Sabah Boustila
email: boustila@unistra.fr

Dominique Bechmann
email: bechmann@unistra.fr

Antonio Capobianco
email: a.capobianco@unistra.fr

E ects of Adding Visual Cues on Distance Estimation, Presence and Simulator Sickness During Virtual Visits Using Wall Screen

Keywords: Human-centered computing → Virtual reality, User studies, Virtual reality, virtual visits, architectural project review, stereo, head tracking, distance estimation, presence, simulator sickness

Stereo and head tracking are considered as distance perception cues in virtual environments. Several studies have investigated their in uence on several tasks. Results were di erent among studies. In this paper, we conducted a complete experiment investigating the in uence of the stereo and the head tracking in the speci c context of virtual visits of houses during architectural project review with clients. We manipulated the stereo and the head tracking in four conditions and we examined the e ects of the two factors on distance estimations (walls size, habitability, etc.), task di culty, presence and simulator sickness. Results revealed a signi cant e ect of the stereo on the judgement of the habitability, the dimensions of the rooms and task di culty. However, for both features, no signi cant e ects were found on the presence and simulator sickness.

judge the size of rooms and living comfort (habitability) to validate (or not) the mockup. us, they need to evaluate distances.

Previous works have found that distances are misperceived in VEs [START_REF] Jones | e E ects of Virtual Reality, Augmented Reality, and Motion Parallax on Egocentric Depth Perception[END_REF][START_REF] Ivelina | Egocentric distance perception in large screen immersive displays[END_REF]. For more realistic perception, VR systems can provide head tracking, stereo, etc.

e stereo has prompted many researchers to study its in uence on distance estimates. Piryankova et al. [START_REF] Ivelina | Egocentric distance perception in large screen immersive displays[END_REF] investigated the e ect of stereo using a large screen, they found less underestimations with a stereoscopic display for distances up to 2.5m. However, distances between 3m and 5.5m were similarly underestimated with or without stereo. Further, Willemsen et al. [START_REF] Willemsen | e E ects of Head-mounted Display Mechanical Properties and Field of View on Distance Judgments in Virtual Environments[END_REF] investigated the stereo with a head mounted display (HMD). In all VE conditions, distances between 5m and 15m were underestimated in comparison to the real world. Otherwise, distance estimation is also in uenced by accommodation and convergence related to stereoscopic display systems. Indeed, Bruder et al. [START_REF] Bruder | Distance estimation in large immersive projection systems, revisited[END_REF] found signi cant asymmetric e ect of these two features on the estimation of distances.

In addition to visual display delity (stereo, FoV, etc), immersive VR systems provide viewing interactions such as head tracking [START_REF] Mcmahan | Evaluating Display Fidelity and Interaction Fidelity in a Virtual Reality Game[END_REF][START_REF] Ragan | Studying the E ects of Stereo, Head Tracking, and Field of Regard on a Small-Scale Spatial Judgment Task[END_REF]. is last allows users to control the visual location in the VE by physical head and body movements. e change induced can help the viewer to judge distances between objects [START_REF] Khanzode | Challenges and bene ts of implementing virtual design and construction technologies for coordination of mechanical, electrical, and plumbing systems on large healthcare project[END_REF]. However, head tracking remains a weak distance cue. Jones et al. [START_REF] Jones | e E ects of Virtual Reality, Augmented Reality, and Motion Parallax on Egocentric Depth Perception[END_REF] found similar underestimations using HMD for distances between 2 and 8 meters whether using head tracking or not.

e combination of the stereo and the head tracking provides several distance informations and can helps the estimation of distances. A previous work indicated that stereo and motion information are very related to each other [START_REF] Domini | Stereo and motion information are not independently processed by the visual system[END_REF]. Some VR systems do not allow important head motions. Indeed, unlike CAVE and HMD, large screens provide display only on one screen in front of the viewer. us, just small head motions are allowed to stay focusing on the virtual scene.

erefore, we were interested in knowing whether this combination is necessary for distance estimations and which factor is more implicated when using large screen.

