Keywords:
CSCW, groupwork, media spaces, video
Introduction
Media spaces are computer-controlled networks of audio
and video equipment designed to support collaboration [2,
4, 6, 14, 17, 25]. They are distinguished from more
common videophone, video-conferencing, and video
broadcasting systems in that they are continuously
available environments rather than periodically accessed
services. Because maintaining high-bandwidth
connections is costly, current video services are typically
used for planned and focused meetings. Media spaces, in
contrast, assume a future in which broadband networks are
commonplace, the data rates needed for high fidelity video
and audio are a trivial fraction of the total available, and
thus the systems can be left "on" all the time. In practice,
this is usually simulated in-house, using dedicated
networks of analog audio and video cables leading from
central computer-controlled switches to equipment in
offices and common areas.
The constancy of connections that characterizes media
spaces has several implications for how they are used.
Because they are not associated with special events (e.g.
meetings), they become part of the everyday work envi-
ronment. It is common always to have a video connection
somewhere, often to a common area, if only because such
views are more pleasant than blank monitors. This im-
plies, finally, that the proportion of time spent using the
media space for meetings is relatively low, and instead
they often are used to support a more informal, peripheral
awareness of people and events. Again, this distinguishes
them from more commonly encountered video systems.
Trouble in Media Space
While there is anecdotal evidence that media spaces can
support professional activities [2, 6, 13, 14, 17], and par-
ticularly long-term collaborative relationships [1, 3],
quantitative data supporting the value of these technologies
have been more difficult to find. Typically studies show
that adding video to an audio channel makes no significant
effect on conversation dynamics or on the performance of
tasks that do not rely heavily on social cues [7, 19, 20].
Even in a more naturalistic media space setting, Fish et al.
[4] found that people usually used the Bellcore system as a
prelude to physically co-present meetings, and concluded
that it did not clearly add to the functionality provided by
telephone or email systems.
These results seem relevant primarily for the role of video
in supporting relatively focused interactions. But one of
the motivating intuitions behind early media space research
was that they might help create and sustain a more infor-
mal sense of shared awareness [e.g., 2, 17, 25]. The fact
that much recent research meant to assess media spaces
seems focused on more formal uses may be because of the
difficulty of finding quantitative data that addresses their
informal possibilities. In any case, it has made it difficult
to assess these original intuitions.
Access to the Task Domain
Observational and analytic studies, on the other hand, have
suggested limitations on the ability for media spaces to
support informal shared awareness. For instance, Heath
and Luff [9] described how co-present collaborators shape
their activities and utterances for their partners, and con-
trasted this with the difficulties they observed in
organizing these sorts of visually mediated activities in and
through media spaces [10]. They concluded that while a
great deal of everyday collaboration is mediated by access
to colleagues in the context of their tasks, this sort of
access is often not provided by current media spaces.
A similar point was made by Nardi et al. [15] in their study
of video used during neurosurgery within operating rooms
and remote offices. Video is important in such settings be-
cause it allows visual access to events that are otherwise
inaccessible (e.g. cameras are pointed into the head during
brain surgery), and thus provides the awareness necessary
for coordination. The emphasis of this sort of application
is on visual access to tasks, not faces; thus Nardi et al. [15]
recommend "turning away from talking heads" and instead
focusing on "video-as-data."
This work is valuable in emphasizing the ability for video
to support awareness of task-related artifacts. It is less
convincing as a case against face-to-face video, however,
since giving access to tasks is not incompatible with giving
access to people. From our perspective, the point is not
that cameras should be focused away from people towards
workbenches (or skulls, as the case may be), but that the
narrow focus of video itself must be broadened.
Extending Affordances
Gaver [5] analysed the affordances of media spaces to
understand how the technologies shape perception and in-
teraction. This analysis emphasized several limitations on
the visual information media spaces convey:
- Video provides a restricted field of view on remote sites.
- Video has limited resolution.
- Video conveys a limited amount of information about
the three-dimensional structure of remote scenes.
- There are discontinuities (or "seams," [13]) on the edges
of scenes and between views from different cameras.
- There are also discontinuities between local and remote
scenes and their geometries.
