evaluation of grounded isometric interface for whole-body navigation in virtual environments

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RESEARCH ARTICLE Evaluation of grounded isometric interface for whole- body navigation in virtual environments Bong-gyu Jang and Gerard Jounghyun Kim * Digital Experience Lab, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, Korea ABSTRACT Whole-body interaction is an effective way to promote the level of presence and immersion in virtual reality systems. In this paper, we introduce G-Bar,a grounded isometric interaction device that naturally induces whole-body interaction without complicated sensing and active haptic feedback apparatus. G-Bar takes advantage of the signicant passive reaction force feedback sensed throughout the body to produce an enhanced level of presence/immersion and possibly even task perfor- mance. For detailed investigation in the contributing factors, two experiments were carried out to assess the comparative effectiveness of G-Bar to the following: (1) grounded but isotonic device (with force feedback and without); and (2) nongrounded handheld devices (both isotonic and isometric). The results showed that the G-Bar induced signicantly higher presence and competitive task performance (xed velocity navigation) than the isotonic (grounded or handheld) and nongrounded isometric interfaces. Compared with the grounded isometric device with active force feedback, G-Bar produced competitive performance. In particular, the analysis of the subjective evaluation revealed a high correlation between the level of presence and whole-body interaction. On the other hand, whole-body experience was not induced as much with just the active force-feedback devices. Thus, for appropriate tasks, the grounded isometric interface can be a viable alternative to expensive and mechanically limiting active force-feedback devices in enhancing user experience. Copyright © 2013 John Wiley & Sons, Ltd. KEYWORDS isometric interface; grounded Interface; whole-body interaction; presence; immersion *Correspondence Gerard Jounghyun Kim, Digital Experience Lab, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, Korea. E-mail: [email protected] 1. INTRODUCTION One of the dening characteristics of virtual reality is the provision of presence,the feeling of being immersed in the content [13]. Many research have investigated the factors that contribute to enhancing the level of presence [1,4,5], and one such element is the whole-bodyinterac- tion, which works by involving as many sensory and motor organs as possible during interaction [6]. In [7], we have introduced an interface called the G-Bar (grounded bar), a two-handed isometric device that is xed to the ground (grounded) for a variety of interactive tasks including navigation and object selection and manip- ulation. An isometric device makes output according to the sensed amount of force but does not move. An isotonic device makes and senses movement and reects output in proportion to the amount of movement [9]. The interface naturally induces whole-bodyinteraction because although the applied pressure is sensed at only four contact points (with two hands), because of the grounded structure of the interface, the user tends to apply and feel pressure naturally using the whole body, and the reactive force propagates back through as well (Figure 1). Moreover, because the device is isometric and senses the users pressure input, the user can express dynamic interactions more naturally [810]. In this paper, we further formally evaluate and validate the projected merits of the whole-body interaction induced by the groundeddevice such as G-Bar. After reviewing other related research, we outline the design, possible interactions and projected merits of the proposed interface. Then, we present two usability experiments and a discus- sion of the results. Finally, we conclude the paper with an executive summary and avenues for future work. This paper is a signicantly extended version of [7] with addi- tional results and analysis to the original experiment (as de- scribed in Section 3.3) and newly added experiments (referred to as experiments I and II). COMPUTER ANIMATION AND VIRTUAL WORLDS Comp. Anim. Virtual Worlds 2014; 25:561575 Published online 24 October 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/cav.1561 Copyright © 2013 John Wiley & Sons, Ltd. 561

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Page 1: Evaluation of grounded isometric interface for whole-body navigation in virtual environments

COMPUTER ANIMATION AND VIRTUAL WORLDSComp. Anim. Virtual Worlds 2014; 25:561–575

Published online 24 October 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/cav.1561

RESEARCH ARTICLE

Evaluation of grounded isometric interface for whole-body navigation in virtual environmentsBong-gyu Jang and Gerard Jounghyun Kim*

Digital Experience Lab, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, Korea

ABSTRACT

Whole-body interaction is an effective way to promote the level of presence and immersion in virtual reality systems. In thispaper, we introduce “G-Bar,” a grounded isometric interaction device that naturally induces whole-body interaction withoutcomplicated sensing and active haptic feedback apparatus. G-Bar takes advantage of the significant passive reaction forcefeedback sensed throughout the body to produce an enhanced level of presence/immersion and possibly even task perfor-mance. For detailed investigation in the contributing factors, two experiments were carried out to assess the comparativeeffectiveness of G-Bar to the following: (1) grounded but isotonic device (with force feedback and without); and (2)nongrounded handheld devices (both isotonic and isometric). The results showed that the G-Bar induced significantlyhigher presence and competitive task performance (fixed velocity navigation) than the isotonic (grounded or handheld)and nongrounded isometric interfaces. Compared with the grounded isometric device with active force feedback, G-Barproduced competitive performance. In particular, the analysis of the subjective evaluation revealed a high correlationbetween the level of presence and whole-body interaction. On the other hand, whole-body experience was not inducedas much with just the active force-feedback devices. Thus, for appropriate tasks, the grounded isometric interface can bea viable alternative to expensive and mechanically limiting active force-feedback devices in enhancing user experience.Copyright © 2013 John Wiley & Sons, Ltd.