Since the perception is more similar to the real world when experiencing depth and point of view changes during body and head movements/shi ing [START_REF] Ijsselsteijn | E ects of Stereoscopic Presentation, Image Motion, and Screen Size on Subjective and Objective Corroborative Measures of Presence[END_REF], these two advanced features are expected to increase the sense of presence, in comparison to desktop tools. Indeed, the sense of presence was be er with stereo and head tracking in [START_REF] Hendrix | Presence in Virtual Environments As a Function of Visual and Auditory Cues[END_REF]. Participants navigate in a furnished virtual room displayed on a rear-projection screen in several conditions: stereoscopy/monoscopy display with and without head tracking.

is result is consistent with the result found in [START_REF] Ijsselsteijn | E ects of Stereoscopic Presentation, Image Motion, and Screen Size on Subjective and Objective Corroborative Measures of Presence[END_REF]. Participants reported a be er sense of presence when seeing a rally racing on a monitor screen using the two features. Besides, most of previous works found that stereo and head tracking a ect the simulator sickness in VEs [START_REF] Mcmahan | Evaluating Display Fidelity and Interaction Fidelity in a Virtual Reality Game[END_REF][START_REF] Schild | Understanding User Experience in Stereoscopic 3D Games[END_REF]. In video games context, Schild et al. [START_REF] Schild | Understanding User Experience in Stereoscopic 3D Games[END_REF] found a signi cant e ect of the stereo on simulator sickness.

e score was higher when using stereo. In this study, participants were si ing without movements in the two conditions (with and without stereo). us, the head tracking was not investigated. Furthermore, in the work of McGee [START_REF] Mcmahan | Evaluating Display Fidelity and Interaction Fidelity in a Virtual Reality Game[END_REF], the author investigated the in uence of head tracking on simulator sickness during a navigation task in a virtual o ce using a HMD and mouse to navigate. Results revealed that the score of simulator sickness increases when using head tracking.

In this paper, we conducted an experiment to investigate the e ect of stereo and head tracking on distance estimations, presence and simulator sickness during virtual visits. To the authors' best knowledge, no such complete user study has been conducted on the in uence of these two features.

EXPERIMENT

e experiment consists of virtual visits of houses at scale 1:1. e visit is carried out with an active guided navigation following a prede ned path represented by a green breadcrumb in the scene.

e participant was standing physically at a distance of 1.5 meters in front of the screen and controlling his virtual movements with a Wiimote. e navigation consists of moving only forward and rotating around using the Wiimote bu ons. In the head tracking conditions, participants were encouraged to move physically their body and head inside the tracking space.

During virtual visits, the participant was asked to answer orally the distance estimation questionnaire as explained in section 2.2.

Method

e experiment used a between-group design with two factors: stereo and head tracking. Each was varied by two levels. is provided a 2 X 2 design, as follow:

(1) Stereoscopic vision + head Tracking (ST) (2) Stereoscopic vision + no head Tracking (SnoT) (3) Monoscopic vision + head Tracking (MT) (4) Monoscopic vision + no head Tracking(MnoT) e between-group design was used in order to avoid any learning e ect biases that may result from visiting the same houses for four times.

Stereo and head tracking are two natural visual cues. Stereo seems to provide depth and distance informations. But in VEs it can increase simulator sickness symptoms. Besides, the changing in the perspective view provided by head tracking helps perceiving distances. Our hypothesis were: H1 Stereo and head tracking provide informations about distances and lead to a good perception of distances. H2 Stereo provided by the 3D glasses may induce eyestrain so, we hypothesis that stereo increases the simulator sickness. of view (FoV) and a 71 • vertical FoV. e resolution is 1400 x 1050. For the interaction, participants used a tracked Nintendo Wiimote.

Stereo was possible through active shu er glasses tracked using a Vicon tracking system based on 6 IR Bonita cameras (200Hz) providing a tracked space of 6 m 2 (see gure 1). e refresh rate was of 60Hz per eye. e virtual eyepoint height was set automatically corresponding to the subject's eye height. Interocular distance was xed to 6.3cm. Our program is developed in C++ and is based on the VRJuggler library for devices management.

e virtual scenes were four houses at scale 1:1 created according to real 2D mockups. ey were furnished with beds, sofa, tables, chairs, etc. Houses were created using 3DMax so ware, with simple textures. Realistic lighting was precomputed to avoid latency during on-line rendering.

