Real Walking through Virtual Environments by Redirection Techniques

Abstract

We present redirection techniques that support exploration of large-scale virtual environments (VEs) by means of real walking. We quantify to what degree users can unknowingly be redirected in order to guide them through VEs in which virtual paths differ from the physical paths. We further introduce the concept of dynamic passive haptics by which any number of virtual objects can be mapped to real physical proxy props having similar haptic properties (i. e., size, shape, and surface structure), such that the user can sense these virtual objects by touching their real world counterparts. Dynamic passive haptics provides the user with the illusion of interacting with a desired virtual object by redirecting her to the corresponding proxy prop. We describe the concepts of generic redirected walking and dynamic passive haptics and present experiments in which we have evaluated these concepts. Furthermore, we discuss implications that have been derived from a user study, and we present approaches that derive physical paths which may vary from the virtual counterparts.

Full text

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Journal of Virtual Reality and Broadcasting, Volume 6(2009), no. 2
Real Walking through Virtual Environments
by Redirection Techniques
Frank Steinicke∗, Gerd Bruder∗, Klaus Hinrichs∗
Jason Jerald‡
Harald Frenz†, Markus Lappe†
∗Visualization and Computer Graphics (VisCG) Research Group
Department of Computer Science, Westf¨ alische Wilhelms-Universit¨ at M¨ unster
Einsteinstraße 62, 48149 M¨ unster, Germany
email: {fsteini,g brud01,khh}@uni-muenster.de
www: viscg.uni-muenster.de
‡Effective Virtual Environments Group
Department of Computer Science, University of North Carolina,
Chapel Hill, North Carolina, USA
email: jjerald@email.unc.edu
www: www.cs.unc.edu
†Psychology Department II
Department of Psychology, Westf¨ alische Wilhelms-Universit¨ at M¨ unster
Fliednerstraße 21, 48149 M¨ unster, Germany
email: {mlappe,frenzh}@uni-muenster.de
www: wwwpsy.uni-muenster.de/Psychologie.inst2/AELappe
Abstract
We present redirection techniques that support explo-
ration of large-scale virtual environments (VEs) by
means of real walking. We quantify to what degree
users can unknowingly be redirected in order to guide
them through VEs in which virtual paths differ from
the physical paths. We further introduce the concept of
dynamic passive haptics by which any number of vir-
tualobjectscanbemappedtorealphysicalproxyprops
having similar haptic properties (i.e., size, shape, and
surface structure), such that the user can sense these
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First presented at the Virtual Reality International Conference (VRIC) 2008,
extended and revised for JVRB
virtual objects by touching their real world counter-
parts. Dynamic passive haptics provides the user with
the illusion of interacting with a desired virtual object
by redirecting her to the corresponding proxy prop.
We describe the concepts of generic redirected walk-
ing and dynamic passive haptics and present experi-
ments in which we have evaluated these concepts. Fur-
thermore, we discuss implications that have been de-
rived from a user study, and we present approaches
that derive physical paths which may vary from the
virtual counterparts.
Keywords:
Redirected Walking, Perception
Virtual Locomotion User Interfaces,
1Introduction
Walking is the most basic and intuitive way of mov-
ing within the real world. Taking advantage of such an
active and dynamic ability to navigate through large
immersive virtual environments (IVEs) is of great
interest for many 3D applications demanding loco-
motion, such as urban planning, tourism, 3D games
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Journal of Virtual Reality and Broadcasting, Volume 6(2009), no. 2
etc.
dimensional and their applications would benefit from
exploration by means of real walking, many such VE
systems do not allow users to walk in a natural manner.
