Unintended Positional Drift and Its Potential SolutionsNiels C. Nilsson∗ Stefania Serafin† Rolf Nordahl‡

Aalborg University CopenhagenA.C. Meyers Vaenge 15, 2450 Copenhagen, DK

ABSTRACT

Walking-In-Place interaction techniques seem particularly useful inrelation to immersive virtual environments where the user’s move-ment is greatly constrained by a limited physical space. However,current techniques may not be particularly useful in combinationwith head-mounted displays since many users unintentionally moveforward while walking in place. We refer to this phenomenonaccidental movement as Unintended Positional Drift. The posterpresents evidence of the phenomenon’s existence and subsequentlydiscusses different design solutions which potentially could circum-vent the problem.

Keywords: walking-in-place, head-mounted display, locomotion

1 INTRODUCTION

One of the biggest challenges facing developers of immersive vir-tual environments (IVEs) is the potential discrepancy between thesize of the virtual and physical space. A virtual space can in prin-ciple be infinite in size and may therefore allow for unhinderedexploration on behalf of the user. However, the user’s real worldmovement is generally confined to a limited physical space. Thus,if the virtual environment offers a larger freedom of movement thanthe the physical environment then we may consider the two spacesto be incompatible. At best the problem of incompatible spacesmay negatively influence the user experience (e.g., if the user exitsthe area where high fidelity motion tracking can be achieved, thismay hamper the sensation of presence within the IVE). At worstthe incompatibility may be dangerous (e.g., a user ”blinded” by ahead-mounted display (HMD) may be unaware of obstacles in thereal world).

One way of approaching the problem is to facilitate virtual loco-motion through so-called Walking-In-Place (WIP) techniques. Ingeneral terms we may describe these as locomotion techniques en-abling users to move within virtual environments by performingbody movements resembling walking while remaining stationaryin the real world [3, 6, 13, 14]. Even though WIP techniques areinferior to real world walking in terms of simplicity, straightfor-wardness and naturalness [13], they have have been demonstratedto surpass ”push-button flight” in regards to subjective ratings ofpresence [6]. While exceptions do exist [11, 12], most WIP tech-niques rely on the same gesture for instigating forward view pointdisplacement within the IVE, namely, leg movements resemblingthose performed when walking up a flight of stairs.

A primary advantage of WIP techniques is that they in princi-ple do not require any movement on behalf of the user. This ar-guably makes such techniques particularly attractive in relation toconsumer IVEs where the spatial constraints may be very promi-nent. However, it would appear that the use of WIP techniques

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

in combination with HMDs is problematic. During our past ex-perimentations, we have informally observed that many individualswearing a HMD while walking in place physically move forward inthe same direction which they are headed within the virtual environ-ment. We refer to this phenomenon as Unintended Positional Drift(UPD). Positional drift has previously been observed on behalf ofblindfolded individuals walking or running in place after they havebeen walking or running on a treadmill[1, 5]. However, this phe-nomenon has to our knowledge not been demonstrated in relationto IVEs combining WIP techniques with HMDs as visual displays.Throughout the following we will present the results of our first at-tempt at measuring UPD which in turn illustrates that UPD indeedmay occur when one relies on a HMD for visual feedback whileusers are walking in place. Subsequently we present a series ofpossible design solutions which may eliminate or minimize UPD.

2 UNINTENDED POSITIONAL DRIFT

If WIP interaction techniques are to be considered meaningful so-lutions to the problem of incompatible spaces, it is crucial that theusers remain stationary. Thus, measurement of UPD should be con-sidered crucial to the evaluation of any WIP technique to be usedin combination with a HMD. We propose that UPD can be opera-tionalized in at least three ways: Maximum drift (the largest phys-ical distance the user has been from the point where the locomo-tion started), Total drift (the total physical distance covered by theuser, and Drift/travel ratio (the ratio describing how far the user hasdrifted in the real world per travelled distance in the virtual world).