2.1.2 Participants. 40 voluntary students (33 males, 7 females) in our university took part in this experiment, 10 for each condition. Average age was about 22 (between 20 and 26 years). All participants had normal or corrected-to-normal vision and had not previously performed experiments in VEs.

Measurements and evaluation

We evaluated the in uence of the stereo and the head tracking on distance estimates using a speci c questionnaire that we developed with architects. Moreover, we evaluated the in uence of the two features on the virtual visits in general using standard presence and simulator sickness questionnaires.

2.2.1

The distance estimation questionnaire. e aim of the evaluation using that questionnaire is to know the tendencies in distance estimates without quantifying the error. We aimed to know if distances are underestimated or overestimated.

Participants answered this questionnaire while immersed in the virtual scene. e questionnaire is divided into four groups of questions inspired by verbal estimation method and a ordance judgements method. e evaluation of the questionnaire is explained at the end of this subsection.

(1) Size estimates: questions to estimate dimensions of the rooms (width and depth) in meter and to estimate the overall area of the house in square meter. Since few people can be accurate in expressing directly distances with a speci c metric, we avoided direct verbal estimation and we preferred using closed-ended questions with 4 possible answers. Participants could choose only one answer among four propositions. As well, our aim was to obtain an estimation tendency (under/overestimation).

Example: choose from the following propositions the width of the room: a) 4.2, b) 4.9, c) 5.6, d) 6.3 m.

To avoid the systematic selection of the middle values, we explained to the participants that the correct answer is not always in the middle. In fact, correct answers were distributed on the 4 possibilities in all the questionnaire.

e three wrong values proposed with the correct answer were computed with an o set of 15% which is the just noticeable di erence threshold for 3D distances [START_REF] De Silva | Just noticeable di erence in depth model for stereoscopic 3D displays[END_REF].

(2) Furnishing judgements: 7-point Likert-type scale questions to assess the possibility of furnishing rooms and pu ing speci c furniture at speci c locations. ese questions depend on furniture present in rooms. In each room, the participant indicates if the place is wide enough to put furniture.

Example: if you had to place the wardrobe next to the bed, do you think that the space would be: 1) insu cient ,2), 3), 4), 5), 6), 7) very su cient.

In this example the right answer is 5. If the participant answers with 7, we consider that the size is overestimated.

(3) Habitability judgements: 7-point Likert-type scale questions to evaluate the capacity of the rooms to accommodate persons and to evaluate the feeling of comfort while imagining particular situations.

Example: When crossing a person in the corridor, the space available would allow you to do it: 1) with di culty ,2), 3), 4), 5), 6), 7) easily.

For the analysis of furnishing and habitability judgements, we just consider if the judgement is positive (overestimate) or negative (underestimate). (4) Task di culty: a 7-point Likert-type scale question to evaluate the di culty of the virtual visits and the estimation of distances in the VE.

Example: Evaluate di culty to visit and to estimate dimensions: 1) easy, 2), 3), 4), 5), 6),7) di cult.

is questionnaire was elaborated with architects. For furnishing judgements and habitability judgements the correct answers were calculated with them according to rules and standards of construction, e.g. the minimum width of a corridor is 90 cm. us, a person needs a minimum width of 40 cm, in order to cross another person in the corridor.

Besides, even if the right answer was based on norms and rules, the questions used do not need an accurate knowledge of them to give an answer.

estions address cases where the right answer can be given just by imagining situations.

The evaluation of questions: For the analysis, we computed the signed gap as follow:

Error gap = participant's answer -correct answer If it is null, the estimation is exact. If it is positive, so the evaluation is overestimated. Otherwise, the evaluation is underestimated. For instance, in the furnishing judgement's example, the right answer is 5. If the participant answers with 2 (insu cient) he underestimates the value. In this case, the error is -3 equivalent to 2-5.

Presence questionnaire ( PQ)

. We used the standard Witmer and Singer questionnaire translated in French by the University of ebec in Outaouais (UQO) Cyberpsychology Lab [START_REF]estionnaire sur l'tat de prsence[END_REF], to evaluate presence. e stereo and the head tracking are usually considered as additional cues for a high visual display delity [START_REF] Ragan | Studying the E ects of Stereo, Head Tracking, and Field of Regard on a Small-Scale Spatial Judgment Task[END_REF]. Our purpose is to investigate their in uence on the sense of presence during virtual visits.