- The medium is anisotropic: the discontinuities between
local and remote geometries are not reciprocal (and thus
not predictable).
- Movement with respect to remote spaces is usually diffi-
cult or impossible.
Each of these attributes has implications for collaboration
in media space. But the inability to move with respect to
remote spaces may be most consequential of all. As
Gibson [8] emphasized, movement is fundamental for per-
ception. We move towards and away from things, look
around them, and move them so we can inspect them
closely. Movement also has implications for the other con-
straints produced by video. If we can look around, we
increase our effective field of view. Moving can compen-
sate for low resolution [21]. It provides information about
three-dimensional layout in the form of movement parallax
[8, 16, 22]. Finally, movement might allow people to
compensate for the discontinuities and anisotropies of
current media spaces.
Allowing Movement in Remote Spaces
One approach to approximating movement within remote
sites was explored using the MTV (for Multiple Target
Video) system, which employed several switched video
cameras in each of two offices [7]. Observations of six
pairs of partners collaborating on two tasks indicated that
the increased access was indeed beneficial. Participants
used all the views, and were often creative, finding unex-
pected ways to gain access to their colleagues and their
working environments. In fact, they accessed face-to-face
views for much less time than views that included places
and objects relevant for the tasks. This supports sugges-
tions that access to task domains may be more useful than
access to colleague's faces [10, 15]. However, participants
did seem to rely on quick views of their colleagues as a
way to assess attention and availability; looking times may
be misleading as a basis for judging the importance of
these views.
Though multiple cameras provided valuable visual access
for collaboration, a number of problems with this strategy
became clear. Despite the proliferation of cameras (and
associated clutter), there were still significant gaps in the
visual coverage provided. In addition, participants seemed
to have problems establishing a frame of reference with
one another, and in directing orientation to different parts
of their environments. One result was that the video
images themselves became the shared objects, rather than
the physical spaces they portrayed, and participants would
point at these images rather than the offices themselves. In
general, the greater access provided by multiple cameras
seemed outweighed by the addition of new levels of
discontinuity and anisotropy.
Despite these problems, increasing visual access to remote
environments seems a clearly desirable goal. In this paper,
we describe another approach, involving the creation of a
Virtual Window that allows true visual movement over
time, rather than a series of views from static cameras. By
providing an intuitive way to move remote cameras, we
believe we can overcome many of the limitations of video
for supporting peripheral awareness without introducing
the problems that come with multiple cameras.
THE DELFT VIRTUAL WINDOW
The basic idea of the Virtual Window is that moving in
front of a local video monitor causes a remote camera to
move analogously, thus providing new information on the
display (see Figure 1). To see something out of view to
the right, for instance, the viewer need only "look around
the corner" by moving to the left; to see something on a
desk, he or she need only "look over the edge," and so
forth. The result is that the monitor appears as a window
rather than a flat screen, through which remote scenes may
be explored visually in a natural and intuitive way.
Figure 1. The Virtual Window: Local head locations are
detected by a tracking camera and used to control a moving
camera in the remote office. The effect is that the image on the
local monitor changes as if it were a window.
Movement Parallax and Depth Television
The Delft Virtual Window was invented originally as a
means for creating depth television, allowing information
for three-dimensional depth to be conveyed on a two-
dimensional screen [16, 22]. The system creates the self-
generated optic flow patterns that underlie movement
parallax. As the head is moved around a focal point
(shown in Figure 1), objects appear to move differently
from one another depending on their distances (this is easy
to see by moving one's head around an object while
focusing on it: objects in the background seem to move
parallel with the head, while those in the foreground move
against it). Movement parallax is well suited for depth
television because it does not require different images to be
presented to both eyes. Indeed, similar methods have been
used for computer graphics [12], but the Delft Virtual
Window is the first system that provides movement paral-
lax around a focal point for realtime video [23].
The Virtual Window has been tested experimentally by
comparing people's accuracy at judging depth in remote
scenes when they were viewed from static cameras, from
moving cameras that they did not control, and from the
Virtual Window system [16, 22]. A clear advantage was
found for the Virtual Window system over static views,
and a significant decrease in variability of depth
judgements when compared with those made from
passively viewed moving scenes. The experimental
evidence thus supports the intuitive impression that the
Delft Virtual Window can do a good job of conveying
depth information.