KEYWORDS

isometric interface; grounded Interface; whole-body interaction; presence; immersion

*Correspondence

Gerard Jounghyun Kim, Digital Experience Lab, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, Korea.E-mail: [email protected]

1. INTRODUCTION

One of the defining characteristics of virtual reality is theprovision of “presence,” the feeling of being immersed inthe content [1–3]. Many research have investigated thefactors that contribute to enhancing the level of presence[1,4,5], and one such element is the “whole-body” interac-tion, which works by involving as many sensory and motororgans as possible during interaction [6].

In [7], we have introduced an interface called the G-Bar(grounded bar),† a two-handed isometric device that isfixed to the ground (grounded) for a variety of interactivetasks including navigation and object selection and manip-ulation. An isometric device makes output according to the

†This paper is a significantly extended version of [7] with addi-tional results and analysis to the original experiment (as de-scribed in Section 3.3) and newly added experiments (referredto as experiments I and II).

Copyright © 2013 John Wiley & Sons, Ltd.

sensed amount of force but does not move. An isotonicdevice makes and senses movement and reflects output inproportion to the amount of movement [9].

The interface naturally induces “whole-body” interactionbecause although the applied pressure is sensed at only fourcontact points (with two hands), because of the groundedstructure of the interface, the user tends to apply and feelpressure naturally using the whole body, and the reactiveforce propagates back through as well (Figure 1). Moreover,because the device is isometric and senses the user’s pressureinput, the user can express dynamic interactions morenaturally [8–10].

In this paper, we further formally evaluate and validatethe projected merits of the whole-body interaction inducedby the “grounded” device such as G-Bar. After reviewingother related research, we outline the design, possibleinteractions and projected merits of the proposed interface.Then, we present two usability experiments and a discus-sion of the results. Finally, we conclude the paper withan executive summary and avenues for future work.

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Figure 1. The G-Bar used for navigating in a virtual environment. The reaction force resulting from the two-hand interaction with thegrounded device propagates throughout the body (left). A more detailed view of the G-Bar prototype (right).

Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

2. RELATED WORK

Employing whole-body interaction is an effective methodto enhance the immersive quality of interactive contents[4,5,11,12]. Possible methods of realizing whole-bodyinteraction include using sensors and haptic/tactile device[13–17]. Although they may be effective, they have theobvious problems of usability and cost.

However, whole-body interaction does not necessarilyrequire separate sensing and feedback mechanisms for allthe body parts involved. Through clever interaction andinterface design, whole-body interaction can be inducedwith minimal sensing. For instance, the arcade game,“Dance Dance Revolution” [18], utilizes a very simple footswitch pad, but the interaction is designed to induce the useof the whole body.

The situation is similar for recent acceleration sensor-based games such as the Nintendo Wii [19]. Whole-bodyinteraction may not necessarily require excessive move-ments of the body parts either. What is more important isthe contraction and relaxation of the muscles, which maynot involve explicit movements of the bones [20]. Forexample, isometric exercises only require stimulation andstrengthening of the muscles rather than limb movementsfor whole-body exercise. In [7], the authors have proposeda device called G-Bar (grounded bar), an inexpensiveisometric device fixed to the ground to induce naturalwhole-body interaction. This paper makes a deeper inves-tigation into the effects of grounded isometric device (suchas G-Bar) toward the level of user-perceived presence andimmersion. A more detailed background is given in thenext section.

“Passive” haptics is another inexpensive and creativeway to take advantage of the natural reaction forcefeedback, for example, by using tangible props. However,

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props are usually of light weight and fragile, limitingthe users in applying a high amount of force. Meehanet al. [21] demonstrated the utility of the passive hapticswith a grounded prop, which had significantly enhancedthe level of presence for their virtual cliff environment.In our case with the grounded device, a large reactionforce is more likely to propagate throughout and thusstimulate the whole body, although the in-depth analysisin this regard was not conducted. Isometric input is alsoknown to increase the overall realism by allowingexpression of the interaction dynamics [8–10]. TheG-Bar combines all these aforementioned elements inhopes of creating effective interaction and compellinguser experience.