In many existing immersive VE systems, the user
navigates with hand-based input devices in order to
specify direction and speed as well as acceleration and
deceleration of movements [WCF+05]. Most IVEs
do not allow real walking [WCF+05].
mains are inherently three-dimensional and advanced
visual simulations often provide a good sense of lo-
comotion, but visual stimuli by itself can hardly ad-
dress the vestibular-proprioceptive system. An obvi-
ous approach to support real walking through an IVE
is to transfer the user’s tracked head movements to
changes of the virtual camera in the virtual world by
means of a one-to-one mapping. This technique has
the drawback that the users’ movements are restricted
by a rather limited range of the tracking sensors and a
small workspace in the real world. Therefore concepts
for virtual locomotion methods are needed that en-
able walking over large distances in the virtual world
while remaining within a relatively small space in the
real world. Various prototypes of interface devices
have been developed to prevent a displacement in the
real world. These devices include torus-shaped omni-
directional treadmills [BS02, BSH+02], motion foot
pads, robot tiles [IHT06, IYFN05] and motion car-
pets [STU07]. Although these hardware systems rep-
resent enormous technological achievements, they are
still very expensive and will not be generally accessi-
ble in the foreseeable future. Hence there is a tremen-
dous demand for more applicable approaches.
As a solution to this challenge, traveling by ex-
ploiting walk-like gestures has been proposed in many
different variants, giving the user the impression of
walking.For example, the walking-in-place ap-
proach exploits walk-like gestures to travel through an
IVE, while the user remains physically at nearly the
same position [UAW+99, Su07, WNM+06, FWW08].
However, real walking has been shown to be a more
presence-enhancing locomotion technique than other
navigation metaphors [UAW+99].
Cognition and perception research suggests that
cost-efficient as well as natural alternatives exist. It
is known from perception psychology that vision of-
ten dominates proprioceptive and vestibular sensation
when different [DB78, Ber00]. Human participants
using only vision to judge their motion through a vir-
tual scene can successfully estimate their momentary
Although these domains are inherently three-
Many do-
direction of self-motion but are not as good in perceiv-
ing their paths of travel [LBvdB99, BIL00]. There-
fore, since users tend to unwittingly compensate for
small inconsistencies during walking, it is possible to
guide them along paths in the real world that differ
from the perceived path in the virtual world. This
so-called redirected walking enables users to explore
a virtual world that is considerably larger than the
tracked working space [Raz05] (see Figure 1).
In this article, we present an evaluation of redirected
walking and derive implications for the design pro-
cess of a virtual locomotion interface. For this eval-
uation, we extend redirected walking by generic as-
pects, i.e., support for translations, rotations and cur-
vatures. We present a model which describes how hu-
mans move through virtual environments. We describe
a user study that derives optimal parameterizations for
these techniques.
Our virtual locomotion interface allows users to ex-
plore 3D environments by means of real walking in
such a way that the user’s presence is not disturbed
by a rather small interaction space or physical objects
present in the real environment. Our approach can be
used in any fully-immersive VE-setup without a need
for special hardware to support walking or haptics. For
these reasons we believe that these techniques make
exploration of VEs more natural and thus ubiquitously
available.
The remainder of this paper is structured as follows.
Section 2 summarizes related work. Section 3 de-
scribes our concepts of redirected walking and present
the human locomotion triple–modeling how humans
move through VEs. Section 4 presents a user study
we conducted in order to quantify to what degree users
can be redirected without the user noticing the discrep-
ancy. Section 5 discusses implications for the design
ofavirtuallocomotioninterfacebasedontheresultsof
the previously mentioned study. Section 6 concludes
the paper and gives an overview about future work.
2 Previous Work
Locomotion and perception in IVEs are the focus
of many research groups analyzing perception in
both the real world and virtual worlds. For exam-
ple, researchers have found that distances in virtual
worlds are underestimated in comparison to the real
world [LK03, IAR06, IKP+07], that visual speed
during walking is underestimated in VEs [BSD+05]
and that the distance one has traveled is also under-
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Journal of Virtual Reality and Broadcasting, Volume 6(2009), no. 2
estimated [FLKB07].
eral difficulties in orienting themselves in virtual
worlds [RW07].