We recently performed an experiment with the intention of de-termining how different gestures for controlling a WIP techniquemeasured up against one another in terms of naturalness [4]. Thecompared gestures were the conventional stair climbing gesture(Marching), a gesture resembling wiping ones feet on a doormatin that the user moved the lower leg backwards in order to producea step (Wiping), and finally a gesture where the user would tap theheels against the ground in order to instigate a step (Tapping). A to-tal of 27 participants (M=29.8, SD=7.1) tried all three techniques.They were instructed to walk naturally along a gravel path in a vir-tual environment. Naturalness was assessed by means of a post testquestionnaire and real and virtual positions were logged from thebeginning to the end of the walk. Even though UPD originally wasintended as a secondary measure, the evaluation of UPD did yieldinteresting results. The results obtained from the three measures ofUPD are summarized in Table 1.

A comparison of the means using repeated-measures analysesof variance (ANOVAs) followed by post-hoc analyses (t-tests usingBonferroni corrected alpha values of 0.017) revealed that Tapping

Table 1: Means ± one standard deviation for the measures of UPD

Marching Wiping TappingMaximum (m) 1.218±0.385 0.993±0.444 0.439±0.143Total (m) 5.297±2.867 4.357±1.678 2.204±0.328Drift/travel (m/m) 0.013±0.004 0.011±0.004 0.005±0.001

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Figure 1: Heat maps (5 x 5 m) generated from the users movementin the horizontal plane. From the left: Marching, Running Tapping

was significantly better than the other two in regards to all threemeasures of UPD. In addition to demonstrating that Tapping entailsthe least drift, these results may be viewed as a testament to theexistence of UPD. From Figure 1, depicting the movement of theusers in the horizontal plane, it is clear that Marching led to themost drift while Tapping led to the least.

3 POTENTIAL SOLUTIONS

In order to find ways of reducing or controlling UPD we may turnto existing redirection techniques for inspiration. Redirection tech-niques generally refer to a collection of techniques for subtly orovertly influencing the orientation or position of the user by contin-uously or discretely manipulating the stimuli used to represent thevirtual world [9].

3.1 Subtle Approaches

It is entirely possible that subtle techniques such as illusory manip-ulation of architecture [10] might be applicable. However, we be-lieve application of rotational gains to the user point of view to beparticularly promising. It has been demonstrated that it is possibleto turn stationary users 49% more or 20% less than the perceivedvirtual rotation and that it is possible to reorient walkers by 13% leftor rightwards while covering a distance of 5m [8]. Considering thata drifting user moves at considerably slower speed than a walker, itshould be possible to apply subtle rotational gains and achieve aneven greater physical reorientation per travel virtual distance. Thus,gradual rotation of the users point of view might make it possibleto achieve controlled UPD.

3.2 Overt Approaches

There exist a series of different overt redirection techniques whichmight be applicable, albeit possibly at the expense of the sensa-tion of presence. E.g., once the user has drifted to far away fromthe initial position one might use freeze-and-turn resetting [15] toreorient the user towards this point again. Alternatively, the usercould be made aware that the edge of the physical space has beenreached by means of Magical Barrier Tape – barrier tape is dis-played when the user reaches a physical boundary and reorienta-tion can be performed by manipulating the tape [2]. Similar resultscould be achieved through a simple Heads-Up Display warning theuser. Finally, we envision that it may be possible to design overtredirection techniques that are less detrimental to the sensation ofpresence. It has been demonstrated that gradual transition into aparticular virtual environment through a portal in an intermediatetransitional environment can increase the sensation of presence [7].Similarly, it might be possible to use a metaphor of the theatre stageto delimit the physical space. If the user unintentionally drifts or de-liberately steps away from the small area where the WIP techniqueis possible, the spot light is exited. The environment gradually be-comes darker and all actions ”on stage” are paused. If user doesnot wish to get ”off stage”, a step backwards will ensure that thisdoes not happen. Alternatively, if the user has stepped off stage, itis possible to look back at the spot light and see the virtual environ-

ment ”inside” the cone of light. Thus making the step on and offthe virtual stage is intuitive and possibly less intrusive.

4 CONCLUDING REMARKS

We have introduced the phenomenon Unintentional Positional Driftwhich may occur if a HMD is used to deliver visual feedback whileusers are walking in place. Notably the traditional walking in placegesture leads to a substantial amount of drift compared to the Tap-ping gesture. Finally, we presented possible design solutions thatmay eliminate or help control UPD. Subtle reorientation of the userand delimitation of the physical space through the use of a theatrestage metaphor appear to be the most promising.

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