As for navigation control in virtual environments,Bowman et al. [22] provided an excellent survey onthe different interaction techniques that employ avariety of metaphors and hardware devices. However,using an isometric device for navigation control hasreceived less attention. Note that there have beennumerous works in using active haptic feedback and itspositive effects for virtual and real navigation control[23–28]. However, as an isometric device, the G-Bar doesnot have any active force feedback, and this paper willinvestigate the G-Bar in comparison with the activefeedback device.

As for the effect of whole-body interaction toward thetask performance, there has been mixed results. Petersonhas found body-based navigation to produce better surveyknowledge, whereas a conventional joy stick interfaceresulted in higher maneuverability [4]. The relationshipbetween presence/immersion (to which whole-body inter-action is related) and task performance is generally viewedas being task dependent [29]. For example, we project that,for body-sized “tight” navigation (passing through a

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Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

narrow maze), whole-body navigation could provide a bet-ter spatial reference and thereby improve performance.

Figure 2. The input/output characteristic of the force/pressuresensor and the interface used for implementing the G-Bar

(recreated from [30]).

3. G-BAR

3.1. Design and Implementation

G-Bar is implemented by installing low-cost pressuresensors and vibrating motors on a bar handle (Figure 1).In addition to the natural reaction force, the four pressuresensors (two at each end of the bar) realize the isometricinput, and the four vibrating motors (laid out along thebar) give directional feedback cues.‡ The pressure sensorused in our implementation was the Interlink FSR model402 (1/2″ circle) [30] and had an input/output characteris-tics as shown in Figure 2. It had a general sensitivity rangeof 0.1–10 kg (or 1.5–150 psi in pressure) with a resolutionof better than 0.5%. Although, at the fullest extent, ahuman user would certainly be able to exert a force muchhigher than just 10 kg, interacting isometrically in therange of, for example, around 2–7 kg is deemed sufficientfor one to feel the reaction force throughout the body(i.e., for the whole-body interaction effect). However, thisrange can affect the time for the user to adapt oneself tothe interface and produce high task performance.

3.2. Operating and Interacting with theG-Bar

To leverage on the isometric nature of the interface, theamount of the pressure applied can be mapped to control-ling the rate/velocity (linear or angular) of the interactionobject, that is, for navigation, the viewpoint; for objectselection, the cursor; and for manipulation, the selectedobject. Because the sensor exhibits a saturated input/outputbehavior, the mapping can be “calibrated” in various formsto produce a linear (e.g., 1:1 or scaled with an offset) [31]or exponential [32] “interface” input/output behavior [22].Figure 2 shows the linearized input/output transfer func-tion as was used in our work with the typical operatingrange between 0.5 and 7 kg, with the maximum speed ofabout 1m/s2.

Although object selection and manipulation might notreally involve whole-body interaction in the real life, byexaggerating the extent of the required body parts involved,we posit that it could be one way to improve the virtualexperience. Figure 3 shows how one might be able tonavigate through the virtual space by combinations ofsimple two-handed isometric push (forward), pull (backward)and twist actions. Note that the G-Bar operates with two pairs(four in total) of pressure sensors, each activated by therespective hand/finger, of which two from each pair mustbe active for a “navigation” command event to occur

‡The vibrator feedback devices were not used in the experi-ments described in this paper.

Comp. Anim. Virtual Worlds 2014; 25:561–575 © 2013 John Wiley & Sons, LtdDOI: 10.1002/cav

(Figure 1). For instance, to make a “push forward” action,sensors 2 and 4 must be active at the same time. To make a“turn right” action, sensors 1 and 4 must be active. For asensor to be “active,” we set a small threshold value to avoidaccidental input, and two sensors are “active”when inputs tothe sensors are above the threshold. For the two sensorvalues to be collectively reflected to velocity (or rate ofturning), the average of the two is used. Note that in theoriginal form, the movement/turning and velocity controlare coupled, although in our subsequent experiments, weremoved the velocity control for simplicity and becauseour main focus was to assess the effect toward presence/immersion rather than task performance.

The G-Bar is particularly appropriate for tasks that involvefrequent and dynamic contact with the environment or inter-action object. A typical example might be navigating andpassing through a bumping crowd or riding and directly con-trolling a vehicle such as a cart, motorcycle or hang glider. Infact, the “bar” resembles the control handles used for some ofthese vehicles (e.g., motorcycle handle), and such metaphorscan be even more helpful. Moreover, many real-life interac-tions do involve the manipulation and shifting of the centerof one’s body mass, for which the G-Bar is able to capture.

For selection, for example, the same navigation interac-tion technique can be used for controlling the virtual ray/cone and then applying the two hand grasp for selection(all four sensors) and final confirmation with the right handgrasp (sensors 3 and 4, and undo with a left hand grasp).Once the object is selected, it can be rotated and moved(manipulated) in a similar fashion (Figure 3).