Simulator sickness questionnaire ( SSQ).

To evaluate simulator sickness we used the questionnaire developed by Kennedy and his colleagues translated in French by the UQO Cyberpsychology Lab [START_REF]estionnaire sur les cybermalaises[END_REF]. e stereoscopic viewing with glasses can lead to eyestrain which is one of the component of simulator sickness. Otherwise, head tracking can cause vertigo and thus disorientation. Our aim is to investigate the impact of experimental conditions on simulator sickness.

Debriefing session.

Participants performed a semi-guided debrie ng at the end of the experiment to explain to the experimenter the methodology used to answer the distance estimation questionnaire.

Procedure

We investigated the four conditions presented in section 2.1. To minimize the impact of subjective appreciation based on furnishing preferences on subjects performance, participants visited four di erent houses. Subjects were randomly assigned to one of the experimental conditions. e total duration of the experiment is 1h20 for visits and questionnaires.

Virtual eyepoint height calibration. e virtual eyepoint height was set automatically according to the subject's eye height. When the head tracking is not used, the participant wears glasses and stands facing the screen, the virtual eyepoint height is se ing automatically. A erword, the experimenter disables the tracking and starts the experiment.

Experiment. Before starting the experiment, the participant reads instructions and trains on the navigation technique in the VE during 5 minutes.

e experiment starts, the participant moves using an active guided navigation following a prede ned path.

e aim of the guided navigation was to avoid navigation issues such as di culties when avoiding obstacles and in order to make sure that all participants had approximately the same visit time. e participant uses arrows on Wiimote to move forward, to stop (by pressing/releasing the forward arrow) and to turn around himself (right arrow to turn right and le arrow to turn le ). Rotations up and down and going back were not possible. When the participant is inside a room, he hears a beep and he gets stuck with only the ability to look around himself. e experimenter starts asking orally questions corresponding to the current room and notes the answers. e participant answers orally. A erwards, the experimenter unlocks the participant to visit the next room.

At the end, the participant answers the presence and the simulator sickness questionnaires and gives a debrie ng.

RESULTS AND DISCUSSION

For the analysis of the distance estimation questionnaire, rst, we computed the error gap for each question. Second, to apply a parametric test, we computed the mean values of error gaps (reported in table 1) for each group of questions.

Our independent variables are the stereo and the head tracking. e dependent variables are: size estimates (SE), furnishing judgements (FJ), habitability judgements (HJ) and task di culty (TD). We also analysed the e ects of the independent variables on the presence (PQ) and the simulator sickness (SSQ).

For the statistical analysis, we rst performed the Shapiro-Wilk normality test and Bartle homogeneity test to verify if our data ful lled the conditions of normality and homogeneity. When these conditions are respected, we realized a multi-factorial ANOVA at a 5% signi cance level. Otherwise, we performed a non-parametric Kruskal-Wallis rank sum test, and pairwise comparisons using the Nemeneyi post-hoc test. All results are reported in table 1.

Size estimates (SE) furnishing judgements (FJ) and habitability judgements (HJ)

As for furnishing judgements, the multi-factorial ANOVA have shown no main signi cant e ect of stereo and head tracking (pvalue = 0.710). Participants answer the furnishing judgements questions similarly in all conditions. Usually answers were underestimated. e mean of gaps for furnishing judgements varies between -0.404 and - No signi cant di erence was found between ST vs SnoT and MT vs MnoT. As shown on the gure 2, sizes are more overestimated when the vision is monoscopic without head tracking.

Trends.

According to the mean values, we note a di erence in the trends in estimations according to questions. Wall sizes were overestimated whereas furnishing possibilities and habitability were overall underestimated. is can be related to the strategy used to answer questions. Indeed, during debrie ng session, some participants mentioned using a computational strategy to evaluate the size of walls (size estimates), they calculated the distance between two corners based on their own size (mainly). However, for the other questions, they gave answers just by imagining situations and based on intuition. is is consistent with previous works who found that the accuracy in the estimation of distances is di erent according to the method used to give judgements (direct walking, verbal reports, etc) [START_REF] Timofey | How Does Presentation Method and Measurement Protocol A ect Distance Estimation in Real and Virtual Environments?[END_REF][START_REF] Richardson | Interaction With an Immersive Virtual Environment Corrects Users' Distance Estimates[END_REF].

