague—often away from the surface of the tabletop itself. It
may be that the more sequential nature of activity in face-toface trials meant that deictic references were as muchtemporal references as spatial ones (i.e., a reference to ―thatone‖ might as easily be a reference to the last tile that had
been touched as the one being explicitly pointed to); in
contrast, with more parallel activity of distributed conditions,such references would be more ambiguous.
Perhaps the most poignant example of this off-surface
gesturing in face-to-face trials was how participants wereable to communicate ideas. In particular, when participants
were attempting to convey an idea during distributed trials,
they would frequently resort to illustrating the ideas using thetile pieces directly (i.e., actually performing the idea). With
face-to-face trials, although we still saw similar instances ofsuch performances, users also mimed ideas: for instance,talking about and acting out (in mid-air) interactions on tiles(without actually using the tiles). Group members seemedcapable of deciphering these mid-air gestures.
Discussion of Study 2This second study allowed us to better understand thefunction of person space, reference space and task space
independent and in concert with one another. Independently,
the results
confirm the theory postulated by Buxton[5], where person space is used to resolve ambiguity and tomaintain social contact (even between tasks, as it turned out),task space for actual work activity, and reference space fordeictic and workspace-relevant gestures. With regard toreference space itself, our implementation of arm shadows
combined with the trace pearls provided both coarse-grained
presence awareness as well as fine-grained manipulation
information, both of which were used by participants indifferent capacities. Perhaps what is most interesting here is
the tight coupling and synergistic relationship between task
space feedthrough and reference space. In concert, they forma powerful source of layered information: the arm shadows
provide awareness of presence, while the feedthrough andpearl traces provide more detailed information about remoteusers’ activities. For example, during a pilot session, theparticipants were able to immediately detect synchronization
errors in the software, even before it became apparent to theexperimenters. The participants stared at the sharedworkspace, visibly puzzled, and then commented, ―What are
you doing? It looks like you’re doing something [referring to
the remote collaborator], but nothing’s moving.‖ This
example of behaviour illustrates that users were attending toboth the arm shadows, as well as interaction in the task space
to understand others’ activities.
As demonstrated in Study 1, the spatial relationship between
reference space and person space is evidently not as critical:

the relative disembodiment between remote users’ arm
shadows and video surrogates did not ever appear to be a
problem.
Our findings align with a long line of inquiry that questions
the value of video in person-space [25]. One conclusion
might be that for the particular task being investigated, it had
negligible value, and that perhaps for other types of tasks(e.g., negotiation [3, 25], or where there is more ambiguity[27,34]), it would provide more value. Such a focus on task
may sidestep the more subtle issue that perhaps we need toreconsider what is being measured with regard to the video.
In this work, coding for number of glances was admittedly
disappointing; however, trying to understand when the
glances occurred provided considerable insight.
Finally, this study raises interesting questions about how
users’ corporeal arms are used differently when collocatedcompared to the arm shadows when distributed. The inherent
nature of arm shadows causes gestures to be fairly explicit,
whereas in face-to-face trials, we saw gestures that weremore vague, yet still communicative. The projected, flattenedarm shadows have some obvious, and perhaps some subtle
property differences that users appropriate differently.
Further, since the arm shadows could not physically impede
on users’ access to the tabletop, distributed setups enabledmore parallel interaction when compared to collocatedsettings.
OPPORTUNITIES FOR DESIGN AND CONCLUSIONS

The work presented here builds on a distributed workspacesystems that provide strong embodiment—both atop theworkspace (reference space) and outside of it (person space).
We investigated, in particular, these spaces as they relate to
one another spatially, and our own reference space design inthree-party collaboration. Our findings suggest several
opportunities for design, as well as promising avenues for
exploration.
Role of Social Coordination. This study reinforces theargument that with sufficient awareness of others’ activities
in a shared workspace, users can smoothly coordinate theiractivities, obviating the need for certain types of ―locks‖ onshared entities at the application level. This was evident inboth participants’ avoidance of collisions, as well as the
micro-coordination required for assisting one another. An
interesting question is the extent to which this coordination
can be achieved using more simple reference spacemechanisms (e.g., telepointers).
Embodiment Presentation and Control. Our use of video forusers’ arm shadows resulted in characteristicallyanthropomorphic embodiments, although it is worth
considering
non-video
and/ornon-anthropomorphic
approaches, which may have lower network bandwith
demands (e.g., [30]). The projection of arms onto the planar
tabletop also fails to capture the proximity information that ispresent in face-to-face scenarios—an awareness cue thatmight be of utility for coordination. Finally, someparticipants asked for more control over the saliency of thearm shadows—suggesting that the saliency of arm shadows
can also be a coordinating mechanism (e.g., more opaque
when performing explicit gesture vs. more transparent whenworking independently). A fruitful avenue may be semiautomated control, where the opacity of the arm shadows is
linked to speech from each workstation (e.g., if you are
speaking, your arm shadows become more opaque).
Configuration and Spatialization. This work provides further
evidence that fluid collaboration can still occur when person
and reference space are divorced. We demonstrated two
configurations of these spaces, suggesting that theappropriate configuration will depend on task characteristics.
Thus, there are likely a whole host of configurations that may
be of differential utility—for example, tools that allow users
to flexibly control their orientation to the workspace.
Spatialization was important to us, and we fixed several
parameters with regard to person space (e.g., we carefullyaligned speaker, LCD and webcam for each surrogate). An
interesting consequence is in how users respond to thisspatialization: in one trial, where the video channel was
removed, we observed a user lean in and chastise the speakerthat was associated with the collaborator he was arguing
with—in effect, responding to the audio source of thatcollaborator.
Finally, users’ desire to identify the owners of embodiments
and actions underscores both the utility of studying threeperson collaboration, and the need for scalable designs.
Spatializing users and tinting the video helped to provide thisidentity cue, but other approaches may also be viable.
In this work, we have sought to build on existing systemdesigns, attempting to extend them for three-way
collaboration. We have explored several design choices, andempirically studied the consequences of these choices. Ourstudies have raised issues of spatial configuration, identity,scalability of embodiment designs, and workspacecoordination, which will continue to be of pertinence in thisresearch space.
ACKNOWLEDGEMENTS

We thank Sara Bly, Carman Neustaedter and Saul Greenberg
for discussions about the ideas in this paper. We also thank
Libby Hanna, Koji Yatani, and Chris Brooks.
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