3.3. G-Bar versus Conventional IsotonicInterface (Keyboard)

In Jang et al. [7], we carried out an initial experiment com-paring the G-Bar interface with a conventional interface,namely the keyboard, to assess the effectiveness of the pro-posed interaction technique. The experiment was designedas a one-factor (two-level) repeated measure within sub-ject, the sole factor being the type of the interface

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Figure 3. Navigation, object selection and manipulation through combinations of push, pull, twist and grasp actions with the fourpressure sensors of the G-Bar.

Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

employed (G-Bar vs. keyboard). The user was asked tonavigate a cart for a fixed path in a virtual maze in afirst-person viewpoint. The task performance, level of pres-ence/immersion and general usability were measured.

Among others, the experiment revealed that the extentof perceived levels of whole-body usage, force feedbackand presence/immersion was all much higher with theG-Bar. As for the user’s opinion in the relationships betweenthe whole-body interaction and presence/immersion andperception of the (passive) force feedback, users attributedthe enhanced experience more toward the whole-bodyinteraction than just the “passive” (reaction) force feedback.In the next two new experiments, we seek the role ofexternal and “active” force feedback as compared with thatof whole-body interaction. As for the task performance,despite the relative familiarity of the keyboard interface,there was no statistically significant difference in the taskperformance. We posit that for navigating in the maze andobstacle course, the whole-body interaction provided abetter spatial reference, calibrating one’s body to that of thesize of the cart to adjust oneself and avoid collision. Thisinitial study showed the promising nature of the G-Bar asan inexpensive way to induce whole-body interaction.

4. EXPERIMENT I

In the previous initial experiment, we have verified thatcompared with a nominal isotonic keyboard interface, theG-Bar induced a high level of presence/immersion andcompetitive navigation performance. However, it is notclear if such a result was simply due to the passive forcefeedback alone or the involvement of the whole body.Thus, the purpose of experiment I is to isolate these twofactors more clearly in terms of their influence to the levelof the user experience and task performance.

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4.1. Experiment Design and Task

In experiment I, we compared three different “grounded”interfaces: (1) the (isometric) G-Bar; (2) an isotonic activehaptic interface; and (3) a conventional isotonic interface(no haptic feedback), applied to a car/cart navigation task.To be more specific, a force-feedback steering wheel(Driving Force GT from Logitech, Morges, Switzerland)was used as the isotonic active haptic interface, and forthe conventional isotonic one, the same device was usedwith the steering force feedback turned off (Figure 8). Thisdevice provides torque on steering proportional to the steerangle, which is also used for directional control. Themovement is controlled separately by a foot pedal.

As already briefly mentioned, in this experiment, thevelocity control was not used. Our main focus was toinvestigate the effects of whole-body interaction topresence and immersion, rather than task performance.The coupled control of direction, motion and velocity canbe difficult or time-consuming to master, and there was adistinct possibility for it (or the ensuing frustration) toaffect the experimental results. The different input–outputcharacteristics for rate control of the G-Bar and the steeringwheel interface would exacerbate the situation evenfurther. Thus, the pressure-mapped (for G-Bar) or pedal-controlled (for Logitech Driving Force) velocity controlfeatures were all disabled, and a preset constant velocitywas used (Table 1).

We emphasize that despite the existence of the activeforce feedback, we view the steering wheel to be anisotonic device, because, as noted earlier, the directionand motion are controlled according to the moving of thesteering wheel and foot pedal. Also, note that the hapticfeedback does not emanate from interaction with the envi-ronment (e.g., colliding with the wall or bumping with an-other car). Although not conclusive, there is a generalindication that the provision of steering force improves

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Table 1. Operation of the G-Bar and Logitech Driving Force in experiment I.

Forward Backward Turn right Turn left

G-Bar Sensors 2 and 3values> threshold

Sensors 1 and 4values> threshold

Sensors 2 and 4values> threshold;turn angle = k* average(0 to 90°)

Sensors 1 and 3values> threshold;turn angle = k* average(0 to �90°)

Logitech DrivingForce

Pedal 1 Pedal 2 Turn angle= steer angle(0–90°)

Turn angle= steerangle (0 to �90 deg.)

Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

driving performance [33,34]. We also expect that, similarto the G-Bar, a grounded interface with active force feed-back will also induce whole-body interaction.

Because the G-Bar in appearance is a “bar” and theother two interfaces are a “wheel” type, we felt that thedriving interface metaphor could influence the outcomeof the experiment. For example, it is plausible to expectthat the G-Bar is more proper to be used for navigating acart or motorcycle whose physical interface resembles abar, whereas the steering wheel is more appropriate fordriving a car. Therefore, to avoid any possible bias, wetested the interfaces under two different visual drivingmetaphors: (1) “cart” type; and (2) “car” type.