The real world remains stationary as we rotate our
heads and we perceive the world to be stable even
when the world’s image moves on the retina. Visual
and extra-retinal cues help us to perceive the world as
stable [Wal87, BvdHV94, Wer94]. Extraretinal cues
come from the vestibular system, proprioception, our
cognitive model of the world, and from an efference
copy of the motor commands that move the respective
body parts. When one or more of these cues conflicts
with other cues, as is often the case for IVEs (due to
incorrect field-of-view, tracking errors, latency, etc.),
the virtual world may appear to be spatially unstable.
Experiments demonstrate that subjects tolerate a
certain amount of inconsistency between visual and
proprioceptive sensation [SBRK08, JPSW08, PWF08,
KBMF05, BRP+05, Raz05]
Redirected walking [Raz05] provides a promising
solution to the problem of limited tracking space and
the challenge of providing users with the ability to
explore an IVE by walking. With this approach, the
user is redirected via manipulations applied to the dis-
played scene, causing users to unknowingly compen-
sate by repositioning and/or reorienting themselves.
Different approaches to redirect a user in an IVE
have been suggested. An obvious approach is to scale
translational movements, for example, to cover a vir-
tual distance that is larger than the distance walked
in the physical space.Interrante et al.
to apply the scaling exclusively to the main walk-
ing direction in order to prevent unintended lateral
shifts [IRA07]. With most reorientation techniques,
the virtual world is imperceptibly rotated around the
center of a stationary user until she is oriented in
such a way that no physical obstacles are in front of
her [PWF08, Raz05, KBMF05]. Then, the user can
continue to walk in the desired virtual direction. Al-
ternatively, reorientation can also be applied while the
user walks [GNRH05, SBRK08, Raz05]. For instance,
if the user wants to walk straight ahead for a long dis-
tance in the virtual world, small rotations of the cam-
era redirect her to walk unconsciously on an arc in the
opposite direction in the real world [Raz05].
Redirection techniques are applied in robotics for
controlling a remote robot by walking [GNRH05].
For such scenarios much effort is required to avoid
collisions and sophisticated path prediction is essen-
tial [GNRH05, NHS04]. These techniques guide users
Sometimes, users have gen-
suggest
on physical paths for which lengths as well as turn-
ing angles of the visually perceived paths are main-
tained, but the user observes the discrepancy between
both worlds.
Until now not much research has been undertaken
in order to identify thresholds which indicate the toler-
able amount of deviation between vision and proprio-
ception while the user is moving. Preliminary stud-
ies [SBRK08, PWF08, Raz05] show that in general
redirected walking works as long as the subjects are
not focused on detecting the manipulation. In these
experiments, subjects answered afterwards if they no-
ticed a manipulation or not. Quantified analyzes have
not been undertaken. Some work has been done in or-
der to identify thresholds for detecting scene motion
during head rotation [Wal87, JPSW08], but walking
was not considered in these experiments.
Besides natural navigation, multi-sensory percep-
tion of an IVE increases the user’s sense of pres-
ence [IMWB01]. Whereas graphics and sound ren-
dering have matured so much that realistic synthe-
sis of real world scenarios is possible, generation of
haptic stimuli still represents a vast area for research.
Tremendous effort has been undertaken to support ac-
tive haptic feedback by specialized hardware which
generates certain haptic stimuli [CD05]. These tech-
nologies, such as force feedback devices, can provide
a compelling sense of touch, but are expensive and
limit the size of the user’s working space due to de-
vices and wires. A simpler solution is to use passive
haptic feedback: physical props registered to virtual
objects that provide real haptic feedback to the user.
Bytouchingsuchaproptheusergetstheimpressionof
interacting with an associated virtual object seen in an
HMD [Lin99] (see Figure 2). Passive haptic feedback
is very compelling, but a different physical object is
needed for each virtual object that shall provide haptic
feedback [Ins01]. Since the interaction space is con-
strained, only a few physical props can be supported.
