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examples of the spatial approach include media spaces
[4],spatially oriented video conferencing [5,6,7], collaborative
virtualenvironments [2,8] and tele-presence systems [9].In contrast
to CSCW systems, di rect collaborative real-timeinteraction leads
to completely new interaction possibilities,especially concurrent
interaction of at least two users with one ormore objects.
Unfortunately there is a lack of application anddesign support for
CVEs. In addition to that most of the CVEsunder investigation are
web based collaborative VirtualEnvironments. Good overviews about
examples of these desktopCVEs can be found in [2,3,8,10,11,12,14].
There exist only a fewapproaches of back projection based VEs and
CVEs. Anoverview of these approaches can be found in [15,16, !
7,18].The work presented in this paper, explores some of the
issuesrelated to human-to-human interaction mediated via
projection-based VE. In particular, issues around the use of video
and audiofor immersive telepresence and input device
representations inCVEs are explored in a number of different
evaluation sessions.In addition, the effects of disturbance factors
typical of projection-based VEs such as cabling, use of shutter
glasses are elaborated.The results and their analysis provides
important insights into theway these issues affect co-presence and
co-working in CVEs andallow for design guidelines to be
developed.3. T H E O R Y O F E V A L U A T I O NWe developed a
distributed Virtual Environment for medicalapplications, that
allows two or more sites to share the samemedical data set and
manipulate it collaboratively in real time.The CVE allows the
distribution of a virtual human data set, thatincludes detailed
body skin, underlying skeleton and heartmodels. The funct ionali ty
of the CVE allows users to cut the skin,pick, manipulate and query
information about the bones o f theskeleton, observe animations of
the heart's function and modifythe transparency of the heart 's
tissue.
Figure 1 . Med ica l V E appl i cat ionIn Figure 1 the
functionality of the VE in single user mode ispresented. The model
is that of a human skeleton covered by skin.The skin can either be
made transparent using a three-dimensionalslider or cut using a
special skin cutter tool. These operations areselectable from the
dynamic ring menu which appears when the
user requests the functionality of the data set. In addition,
bonescan be positioned and their names can be queried [24].The CVE
is implemented in AVANGO [28]. Therefore, we areable without any
modifications to use a variety of input and outputdevices. In this
paper, we use two Collaborative Responsiveworkbenches [26,27], with
a stylus and a three-button tool asinteraction device at each site.
The two sites are connectedthrough a 100Mbps fast Ethernet. An
Onyx2 Sil icon Graphicscomputer at each site renders and drives the
two output devices.One 02 Silicon Graphics workstation captures and
sends real-time audio and video of the local participant while
another one isreceiving the real time audio and video of the remote
participantat each site. Obviously, lower cost hardware can be used
both forrendering and for the audio/video communication.
Theavailability of the particular equipment and its high
performanceprovide us wi th an easy choice.Usabil ity sessions
carried out by VR experts not involved in theinitial design,
resulted in an improved system. This system hasalso gone through an
initial evaluation in a single user evaluationsession. The system
is improved in terms of menus and toolrepresentations and overall
functionality as a single user virtualenvironment. Thus, any
usability issues related to the design andfunctionality of the VE
in single user mode have been eliminatedprior to the evaluation of
the co-presence and co-working in thedistributed scenarios.We focus
in this paper on the two evaluation sessions that aremore closely
linked to co-presence and co-working. Threeadditional ones are also
included in the variation group analysisfor cross-checking of the
analysis results.3 . 1 Ev a l ua t i on S e s s i onsCo-presence
SessionThe purpose of this co-presence session is to evaluate the
CVE i nterms of its support to the learning experience using
immersivetelepresence only. The scenario for this session is that
of adistance learning paradigm. A teacher and a learner
arecommunicating via a video/audio connection each working on
aCollaborative Responsive Workbench (two-sided
responsiveworkbench). The vir tual data set is that of a human
skeleton and itis distributed to the two sites, so that changes
made by the studentare immediate ly visib le on the teacher's
site.In this session, the learner is also the evaluator of the CVE.