Our hypothesis was that although the “active” forcefeedback alone might play an important role in user expe-rience and task performance, the G-Bar (which does notprovide active force feedback) would be still be

Table 2. Design o

G-Bar F-Handle (Force

Isometric

Visual Metaphor Cart T1Car T4

Figure 4. Combinations of three interface types and two visual met

Comp. Anim. Virtual Worlds 2014; 25:561–575 © 2013 John Wiley & Sons, LtdDOI: 10.1002/cav

competitive and thus offer an inexpensive alternative tothe isotonic active force-feedback device. This wasbecause the isotonic aspect of the latter limits the degreeof whole-body experience, for example, to the arms andupper body only.

Thus, to summarize, as shown in Table 2, there weretwo factors: (1) the interface type with three levels (“G-Bar,” grounded isometric; “F-Handle,” isotonic steeringwith force feedback; and “NF-Handle,” isotonic steeringwithout force feedback); and (2) metaphor type with twolevels (“cart” and “car”). These two factors and levels pro-duced six combinations of treatments to be tested (Table 2and Figure 4). The experiment was run as 3 × 2 within-subject repeated measures. As for the dependent variables,we measured general usability, subjective user experience(presence and immersion), and task performance. Moreprocedural details are presented in the next sections.

f experiment I.

Interface type

feedback handle) NF-Handle (No force feedback handle)

Grounded

Isotonic

T2 T3T5 T6

aphors (six in total) applied to navigation of an obstacle course.

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Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

4.2. Experimental Procedure

A total of 16 subjects participated in experiment I (eightmen and eight women, with an average age of 25 years).After their background information was collected, theywere given a short amount of training for familiarizingthemselves to three interfaces. The subjects were givenproper compensations for their participation.

Again, the G-Bar was operated as described in Section3, and the two other steering wheel interfaces in the con-ventional way (i.e., turning the handle for directional con-trol and stepping on the two footpads for moving forwardand backward). As shown in Figures 4 and 5, with the“cart” metaphor, a first-person view of a cart was visuallyshown, whereas with the “car,” the frontal interior of acar with the virtual steering wheel turning according tothe user input was displayed.

The subject navigated the virtual obstacle course usingthe three interfaces and two metaphors presented in acounterbalanced order. The virtual obstacle course is illus-trated well in Figure 4, and it was marked with directionalguidance. Subjects were asked to follow and navigate the

Figure 5. Experiment I setup: (a) the G-Bar with the cart metaphor,with the cart metaphor and (d) the F-Hand

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path as fast as possible but without colliding with wallsor obstacles as much as possible.

The subject tried out each treatment (T1–T6 in Table 2)three times over the same course. Note that as was with theG-Bar, the handle interfaces were also grounded (fixed tothe table). Because of the structure of the interface, theG-Bar was operated standing (to maximize the whole-bodyexperience), whereas the two handle interfaces were oper-ated in a sitting position. The G-Bar could have been usedwhile seated, but this would have eliminated the supportvia one’s legs and diminish the whole-body experience.On the other hand, using the steering wheel was mostnatural while seated. We opted to use the operation pos-ture most natural and familiar for the given interface type(e.g., standing for cart pushing and sitting for driving).Nevertheless, we acknowledge that this difference in theoperating could have an impact to the experimental results,if any, mostly to the task performance, rather than theeffects of whole-body interaction, which is our main focusin this exposition.

The dependent variables measured included the taskcompletion time, accuracy (e.g., number of collision),

(b) the G-Bar with the car metaphor, (c) the F-Handle/NF-Handlele/NF-Handle with the car metaphor.

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Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

subjective presence/immersion score and general usabilitycollected through a survey shown in Table 3.

4.3. Results

Figure 6 shows the level of presence/immersion felt, andthe G-Bar showed a statistically significant edge over theother two conditions, whereas the F-handle was rated tobe the easiest to use. Figure 7 shows the perceived levelof whole-body interaction, and an analysis of variancerevealed statistically significant differences between theG-Bar and the two other interfaces, and similarly for theirperceived influences toward presence/immersion. Eventhough the F-Handle was grounded and there was animplicit (isotonic or position/angle-based) force-basedinteraction, users did not feel the whole-body interactionto the same extent as with the G-Bar.