Affordances for Increased Access
It is difficult to implement Virtual Window systems with
the speed and accuracy necessary to give very good
impressions of depth. But the technique gives rise to a
number of other, serendipitous affordances that make even
less-ambitious versions potentially beneficial for media
spaces.
Field of View
Because the camera moves around a focal
point, it provides access to a much larger area of the
remote scene than stationary cameras do (see Figure 2).
The distance of the focal point from the camera determines
the effective field of view. If it is set at infinity, for
example, the camera moves only laterally and relatively lit-
tle is added to the field of view. At the opposite extreme,
if the focal point is set at the front of the camera itself,
there is no lateral movement and the camera movement is
equivalent to that provided by a pan-tilt unit. The field of
view is greatly expanded, but parallax information for
depth is lost.
Resolution
As Gaver [5] pointed out, for static cameras
there is an inherent tradeoff between field of view and
resolution. This conflict does not exist for moving
cameras: Not only is the effective field of view increased
by allowing movement, but Smets et al. [21] have shown
that information for fine details can be obtained over time
from a moving camera: effective resolution is increased as
well.
Continuity of the Remote Scene
Although the greater field
of view offered by the Virtual Window must be accessed
over time, new views are linked continuously. Instead of
jumping from one view to another, one moves smoothly
among views, making it easy to understand how they relate
to one another. This contrasts with the MTV system, in
which jumps among views introduced gaps and discontinu-
ities that seemed to impede orientation [7, 11].
Figure 2. If cameras are stationary (A), local movements do not
change the field of view, but do introduce discontinuities
between local and remote spaces. In the Virtual Window system
(B), local moves can provide a greater field of view continuously
with local visual changes.
Continuity with the Local Scene
If visual movement within
the remote scene appears continuous with movement-
induced shifts of perspective on the local one, the sense of
continuity in and through media space should be increased.
The glass screen of the video monitor will continue to act
as a barrier between local and remote spaces, of course, but
no longer spaces with different physics (i.e., in which head
movements produce different visual consequences).
Control and Coordination:
Finally, local control over re-
mote cameras has several implications for perception and
interaction in media space. Not only does it imply a larger
field of view, but one available for active exploration
rather than one depending on passive presentation. This
may help support coordination with remote colleagues. As
a simple example, it is common to hold something up to
show a remote colleague, only to misjudge and hold it par-
tially off-camera. Correcting the error usually requires ex-
plicit negotiation ("a little to the left...no, my left!"). The
Virtual Window system allows the remote viewer to com-
pensate for his or her partner's mistake simply by moving,
without requiring any explicit discussion about the me-
chanics of the situation.
The combination of these affordances - the ability to
expand the field of view, to raise the effective resolution,
to increase the continuity within and between spaces, to
support control and coordination, and to provide depth
information - make the Virtual Window concept appealing
for media space research. In the following sections, we de-
scribe our approach to implementing such a system and
our experiences with the prototype we built.
EXPERIENCES WITH A VIRTUAL WINDOW
We collaborated to design, build, and assess an instantia-
tion of the Virtual Window system. Most of the design,
implementation, and initial programming were done at the
Delft University of Technology. Two of the three devices
were then installed, the software ported and developed, and
the results tested at Rank Xerox Cambridge EuroPARC.
There are three separate aspects involved in instantiating a
Virtual Window system:
- Head-tracking The location of the viewer's head with re-
spect to the monitor must be determined.
- Camera-moving The camera must be moved in the re-
mote site.
- Mapping The head location must be mapped to a desired
camera location.
A number of approaches may be taken to these issues [16,
22]. The prototype we built depended on a combination of
idealistic goals (e.g., hands-free operation) tempered -
sometimes betrayed - by pragmatic realities (e.g., cost of
implementation). In the end, the process of designing,
building, and trying it ourselves taught us, at least as much
as watching it in use, both about the fundamental issues at
stake and about the realities of implementation. Here we
describe our tactics in some detail, and discuss some of the
implications for our experiences with the system.