Thetask is for the learner to position three bones precisely in
theircorrect location on a human skeleton. These bones lie in front
ofthe evaluator and look very similar to each other so that it is
notobvious where they have to be added to the skeleton. The
teacherexplains the task, the data set, the input devices and the
toolsremotely to the evaluator. If the evaluator does not know how
toachieve the goal the teacher g ives advice about which tool
shouldbe used, how to query in formation about the bones, how to
changethe viewpoint etc. In this session the remote teacher does
not useany input devices or tools. Only gestures and verbal
instructionsare used.When the task is completed the co-presence
session is ended and aquestionnaire addressing co-presence
assessments is handed out tothe learner. After the questionnaire is
filled out the leaner has a
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f ive minutes recovery t ime before start ing the co-work
evaluationsession.C o - w o r k S e s s i o nThe purpose of the
co-work session is to evaluate the CVE interms of i ts support for
collaborative work. In this session there isagain a local learner
and a remote teacher (see Figure 2).However, both partners have
equal r ights concernin g decisions,manip ulations and the access
to tools. I t is interesting to note thatboth users can comp lete
the task autonomo usly as well. Therefore,this session tests if
CVEs encourage team work even though suchwork is not essential for
completing this task.
Figure 2. Collaborative Medical Respo nsiveWorkbenches (RWB)The
learner is again the evaluator of the CVE. The task is sl
ightlydifferent from the one of the co-presence session. Together
theusers have to posit ion six bon es belo nging to three different
pairsto complem ent the human ske le ton . Each bone in a pa i r be
longs tothe left or r ight side of the skeleton (i .e . the femur
bone of theright and the left leg). A set of three o f these bones
l ie in front ofeach user. As the users stand opposite each other
on differentsides of the skeleton they have to f ind out which
bones belong totheir side as the bones are mixed. This can be done
by queryingthe name o f the bone .If a user f inds out that a bone
belongs to the partner 's side thisbone can be exchanged by pass
ing i t over . Af te r a bone has been
pos i t ioned in the ske le ton the user can make use o f a
snap-backtool which allows the selected bone to snap back to the
correctposit ion. This facil i ty enables users to evaluate and
verify theirown o r their partner 's work.In order to complicate
the task the hum an skeleton is covered withthe skin. For posit
ioning the bones the part icular part of skeletonneeds to be mad e
visible by cutt ing away the skin in this region.For doing so one
user selects the cutt ing tool from the r ing menuand applies this
to the interesting part of the human body.However, i t is not
possible to cut the skin in this regionperm anen tly and therefore
the user applyin g the cutt ing tool has tohold the tool while the
other user posit ions the bone.When the six bones are posit ioned
correctly into the skeleton, thesess ion is comple ted and the
co-work ques t ionna i re is handed outto both users.U s e r P r o
f i le sAnalys is o f an in t roduct ion ques t ionna i re a l
lowed us to produce auser profi le. The age of the 60 evaluators is
from a minimum 17years to a maximum 58 years. The majority are
between 22 and27 years old. Most of these evaluators are universi
ty studentswhereas the diversi ty of the other users ' professions
reaches frompersonal assistants, journalists and workers, to
technicians andcomputer sc ience or non-computer sc ience univers i
ty professorsand researchers.Al l eva lua tors a re no t V i r tua
l Envi ronm ent exper ts . However ,the i r knowledge concern ing
computer hardware and sof twarediffers substantial ly. The group of
22-27 year old uses thecomputer mos t ly for web sur f ing as wel l
as computer games ,whereas he older evaluators use i t mostly for
edit ing with textprocessing software. Therefore, the f irst group
is moreexperienced with hardware devices, such as game
joysticks,steering wheels inclu ding force feedback. This ob
servation isindependent from the subject ' s profession or f ield
of studies. Acontrary result is that the older evaluators use a
computer almosttwice as long per week as the group of the 22-27
years old. Noother significant differences betwe en the evaluators
that mighthave an imp act on the analysis of the evaluation results
are found.3 . 2 Ev a l ua t i on Me t hod sThere exist three
different evaluation methods which can beappl ied to Col labora t
ive Vir tua l Envi ronments : the exper theurist ic, the formative
and the summative evaluation [21,22,23].The expert heuristic
evaluation is an analytical method, while theformat ive and summat
ive eva lua t ions a re empir ica l andobservational methods
[25].In the expert heurist ic evaluation the evaluator is a f ield
expertwho de te rmines problems wi th usabi l i ty in the des ign
phase of theCVE.The output of the format ive eva lua tion i s a
combina t ion ofquali tat ive and quanti tat ive results . The
quanti tat ive dataeva lua tes the amoun t of t ime , the num ber
of t r ial s , the nu mber ofmistakes etc. while perform ing a
special task. The quali tat ive datacan be obtained by observing
so-called cri t ical incidents [21]. Acrit ical incident is a
problem that occurs wh ile a user is interacting
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