On the other hand, Figure 8 shows the perceivedamount of force feedback. The G-Bar showed no statisticaldifference to the F-Handle. Note that only passive forcewas present with the G-Bar, whereas both active andpassive forces were present with F-Handle. Both interfaceswere differentiated (regardless of whether the force feedback

Table 3. The usability and user experience surv

Q1 Ease of use How easy w(1: difficult, 7

Q2 Whole-body interaction (WBI) To what extethe given int(1: low, 7: hi

Q3 Presence/immersion To what exte(1: outside, 7

Q4 Force feedback To what exte(1: none, 7: v

Q5 WBI and presence/immersion Did you thinkwere involve(1: not at all,

Q6 WBI and force feedback Did you thinkfeedback pe(1: not at all,

Figure 6. (a) Presence was felt the highest with the G-B

Comp. Anim. Virtual Worlds 2014; 25:561–575 © 2013 John Wiley & Sons, LtdDOI: 10.1002/cav

was real or not) to the NF-handle. Thus, we conclude thatthe existence of the active force feedback did not add toimproving the whole-body experience nor to improvedperception of force feedback.

Figure 9(a) shows the experiment results with the taskperformance (task completion time and number of wallcollisions). Statistically significant differences appearedonly between the G-Bar/F-Handle and NF-Handle for taskcompletion time (both p< 0.001). No statistical differencewas found with regard to the metaphor appropriatenessacross the six combinations (Figure 9(b)).

Finally, a nonparametric correlation analysis was carriedout among the level of presence, whole-body experienceand perception of force feedback (Table 4). Although allthree elements show a high mutual correlation, thatbetween the whole-body experience and force feedback isthe least.

We acknowledge that the results are possibly con-founded by two aspects. One is the difference in the userposture or stance (seated or not). Because our focus wasto leverage upon the merits of each tested interface, itwas unavoidable to test the G-Bar while standing andthe steering interfaces while sitting. On the basis of

ey (all answered in the 7-point Likert scale).

as the given interface for navigation?: easy)nt did you feel the body was involved while navigating witherface?gh)nt did you feel you were physically immersed in the maze?: inside)nt did you feel or perceive force feedback?ery much)immersion or presence was affected by how much body partsd during interaction?7: very much so)immersion or presence was affected by the amount of force

rceived?7: very much so)

ar and (b) ease of use the highest with the F-handle.

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Figure 8. Perceived levels of force feedback and influence toward presence/immersion.

Figure 7. Perceived levels of whole-body interaction and its influence toward presence/immersion.

Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

careful observation and post-briefing, we claim that thisdid not affect much our experimental results or conclu-sions. However, we feel that the task performance couldhave been biased by the difference in interface coupling.That is, with the G-Bar, the movement and directionalcontrol were coupled, whereas with the steering wheel,the movement was controlled separately by the footpedal. This is likely to give an advantage to the G-Barwith regard to task performance. Thus, we put a higheremphasis on the findings regarding the relationship betweenthe whole-body experience, presence/immersion and activeforce feedback.

§Wii-mote is a registered trademark of Nintendo, Co., Ltd.

5. EXPERIMENT II

Experiment I has shown that whole-body interaction withG-Bar, a grounded isometric device, produced high perfor-mance, immersion and perception of force feedback

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comparable with those produced using an active force-feedback device. Experiment II was conducted to furthersingle out or assess the more important factor betweenthe two, namely, the G-Bar being isometric or grounded.Another objective was to make comparisons to a whole-body interaction using an ungrounded isotonic device(a Wii-mote§ [19] was used).

5.1. Experiment Design and Task

In experiment II, we compared three different interfaces:the G-Bar (isometric and grounded), NG-Bar (isometricbut handheld/nongrounded device, custom-built for theexperiment) and the Wii-mote (isotonic and handheld/nongrounded). Figure 10 shows the three interfaces.

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Time

0

20

40

60

80

100

120

140

G-bar F-handle NF-handle

Collision

0

0.5

1

1.5

2

2.5

3

3.5

4

G-bar F-handle NF-handle

Improper

Proper

Car

Cart

1

2

3

4

5

6

7

Metaphor Appropriateness

(a)

(b)

Figure 9. (a) Task performances between the three interfaces and (b) metaphor appropriateness among the six treatments.

Table 4. The correlation coefficients among whole-bodyinteraction (WBI), force feedback and presence.

Kendall’s tau-b Spearman’s rho

WBI and force feedback 0.457 0.555Presence and WBI 0.569 0.657Force feedback and presence 0.651 0.732

Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

On the basis of the results from experiment I, whichindicated no particular effects due to the visual metaphor(e.g., using the cart or car metaphor), Experiment II was

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run using a commercial racing game, called the “Needfor Speed” (i.e., car metaphor), to also relate our resultsto a real-world application (Figure 11). The course trackshown in Figure 11 was used. As was with experiment I,the racing task required only directional and movementcontrol. The velocity control was disabled as with previousexperiments for the same aforementioned reasons. TheG-Bar was operated as described in Table 1. The NG-Barwas implemented and operated in the same way as theG-Bar, except that it was not grounded. The Wii-motewas operated by rotating it (sensed by the accelerometer)for direction steering and using the buttons for movementor braking (Table 5 and Figure 11(c)).