Head-tracking
We decided at the outset of the project that head-tracking
should be accomplished without requiring users to wear
any special devices or clothing. This seemed crucial if the
Virtual Windows were to be used as casually as the rest of
EuroPARC's media space. However, this precluded the
use of commercially available devices such as Polhemus
sensors or infrared trackers. Instead, our version of the
Virtual Window uses image processing on a video signal to
determine head location.
For our implementation, a "tracking camera" is mounted
on the local video monitor (Figure 1) and the incoming
video stream is processed to extract the viewer's head loca-
tion. The basic image processing strategy is shown in
Figure 3. First, a single frame is digitized from the head-
tracking camera when nobody is in view; this is used as the
reference image. While the system is running, the refer-
ence image is subtracted from each incoming video frame,
leaving a difference image that is processed to find an area
of large differences assumed to be the viewer's head.
Finding such an area is at the heart of the image processing
algorithm. First the differences along the rows of the im-
age are summed, giving a difference profile for the height
of the image. A threshold is set between the overall aver-
age of the differences and the greatest difference, and the
top of the head is taken to be the first row of the image
from the top that crosses the threshold (the head is
assumed to be upright in the image). Then a horizontal
difference profile is taken from a row on or just below the
supposed top of the head, and a new threshold is set. The
first cells to exceed this threshold from the right and left
are assumed to be the sides of the head, and the center of
the head to be halfway between the two.
Figure 3. Head-tracking is accomplished by looking for values
over threshold in a difference image produced by subtracting a
reference image from each incoming frame.
A number of small variations can be used to improve this
basic algorithm. For instance, it is useful to set a threshold
for the minimum distance required before moving the
camera. This helps to avoid spurious camera jitter caused
by small fluctuations between successive frames.
This algorithm is simplistic in a number of ways. For in-
stance, it does not recognise a head per se, but only areas
where the incoming image is very different from the refer-
ence image. This means that the algorithm will track any
source of change, such as a moving hand. It also means
that the algorithm is very sensitive to changes in the ambi-
ent light, since these tend to introduce spurious differences
between the incoming and reference images. Finally, it
implies that more than one source of difference - such as
two people in the tracking camera's field of view - may
cause it to return inaccurate values (it tends to track who-
ever is higher in the tracking image, and returns an average
horizontal value if they are at the same level). This is a
manifestation of the more fundamental problem of scaling
the Virtual Window to provide the correct visual informa-
tion to more than one viewer.
Nonetheless, the algorithm works surprisingly well for all
its simplicity. When conditions are good, the algorithm
produces generally accurate values allowing a viewer's
head to be tracked even against a cluttered background.
Clearly there are more sophisticated approaches that might
be used for this task, but there are severe constraints on the
amount of processing that can be done while maintaining
reasonable system latency. Even using this simple
algorithm, we only achieved rates of about 3 - 7 frames per
second on a Sparcstation 2; more accurate algorithms
might not be worth still slower rates.
Camera-Moving
To move a camera around a focal point, recreating the
optics of looking through a window, it is necessary both to
rotate it and to move it laterally. This means that
commercially available pan-tilt units are inadequate, unless
the focal point is set to the front of the camera and no lat-
eral movement is required.
We constructed our camera-moving apparatus from two
A3 size flat-bed plotters that originally used software-
controlled stepper motors to move pens over paper. We
modified them extensively, cutting away most of the flat
bed to reveal the basic frame, moving the control boards,
and mounting them together so they would stand vertically
(see Figure 4). The two pen transports are used to move
the front and back of a Panasonic thumb-sized camera
separately; each is powered by two stepper motors
controlled over an RS232 link by the host computer.
Though we had originally planned to use the built-in
hardware and software to control the motors, this produced
only instantaneous acceleration and deceleration, which led
to unacceptably shaky camera movement. We hired an
electronics contractor to develop new control hardware and
software, which greatly enhanced the system by allowing
smooth acceleration and deceleration of each motor
separately.
Figure 4. The camera transport mechanism uses two transport
arms to move the front and back of a thumb camera separately.
The camera transport mechanism uses two transport
arms to move the front and back of a thumb camera separately.
