A Comparison of Four Different Approaches to Reducing UnintendedPositional Drift During Walking-In-Place Locomotion

Niels Christian Nilsson∗

Aalborg UniversityStefania Serafin†

Aalborg UniversityRolf Nordahl‡

Aalborg University

ABSTRACT

Users wearing a head-mounted display while relying on Walking-In-Place techniques for virtual locomotion tend to physically driftin the direction which they are headed within the virtual environ-ment. It has previously been demonstrated that different types offeedback may be used to constrain the movement of the user. Thisposter presents a within-subjects study comparing four methods forensuring that the user remains within a certain area. The partic-ipants were asked to determine which method the generally pre-ferred and assess the perceived helpfulness and intrusiveness of thedifferent methods. The results indicate that passive haptic feedback(a carpet) was preferred and also was regarded as the most helpfuland the least intrusive. However, gathered qualitative data suggestthat this method might be used in combination with feedback inother modalities.

Index Terms: H.1.2 [Information Systems]: User/MachineSystems—Human factors; I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Virtual Reality;

1 INTRODUCTION

Walking-in-place (WIP) locomotion constitutes a promising solu-tion to the problem occurring when a virtual environment (VE) of-fers a larger freedom of movement than the physical interactionspace.However, it has been documented that WIP locmotion maylead to Unintended Positional Drift (UPD), that is, users walking inplace while wearing a head-mounted display may physically driftin the direction which they are headed within the VE [3]. If a WIPtechnique is to be considered a viable locomotion method it is cru-cial that the user remains relatively stationary. Nilsson et al. [2]performed a study suggesting that alternative forms of gestural in-put may considerably reduce the amount of UPD. Williams et al.[7] proposed that UPD may be controlled by placing the user ona physical object delimiting the area where walking is possible viapassive haptic feedback. However, no formal evaluation of this ap-proach was performed. Moreover, it has been proposed that ex-isting redirection techniques [6] may also be used to control UPD[3]. Recently, Nilsson et al. [4] performed a study investigatinghow different types of feedback influenced UPD. They compared13 methods involving different feedback indicating to the user that acertain amount of drift had occurred and a control condition devoidof feedback. The feedback varied in terms of sensory modality (au-ditory, visual or audiovisual), onset mode (gradual or sudden) andpresentation mode (a warning or a deprivation of the stimuli usedto represent the VE). Finally, a condition provided passive hapticfeedback (a circular carpet). The results suggested that both pas-sive haptic feedback and audiovisual feedback with a gradual onsetwere the most efficient at controlling UPD. However, passive haptic

∗e-mail:ncn@create.aau.dk†e-mail:sts@create.aau.dk‡e-mail:rn@create.aau.dk

feedback was experienced as the most helpful and the least disrup-tive for the experience of being in the VE. Notably, the study didnot involve gradual onset of passive haptic feedback. The studyperformed by Nilsson et al. [4], focused on assessing the utility andperceived ease of use of the individual types of feedback. How-ever, it seems likely that other factors, such as the choice of interac-tion metaphor, might also influence what methods users will prefer.Consequently, the current study was performed with the intention ofdetermining how passive haptic feedback compares to methods thatprimarily vary in terms of aesthetics in regards to user’s preference.

2 USER STUDY

Seventeen individuals (15 males, 2 females) aged between 19-41years (M=25.5 years, SD=6.5) took part in the study. They were re-cruited via a mailing list comprising volunteers from Aalborg Uni-versity Copenhagen.The study employed a within-subjects designand involved four different conditions presented in randomized or-der. Each condition corresponded to a unique method designed toalert the participants whether they were within a predefined walk-ing area (WA) with a diameter of 80cm. The WA was divided intotwo zones (see Figure 1).When the participant remained within thesafe zone no feedback was presented. When entering the warningzone the feedback gradually became more intense. The intensitywas mapped to the distance between the participant and the inneredge of the warning zone. As this distance increased the intensityof the feedback increased proportionally and reached its maximumwhen the distance to the center was 40 cm or higher. If the partici-pants stepped out of the WA, the area was visualized within the VE,thus making it possible to relocate it. The four methods were:

Warning: When the participants exited the safe zone a warningsign and warning sound were presented. They got gradually moreintense (lower transparency and higher amplitude) as the partici-pants got closer to the edge of the WA. This method was inspiredby a metaphor for safe navigation within VEs [1].

Spotlight: As the participants got closer to the WA’s edge theygot gradually deprived of the stimuli representing the VE (the vi-suals got darker and the sound decreased in amplitude) as if theystepped outside of a spotlight on a stage. This metaphor was con-sidered as it has been demonstrated that intermediate transitionalenvironment can increase the sensation of presence [5].

Greyscale: When the participants exited the safe the visualsgradually got more desaturated as the participants got closer to theedge of the WA.

Figure 1: Illustration of the walking area.

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Figure 2: The visual feedback of three of the methods. The inten-sity varied depending on the distance (d) between the user and thecenter of the WA. From the top: Warning, Spotlight and Greyscale.

Carpet: The participants stood on a circular carpet with a diam-eter corresponding to the one of the WA. The physical edge of thecarpet provided passive haptic feedback delimiting the WA.

Figure 2 illustrates the visual feedback used for Warning, Spot-light and Greyscale. The participants explored the VE and experi-mented with the four methods for as long as they wanted to. Aftereach trial they filled out a questionnaire regarding what method theypreferred and the perceived helpfulness and intrusiveness of eachmethod (rated on a 9 point scale). Finally, the participants wereencouraged to qualify their responses.

3 RESULTS

Repeated-measures ANOVAs were used to compare the results re-lated to helpfulness and intrusiveness (Figure 3). Significant differ-ences were found for both helpfulness (F(16,67) = 20.95, p < .01)and intrusiveness (F(16,67) = 71.49, p < .01). Post-hoc analyseswere performed by means of paired sample, two tailed t-tests usingBonferroni corrected alpha values. Pearson’s chi-square goodness-of-fit test was used to compare the results pertaining to preference(X2(3) = 16.2, p < .01) (Figure 4).

4 CONCLUSION

The results generally support the findings of Nilsson et al. [4], thatis, the carpet was experienced as the most helpful and the leastintrusive. Moreover, the study indicated that the passive hapticfeedback generally was preferred. When asked to qualify their re-sponses, all the 11 participants who preferred the carpet stated thatthey preferred this form of feedback due to the ease of use. Sixstated that they liked this method since it did not interfere with theaudiovisual feedback used to represent the virtual environment. Theparticipants who preferred Spotlight and Greyscale generally didso because they found it useful to be informed of when they weregetting closer to the edge of the WA before actually reaching it.Moreover, they preferred these approaches over Warning becausethey found the visual feedback aesthetically pleasing. Thus, whileperceived helpfulness and intrusiveness seemingly had the great-est influence on the participants’ preferences, the metaphor form-ing the basis for the feedback may also be of influence. Notably,several participants highlighted that even though they preferred thecarpet because it was more helpful and less intrusive, they did likethe possibility of pairing it with feedback in other modalities. In-deed, neither Spotlight nor Greyscale would presumably be usefulin isolation if the VE was dimly lit or involved desaturated colors.

(a) Perceived helpfulness of the methods

(b) Perceived intrusiveness of the methods

Figure 3: Results pertaining to perceived helpfulness and intrusive-ness. The dotted lines indicate significant differences (α = .008).

Figure 4: Results pertaining to the self-reported measure of prefer-ence.

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