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Figure 10. The three interfaces tested in experiment II: (a) G-Bar, (b) NG-Bar (nongrounded isometric device) and (c) Wii-mote (anongrounded isotonic device inducing whole-body interaction).

Figure 11. (a) A snapshot of the test environment (“Need for Speed”, a commercial racing game) and (b) the course track used for theracing task and (c) the Wii-mote operation for the racing task.

Table 5. Operations of the interfaces in experiment II.

Forward Backward Turn right Turn left

G-Bar/NG-Bar Sensors 2 and 3values> threshold

Sensors 1 and 4values> threshold

Sensors 2 and 4values> threshold;Turn angle = k* average(0° to �90°)

Sensors 1 and 3values> threshold;Turn angle = k * average(0° to �90°)

Wii-mote Button A Button B Turn angle = sensor output(0° to �90°)

Turn angle = sensor output(0° to �90°)

Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

As shown in Table 6, three treatments were tested withregard to the subjective usability and experience andnavigational performance (time and number of collisions),similar to experiment I (within-subject repeated measure).To reiterate, we expect the handheld interfaces to implicitlyinduce whole-body interaction also added by the applica-tion domain (e.g., racing).

Table 6. Design of experiment II.

Factor 1

Grounded Non-Grounded (Hand-held)

Factor 2 Isometric T1 (G-Bar) T2 (NG-Bar)Isotonic - T3 (Wii-Mote)

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5.2. Experimental Procedure

A total of 16 subjects participated in experiment III (eightmen and women each, with an average age of 24.2 years).After their background information was collected, theywere given a short amount of training for familiarizingthemselves to the three interfaces. Subjects were givenproper compensations for their participation.

Subjects were asked to complete the racing course twotimes, as fast as possible and with as few collisions aspossible, using each interface in a counterbalanced order.All three interfaces were operated in standing positions(Figure 12). The course completion time was measured(or read off the game display). Because the source codefor the racing game was not available for automaticallyrecording the number of collisions, it was counted manually

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Figure 12. Setup for experiment II: (a) G-Bar, (b) NG-Bar and (c) Wii-mote for playing the “Need for Speed”, a commercial racing game.The large grounding fixture for G-Bar was used for (b) and (c) to make the operating condition as equal as possible.

Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

under the discretion of the administrator. For instance, when therace car collided, it sometimes made more than just one contactbecause of the geometry of the wall, obstacle or the car itself. Insuch a case, only one collision was counted.

Usability survey similar to the one used in experiment I,asking of subjective presence/immersion, general usabilityand satisfaction and user experience were given after theuser tried all three treatments. Please see the SupportingInformation.

5.3. Results

Figure 13 shows the subject’s responses to the level ofwhole-body interaction experience and its possible influ-ence toward the feeling of presence and immersion. TheG-Bar clearly showed significant statistical difference tothe other two, demonstrating the important “collective”contribution of whole-body interaction and isometric inter-face. That is, although interacting with the NG-Bar or Wii-mote induced whole-body interaction, the users did notperceive to it be so as much as with the G-Bar.

Figure 13. Subjective responses regarding the whole-body intera

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Figure 14 shows the subjective rating of the perceivedforce feedback and its possible influence toward presence.Naturally, the two isometric devices showed a clear edge inthe perception of force feedback. However, in terms of its ef-fect toward presence, there was statistical difference betweenthe two. Table 7 also confirms the high correlation amongthese variables. In summary, these results collectively showedthat the force-based (either active or passive) interaction had asignificantly important role for enhanced user experience thanmerely getting many parts of the body involved.

Figure 15 shows the experiment results with the taskperformance (task completion time and number of colli-sions). Statistically significant differences appeared betweenthe G-Bar/NG-Bar and Wii-mote for both the task comple-tion time and number of collisions, showing the advantagesof the isometric interfaces.

Finally, Figure 16 shows responses to two representative(others omitted) general usability questions, namely the easeof use and fatigue. Consistent with the quantitative measures(i.e., task performance), subjects generally reported that theisometric interfaces were easier to use, but more tiring, prob-ably because of the need to exert force.

ction experience and its effect toward presence/immersion.

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Figure 14. Subjective responses regarding the force feedback and its effect toward presence/immersion.

Table 7. The correlation coefficients among whole-bodyinteraction, force feedback and presence.

Kendall’s tau-b Spearman’s rho

Presence andwhole-body interaction

0.644 0.764

Force feedback andpresence

0.547 0.635

Evaluation of grounded isometric interface for VE navigation B.-g. Jang and G. J. Kim

6. DISCUSSION

The experiments have revalidated the established fact thatforce-based, passive or active, haptic interaction andwhole-body interaction improve user experiences in virtualenvironments [4,21,35,36]. However, force-based interac-tion may not necessarily involve the whole body even ifthe haptic devices were grounded. In particular, mostactive haptic devices are not strong enough to support thehigh amounts of force or inertia of the whole body and

Figure 15. Task performances between

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often only provide localized contact interaction, therebylimiting the extent of the whole-body involvement andexperience.