The camera-moving devices are successful in being able to
move a camera relatively quickly and smoothly over an
area of about .35 X .2 meters. However, when two of them
were moved from the large workshop in Delft where they
had been designed and initially tested to the smaller,
quieter office environment in Cambridge, it quickly
became apparent that they are far too large and noisy to be
acceptable for office use. Each of the devices takes up a
volume of about .7 X .5 X .2 meters, and has a footprint of
roughly .8 X .5 meters, larger than most of the video
monitors being used. In addition, the motors cause audible
vibrations in the frame. When we changed the system to
allow each of the four motors to accelerate and decelerate
independently, as described earlier, the noise problem was
greatly exacerbated because each motor introduced its own
independently changing frequency component. The
resulting noise, though sounding impressively like a
science fiction sound effect, is clearly too intrusive to be
used in an office environment. In sum, the camera-moving
devices have been adequate for our initial research, but a
different design would be necessary for longer-term use.
Mapping Head Location to Camera Movement
A final issue for implementing a Virtual Window is the
mapping between head and camera location. We discuss
two aspects of this here: the determination of the focus
point and errors caused by the expression of location as a
point in the tracking camera's picture plane.
Determining the Focal Point
One difficulty in implementing the virtual window system is in determining the focal
point about which the remote camera is to move. Ideally,
the viewer's actual focus could be determined by
measuring gaze direction, convergence and
accommodation. In practice, this seems difficult at best,
not clearly necessary depending on the aims of the system,
and almost certainly unfeasible if the system is to be used
casually.
For our prototype, then, the focal point was set by the user
using a simple graphical interface. We assumed that the
focal point is always on a line extending from the center of
the camera moving device. By taking the origin of our
movement coordinates at that point, we can express the
focal point simply as the ratio of front and back camera
movements (see Figure 5). If the ratio is 1, the focal length
is infinite, the front of the camera moves as much as the
back, and the effect is one of lateral movement with no
rotation. If the ratio is 0, the focal point is the front of the
camera, and the camera only rotates without moving
laterally, just like a pan-tilt unit. Intermediate ratios give
intermediate focal lengths.
Figure 5. The focal point, f, can be expressed as the ratio of front
to back movement. When f is 1, the focal point is at infinity and
the camera only moves laterally. When f is 0, the focal point is
at the front of the camera and the effect is like a pan-tilt device.
Here the camera is shown as it moves around an f of .5 from top
to bottom.
Angular Locations and Visual Information
For our prototype, we simply mapped the pair of coordinates returned
by the head-tracking software to a new location for the
back of the camera so that the maximum values of each
would map to one another. This seems satisfactory in
practice, but in reality it leads to systematic differences
from the optical changes that movement in front of a
window would make. In Figure 6, for instance, the two
heads are both on the edge of the tracking camera's field of
view, and so would return the same head locations and
receive the same view from the remote camera. But if the
monitor were really a window, the views would be
different, as indicated by the lines of sight shown in the
figure. This disparity arises because the edges of the
tracking camera's image plane do not map to the edges of
the monitor. The practical consequences of this disparity
are unclear - again, our simple mapping seems satisfactory
- but the issue bears consideration.
As we suggested earlier, implementing a Virtual Window
that can move a remote camera with the speed and
accuracy necessary for veridical depth perception is
difficult; some of the issues we have just discussed should
make clear why this is so. We relaxed a number of the
requirements for our prototype, since we were less
interested in producing convincing depth information than
we were in exploring the other affordances offered by the
Virtual Window. Nonetheless, in many cases the changing
scene provided by our implementation does evoke a good
impression of depth (albeit at the wrong scale: often the
remote office seems like a relatively small box). More
importantly, the prototype has allowed us to explore some
of the possibilities of using the Virtual Window to provide
greater access to remote sites.
Figure 6. Equal locations in the tracking camera's picture plane
should sometimes map to different camera positions.
Observing the Virtual Window in Use
To observe the system in use, we had six pairs of partici-
pants use it in pursuing two simple collaborative tasks.