Moreover, the presence of physical props in the inter-
action space prevents exploration of other parts of the
virtual world not represented by the current physical
setup. For example, a proxy prop that represents a vir-
tual object at some location in the VE may occlude the
user when walking through a different location in the
virtual world. Thus exploration of large scale environ-
ments and support of passive haptic feedback seem to
be mutually exclusive.
Redirected walking and passive haptics have
been combined in order to address these chal-
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Journal of Virtual Reality and Broadcasting, Volume 6(2009), no. 2
lenges [KBMF05, SBRK08]. If the user approaches
an object in the virtual world she is guided to a corre-
sponding physical prop. Otherwise the user is guided
aroundobstacles inthe workingspace inorder toavoid
collisions. Props do not have to be aligned with their
virtual world counterparts nor do they have to provide
haptic feedback identical to the visual representation.
Experiments have shown that physical objects can pro-
vide passive haptic feedback for virtual objects with a
different visual appearance and with similar, but not
necessarily the same, haptic capabilities, or the same
alignment [SBRK08, BRP+05] (see Figure 2 (a) and
(b)). Hence, virtual objects can be sensed by means
of real proxy props having similar haptic properties,
i.e., size, shape and texture and location. The map-
ping from virtual to real objects need not be one-to-
one. Since the mapping as well as the visualization
of virtual objects can be changed dynamically during
runtime, usually a small number of proxy props suf-
fices to represent a much larger number of virtual ob-
jects. By redirecting the user to a preassigned proxy
object, the user gets the illusion of interacting with a
desired virtual object.
In summary, substantial efforts have been made to
allow a user to walk through a IVE which is larger
than the laboratory space and experience the environ-
ment with support of passive haptic feedback, but the
concepts can be improved on.
3 Generalized Redirected Walking
Redirected walking can be implemented using gains
which define how tracked real-world motions are
mapped to the VE. These gains are specified with re-
spect to a coordinate system. For example, they can be
defined by uniform scaling factors that are applied to
the virtual world registered with the tracking coordi-
nate system such that all motions are scaled likewise.
3.1Human Locomotion Triple
We introduce the human locomotion triple (HLT)
(s,u,w) by three normalized vectors—strafe vector s,
up vector u and walk-direction vector w. w can be
determined by the actual walking direction or using
proprioceptive information such as the orientation of
the limbs or the view direction. In our experiments
we define w by the current tracked walking direction.
The strafe vector s, a.k.a. right vector, is orthogonal
to the direction of walk and parallel to the walk plane.
Figure 1: Redirected walking scenario: a user walks
through the real environment on a different path with
a different length in comparison to the perceived path
in the virtual world.
We define u determining the up vector of the tracked
head orientation. Whereas the direction of walk and
the strafe vector are orthogonal to each other, the up
vector u is not constrained to the crossproduct of s
and w. Hence, if a user walks up a slope the direc-
tion of walk is defined according to the walk plane’s
orientation, whereas the up vector is not orthogonal
to this tilted plane. When walking on slopes humans
tend to lean forward, so the up vector remains parallel
to the direction of gravity. As long as the direction of
walk holds w ?= (0,1,0), the HLT composes a coor-
dinates system. In the following sections we describe
how gains can be applied to the HLT.
3.2Translation gains
The tracking system detects a change of the user’s real
world position defined by the vector Treal = Pcur−
Ppre, where Pcuris the current position and Ppreis
the previous position, Trealis mapped one-to-one to
the virtual camera with respect to the registration be-
tween virtual scene and tracking coordinates system.
A translation gain gT ∈ R3is defined for each com-
ponent of the HLT (see Section 3.1) by the quotient
of the mapped virtual world translation Tvirtualand the
tracked real world translation Treal, i.e., gT :=Tvirtual
Hence, generic gains for translational movements can
be expressed by gT[s],gT[u],gT[w], where each com-
ponent is applied to the corresponding vector s, u and
w respectively composing the translation. In our ex-
periments we have focussed on sensitivity to transla-
tion gains in the walk direction gT[w].
Treal.