Furthermore, the overestimation is probably due to the computational strategy. Indeed, if the participants imagine a segment less than their height and consider it equal to their height they will put more segment than necessary which leads to an overestimation.

is is consistent with the earlier nding [START_REF] Boustila | Evaluation of Factors A ecting Distance Perception in Architectural Project Review in Immersive Virtual Environments[END_REF][START_REF] Boustila | A hybrid projection to widen the vertical eld of view with large screens to improve the perception of personal space in architectural project review[END_REF][START_REF] Zikic | Evaluating Relative Impact of VR Components Screen size[END_REF]. In Nevena [START_REF] Zikic | Evaluating Relative Impact of VR Components Screen size[END_REF] the dimensions of rooms width and length indicated in feet, were also overestimated.

Stereo and head tracking.

For both size estimates and habitability we observed no signi cant di erence between ST, SnoT and MT, MnoT. We con dently interpret this as an absence of effect for head tracking. Probably the amount of head motion was much smaller leading to no signi cant e ect. Indeed, large screens provide display only on one screen in front of the view. us, even if participants were encouraged to move in the tracked space, they performed just small motions. Jones et al. [START_REF] Jones | e E ects of Virtual Reality, Augmented Reality, and Motion Parallax on Egocentric Depth Perception[END_REF] also found no signi cant e ect of head tracking on distance estimations.

As for stereo, we found an e ect on size estimates and habitability judgements only when the head tracking is used in ST and MT. When it is removed, the absence (MnoT) or the presence (SnoT) of the stereo do not a ect estimations. Similarly, Willemsen et al. [START_REF] Willemsen | E ects of Stereo Viewing Conditions on Distance Perception in Virtual Environments[END_REF] have found no signi cant e ect of stereo on distance estimation when head tracking was not used. Besides, even if the signi cant in uence of the two factors was the same between conditions for size estimates and habitability, the e ect was di erent. In fact, according to mean values (table 1), size estimates were more accurate when using stereo in ST and SnoT.

is means that adding stereo when head tracking is used improves size estimates. Nevertheless, this was the opposite for habitability judgements. e habitability was more underestimated in ST and SnoT.

Probably, the stereo with the head tracking help and improve estimations in the computational strategy used to evaluate dimensions of the walls whereas the visual impression of distances is not improved (for the habitability). However, since the strategy in furnishing judgements and habitability judgements was the same, the absence of signi cant in uence of the stereo and head tracking on furnishing judgements do not allow us to con rm this hypothesis. Unfortunately, we did not nd previous experiments that investigate stereo and head tracking on the estimation of distances using similar strategies. Besides, these results cannot validate or invalidate our rst hypothesis H1.

e e ect of the two factors is di erent from the results found in previous works. In fact, some previous works did not nd improvement in distance judgements using stereo [START_REF] Ivelina | Egocentric distance perception in large screen immersive displays[END_REF]. We emphasize that the nature of the task in our study is di erent from previous works. is di erence in the in uence of the stereo according to the task to perform was already mentioned [START_REF] Hu | Visual Cues for Perceiving Distances from Objects to Surfaces[END_REF]. Ragan et al. [START_REF] Ragan | Studying the E ects of Stereo, Head Tracking, and Field of Regard on a Small-Scale Spatial Judgment Task[END_REF] conducted an experiment where participants counted the connections (intersect or not) between horizontal and vertical lines based on depth. Stereo did not improve results. However, Hu et al. [START_REF] Hu | Visual Cues for Perceiving Distances from Objects to Surfaces[END_REF] found a signi cant e ect of stereo on task where participants were instructed to approach a virtual block towards a virtual table as close as possible without touching it.