Subjects sat in separate offices, each controlling camera
movement in his or her partner's office using the Virtual
Window. The first task was called the Room-Draw Task,
and required each participant simply to draw a floor-plan
of his or her colleagues' office. The second task was the
Overhead Projector Design Task, which asked the partners
to redesign an overhead projector so that the lens-carrying
arm would not block the audience's view. These tasks
were modelled after similar ones used previously to assess
collaboration in media spaces [7, 11]. They are designed
to be simple, easily understood and motivated, and to focus
on participants' access to their remote colleagues' environ-
ment.
Our observations tended to confirm the advantages, and
emphasize the deficiencies, that we had noticed in develop-
ing the system. In the following, we briefly describe the
problems that participants had with the system, then the
advantages it provided.
When It Was Bad…
The first two pairs of participants used the system on a
beautiful spring day, with white clouds racing over a bright
blue sky. Unfortunately, this provided a compelling
demonstration of the head-tracking algorithm's susceptibil-
ity to variations in ambient light. The reference images we
used could not be representative of the wide ranges of
room illumination, and so the cameras often moved errati-
cally as the head-tracking algorithm located the areas of
greatest momentary difference, even though these were of-
ten due to the shifting light.
The results were extremely puzzling and frustrating to the
participants in the study, who had not used the Video
Window before, and who for the most part were relatively
naive about media spaces in general. The movements of
the view were only partially related to their own move-
ments, and it seemed that because they were new to the
system they had little comprehension of what or whether
anything was going wrong. In effect, they became passive
rather than active observers of the remote scene, a situation
that has been shown experimentally to produce worse
performance than if no motion were provided [21].
In any case, there was little that participants could do to
correct tracking problems except to take a new reference
image, which required ducking under the table so that they
would not be in view of the tracking camera. On occasions
when the view would show an area of the remote office
that was useful, participants would often freeze in an
attempt to keep the camera from moving. Ironically, in
these circumstances a stationary camera would have given
the participants better access to the remote site than a
moving one - a point to which we return.
But When It Was Good...
Fortunately, the remaining participants were tested on
cloudy days more typical of England, which meant that the
systems were relatively accurate and stable. In these
conditions, several advantages of the Virtual Window
became clear. For example, there were several instances in
which a participant would move slightly to achieve a better
view on something his or her partner was displaying; thus,
as we had expected, the system appeared to allow subjects
mutually to negotiate orientation. In addition, there were
occasions in which the system seemed to help participants
maintain awareness of their partner's field of view, by
increasing their awareness of the camera and its orientation
(though this may in part have been due to the salience of
the camera-moving device).
Most importantly, though, the Virtual Window did succeed
in allowing participants to explore their partner's office
visually, and the mapping between local movements and
remote views appeared natural to the users. It seems
difficult to convey the force of this result because of its
simplicity. For instance, when one participant wanted to
look down and to the side, he simply stood up and moved
to the side. This sort of observation seems easy to
overlook in the midst of the many difficulties people had
with the current system. But the fact that this is possible at
all, and that it seemed so natural, is a major success of the
Virtual Window system.
CONCLUSIONS
Providing the ability to move with respect to remote spaces
seems a clearly desirable goal. But our experiences with
the Virtual Window, as well as with the earlier MTV
system [7] suggest that the vague notion of "remote
movement" should be decomposed. From this perspective,
experiencing a monitor as a window requires:
- user access to new views of the remote site
- linked continuously in space and time
- produced by local head movement
- with enough speed and accuracy for movement parallax.
This decomposition is useful in comparing strategies for
providing greater access to remote scenes. For instance,
the original MTV system [7] provided new views of
remote sites, but they were not linked continuously in
space or time. A later version, which replaced switching
with multiple monitors [11], allowed continuous access
over time, but there were still discontinuities (gaps) in
spatial coverage. Pan-tilt-zoom units provide both sorts of
continuity, but are typically controlled by joysticks and
similar devices. Finally, the Virtual Window we built
enables head-tracked camera movement, but not true
movement parallax.
Though the prototype we built is too slow and inaccurate
to provide good movement parallax, and too large and
noisy for everyday use, many of the problems we encoun-
tered seem less like inherent failings of the concept and
more like challenges for iterative design. We may have
been too ambitious in our design, rejecting reliable off-the-
shelf equipment and using less-reliable custom solutions in
an attempt to avoid compromising our ideals about how
the system should work. Nonetheless, the prototype does
illustrate some of the potential advantages of the Virtual
Window approach. In addition, it opens a space of possi-
bilities for the design of systems that allow much richer
access to remote sites.