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3.3 Rotation gains
Real-world head rotations can be specified by a
vector consisting of three angles, i.e., Rreal
(pitchreal,yawreal,rollreal).
change is applied to the virtual camera. Analogous to
Section 3.2, rotation gains are defined for each compo-
nent (pitch/yaw/roll) of the rotation and are applied to
the axes of the locomotion triple. A rotation gain gRis
defined by the quotient of the considered component
of a virtual world rotation Rvirtualand the real world
rotation Rreal, i.e., gR:=Rvirtual
gRis applied to a real world rotation α the virtual cam-
era is rotated by gR· α instead of α. This means that
if gR= 1 the virtual scene remains stable considering
the head’s orientation change. In the case gR> 1 the
virtual scene appears to move against the direction of
the head turn, whereas a gain gR< 1 causes the scene
to rotate in the direction of the head turn. A system
with no head tracking would result in gR= 0. Rota-
tional gains can be expressed by gR[s],gR[u],gR[w],
where the gain gR[s] specified for pitch is applied to
s, the gain gR[u] specified for yaw is applied to u, and
gR[w] specified for roll is applied to w. In our ex-
periments we have focussed on rotation gains for yaw
rotation gR[u].
:=
The tracked orientation
Rreal. When a rotation gain
3.4Curvature gains
Instead of multiplying gains with translations or rota-
tions, offsets can be added to real world movements.
Camera manipulations are used if only one kind of
motion is tracked, for example, user turns the head,
but stands still, or the user moves straight ahead with-
out head rotations. If the injected manipulations are
reasonably small, the user will unknowingly compen-
sate for these offsets resulting in walking a curve.
The curvature gain gCdenotes the resulting bend of
a real path. For example, when the user moves straight
ahead, a curvature gain that causes reasonably small
iterative camera rotations to one side enforces the user
to walk along a curve in the opposite direction in or-
der to stay on a straight path in the virtual world. The
curve is determined by a segment of a circle with ra-
dius r, and gC :=1
it is r = ∞ ⇒ gC = 0, whereas if the curvature
causes the user to rotate by 90◦clockwise afterπ
the user has covered a quarter circle with radius r = 1
⇒ gC= 1.
r. In case no curvature is applied
2m
3.5 Displacement Gains
Displacement gains specify the mapping from phys-
ical head or body rotations to virtual translations.
In some cases it is useful to have a means to in-
sert virtual translations while the user turns the head
or body but is stationary at the same physical posi-
tion. Hence, with displacement gains translations are
injected with respect to rotational user movements,
whereas with curvature gains rotations can be in-
jected corresponding to translational user movements
(see Section 3.4). Three displacement gains are in-
troduced: (gD[w],gD[s],gD[u]) ∈ R3, where the
components define the contribution of physical yaw,
pitch, and roll rotations to the virtual translational dis-
placement. For an active physical user rotation of
α := (pitchreal,yawreal,rollreal), the virtual position
is translated in the direction of the virtual vectors w, s,
and u accordingly.
3.6Time-dependant Gains
Uptonowonlygainshavebeenpresented, thatdefined
the mapping of real-world movements to VE motions.
However, not all virtual camera rotations and trans-
lations have to be triggered by physical user actions.
Time-dependant drifts are an example for this type of
manipulation. They can be defined just like the other
gains described above. The idea behind these gains is
to introduce changes to the virtual camera orientation
or position over time. The time-dependant rotation
gains are ratios in degrees/sec and the corresponding
translation gains are ratios in m/sec. Time-dependant
gains are given by gX= (gX[s],gX[u],gX[w]), where
X is replaced by T/t for time-dependent translation
gains and R/t for time-dependent rotation gains.
4 Experiments
In this section we present three sub-experiments in
which we quantify how much humans can unknow-
ingly be redirected.We analyze the appliance of
translation gT[w], rotation gR[u], and curvature gains
gC[w].
4.1Experimental Design
4.1.1Test Scenario
Users were restricted to a 10m × 7m × 2.5m track-
ing range. The user’s path always led her clockwise
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