Furthermore, our experiment has shown that even incases where the haptic device was able to support (e.g.,F-Handle) the whole body, the active feedback did notadd to improving the user experience or feeling of presencecompared with when relying only on the passive force in-teraction (e.g., G-Bar). This is mostly probably due to thenature of the given task in which the fidelity of the forcewas not too important. That is, it seems that the existenceof force interaction itself was important (for safe naviga-tion) rather than perceiving the exact amounts of force.We expect different results for tasks in which the perfor-mance is dependent on the correct perception of theamounts of the force (e.g., playing tennis and sculpture).

Reversely, our experiments also have shown thatwhole-body interaction can be significantly improved withthe element of force, active or passive. For instance, a free-moving Wii-mote interface induces a whole-body experi-ence but can be further improved significantly with the

the three interfaces in experiment II.

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Figure 16. Results for usability and fatigue. Contrary to the literature [10], isometric interfaces were found to be more tiring to use.

Table 8. The correlation coefficients among whole-body interaction, force feedback and presence.

Interaction design Examples Cost Effect

Interacting with environment objects (floor, wall and tangibleprops) using the hands/feet

DDR (Konami 2011) Low Med

Interacting using the handheld devices equipped withtactile/inertial feedback

Wii-mote (Nintendo 2011) Low/med Med

Interacting using the handheld isometric devices NG-Bar Low/med MedInteracting with grounded isometric devices G-Bar Low/Med HighInteracting with grounded active haptic devices Haptic device Expensive High

Evaluation of grounded isometric interface for VE navigationB.-g. Jang and G. J. Kim

element of force. With this in mind, several interactiondesigns are possible as summarized in Table 8.

7. CONCLUSION

In this paper, we presented the G-Bar, a low-cost, two-handed isometric whole-body “inducing” interface fornavigating in the virtual space. The induced whole-bodyinteraction and the passive force feedback proved to bethe critical contributing factor to enhancing the user expe-rience, namely the feeling of presence and immersion. Inaddition, despite the novelty of the technique, after mini-mal training, the users were able to achieve the level oftask performance and generally higher usability compara-ble with the nominal isotonic device as well.

In particular, our experiments have also found the syn-ergistic effect of the whole-body interaction and isometricinterface. In addition, the analysis of the subjective evalu-ation revealed a high correlation between the level of pres-ence perceived and that of whole-body interaction, whichwas not perceived as much with the active force-feedbackinterface was used.

The G-Bar may not be appropriate for all types ofvirtual tasks (e.g., those that require perception of theamounts of force, for interacting with fast-moving light

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objects with relatively little reaction force or for tasks thatare more natural with one hand), but we project that manyconventional interfaces could achieve the same benefits bythe “grounding” and/or adding isometric element. Our futuredirections include further enriching the virtual experience byadding multimodal interaction in an inexpensive way (suchas using vibration feedback and body gesture input).

ACKNOWLEDGEMENTS

This research was funded by the Forensic Research Pro-gram of the National Forensic Service (NFS), Ministry ofSecurity and Public Administration, Korea and the Na-tional Research Foundation of Korea (NRF) grant fromthe Korea government (MEST) (No. 2013-067321).

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SUPPORTING INFORMATION

Additional supporting information may be found in the onlineversion of this article at the publisher’s web site.

The video shows scenes from the Experiment III, illustratingthe three tested devices: (1) grounded isometric (G-Bar),(2) Non-grounded isometric (hand-held Bar), and (3) non-grounded isotonic (Wii mote).

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Authors’ biographies:

.

Bong-gyu Jang is currently workingas a research staff at the POSCO ICTin Korea. He studied virtual realityand HCI as a graduate student ofComputer Engineering at KoreaUniversity (master’s degree, 2011)under the guidance of Prof. GerardKim. He received his bachelor’sdegree from Kyunghee University,Korea (Computer Engineering, 2009).

Gerard Jounghyun Kim is cur-rently a professor of the Collegeof Information and Communica-tions at Korea University. Prior tojoining Korea University in 2006,he spent 10 years as a professor atthe Pohang University of Scienceand Technology (POSTECH). Hereceived a bachelor's degree inElectrical and Computer Engineer-

ing at the Carnegie Mellon University (1987) andmaster's (Computer Engineering, 1989) and PhD(Computer Science, 1994) degrees at the Universityof Southern California. Gerard’s research interestsinclude various topics in virtual and mixed reality,human computer interaction, and computer music.

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