For instance, the inaccuracy of the head-tracking algorithm
was clearly due to its reliance on an accurate reference
image. There are several possibilities for increasing the
robustness of this algorithm. If the overall differences be-
tween the incoming and reference pictures are consistently
large, for example, it might be assumed that the reference
image is out of date and the user could be notified. More
fundamentally, greater accuracty can be obtained if the
reference frame is updated adaptively [e.g., 24]. Another
possibility is to replace the reference image with the results
of low-pass filtering the current stream of images; this
would have the effect of blurring out any movement (e.g.,
of the head) and helping to compensate for shifts in light.
Finally, other head-tracking techniques might fruitfully be
explored, including those which require users to wear
special devices.
Similarly, we might expect that further iterations of the
camera-moving system would greatly help with its size and
noise. One possibility is to shift priorities from providing
movement parallax towards providing a greater field of
view. This would imply that lateral movement is unneces-
sary and allow the use of a commercially available pan-tilt-
zoom unit. An additional advantage of using an off-the-
shelf unit would be the opportunity to incorporate zoom as
well, so that leaning towards the monitor might cause the
camera to enlarge the image around the focal point. In
fact, we are currently exploring such a system with
Koichi'ro Tanikosi, Hiroshi Ishii, and Bill Buxton at the
University of Toronto.
A more radical design option is to avoid moving a camera
at all, and instead to produce a shifting view on remote
scenes by moving a window over, and then undistorting,
the view from a fish-eye lens. Apple Computer has devel-
oped a similar strategy for creating Quicktime "virtual
reality" [18], but not for use with realtime video. The pro-
cessing demands of such a strategy are quite high, but it
has a number of advantages. Not only would it eliminate
the difficult problems of mechanically moving the mass of
a camera very quickly with no discernible vibrations, but it
would also do away with the problem of scaling the system
to deal with multiple, distributed remote viewers. It is not
clear that the strategy could be extended to produce lateral
as well as rotary camera movement, but it seems well
worth further investigation.
Finally, it is also desirable to design for the enduring
differences between Virtual Windows and real ones. For
example, a clear finding of our user study was the need to
distinguish and allow separate control over movement in
local and remote spaces. Once participants had achieved
good views of remote spaces, they often seemed reluctant
to move for fear of losing them. This problem is partially
an effect of the current system's limitations. When work-
ing in front of a real window, moving away to achieve
some local goal is easily reversed simply by moving back
again. Using the current implementation of the Virtual
Window, in contrast, moving back is no guarantee of re-
covering the original view. Though future versions should
alleviate this problem, it may actually be desirable to
maintain the dissociation. A foot pedal could be added to
the system, for instance, allowing people to stop the
Virtual Window so that local movement would not disturb
a good view of the remote site.
In sum, the prototype Virtual Window is useful in opening
up a wide space for the design of new video systems.
Perhaps none will succeed in fully creating the experience
of looking through a window into an office thousands of
miles away, but many are likely to be useful in overcoming
the limitations of existing systems. In the end, perhaps the
most important contribution the Virtual Window makes is
as a concrete reminder that media spaces need not be
constrained to single, unmoving cameras left sitting on top
of video monitors.
ACKNOWLEDGEMENTS
We thank Rank Xerox Cambridge EuroPARC -
particularly Bob Anderson and Allan Maclean - and the
Faculty of Industrial Design Engineering at Delft TU for
supporting this collaboration. Pieter Jan Stappers was an
invaluable guide to Virtual Window design, particularly
the head-tracking algorithm. We thank Ronald Teunissen
for work on the camera-moving apparatus and Jeroen
Ommering for the "Cameraman" motor-control software.
Finally, we are extremely grateful to Abi Sellen for helping
with the study reported here, and to her and Christian
Heath, Paul Luff, Anne Schlottmann, Paul Dourish, Sara
Bly and Wendy Mackay for valuable discussions about this
work.
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