Task di culty (TD)

Concerning task di culty we found signi cant results (p-value = 0.016). Task di culty was estimated as being average with a mean values around 4/7 (7 represent a di cult task). Pairwise comparisons revealed a signi cant di erence only between SnoT and MnoT (p-value = 0.016). e task was more di cult when the two factors were removed (MnoT) with a mean of 4.3/7 while easier with stereo (SnoT) with a mean of 3.4/7, see gure 4. is means that the head tracking alone do not in uence task di culty while the stereo do only when the head tracking is removed. e task di culty assesses the di culty of the experiment in the VE and the di culty of the distance estimation task. us, for head tracking, the result seems logical because it did not a ect the estimation tasks. As for stereo, previous work found that the importance of the stereo depends on the di culty of the task to perform in the VE [START_REF] Miller | Some e ects on depth-position and course-prediction judgments in 2-D and 3-D displays[END_REF].

Presence (PQ) and simulator sickness (SSQ)

Results reveal no signi cant e ect of stereo and head tracking on presence (PQ) (p-value = 0.576). e sense of presence was good in all conditions with means between 4.2 and 4.5 on 7 (7 is a good sense of presence), see table 1.

In contrast to previous work that found improvement in presence when using stereo and head tracking [START_REF] Ijsselsteijn | E ects of Stereoscopic Presentation, Image Motion, and Screen Size on Subjective and Objective Corroborative Measures of Presence[END_REF], in our study, results have shown a similar sense of presence in all conditions. Probably, the importance of the two factors depends on the nature of the task. In our study, participants had to visit houses and since the majority of them have already performed this task in real life, virtual visits seemed natural and probably led to a good sense of presence.

As for presence, no signi cant e ect of stereo and head tracking was found on simulator sickness (SSQ) (p-value = 0.144). e obtained simulator sickness scores (see table 1) revealed signi cant symptoms ( [START_REF] Hendrix | Presence in Virtual Environments As a Function of Visual and Auditory Cues[END_REF][START_REF] Hu | Visual Cues for Perceiving Distances from Objects to Surfaces[END_REF][START_REF] Ijsselsteijn | E ects of Stereoscopic Presentation, Image Motion, and Screen Size on Subjective and Objective Corroborative Measures of Presence[END_REF][START_REF] Jinjakam | Comparative Study of Virtual Sickness between a Single-screen and ree-screen from Parallax A ect[END_REF][START_REF] Jones | e E ects of Virtual Reality, Augmented Reality, and Motion Parallax on Egocentric Depth Perception[END_REF][START_REF] Kennedy | Con gural Scoring of Simulator Sickness, Cybersickness and Space Adaptation Syndrome: Similarities and Di erences? Virtual and Adaptive Environments: Appplications, Implications, and Human Perfoormance[END_REF]) for ST, SnoT and MnoT, according to the interpretation of scores in [START_REF] Kennedy | Con gural Scoring of Simulator Sickness, Cybersickness and Space Adaptation Syndrome: Similarities and Di erences? Virtual and Adaptive Environments: Appplications, Implications, and Human Perfoormance[END_REF] and minimal symptoms for MT ( [START_REF] Dodgson | Variation and extrema of human interpupillary distance[END_REF][START_REF] Domini | Stereo and motion information are not independently processed by the visual system[END_REF][START_REF]estionnaire sur les cybermalaises[END_REF][START_REF]estionnaire sur l'tat de prsence[END_REF][START_REF] Timofey | How Does Presentation Method and Measurement Protocol A ect Distance Estimation in Real and Virtual Environments?[END_REF][START_REF] Hendrix | Presence in Virtual Environments As a Function of Visual and Auditory Cues[END_REF]). However, this di erence in simulator sickness scores remains not signi cant.

Contrary to our expectations, we did not nd an e ect of stereo and head tracking on simulator sickness. is e ect is di erent from previous work that found a signi cant in uence when investigating simulator sickness in stereo video game visualisation and virtual navigation tasks [START_REF] Mcgee | Assessing Negative Side E ects in Virtual Environments[END_REF][START_REF] Schild | Understanding User Experience in Stereoscopic 3D Games[END_REF].

Lack of measured inter-pupillary distance (IPD)

Our project review tool is used by architects and their clients. For practical reasons and to save time, the architects preferred using a xed IPD for all clients. e IPD can a ect distance estimates and simulator sickness. For the simulator sickness, the score of the SSQ should increase when the IPD is not suitable. Indeed, Jinjakam et al. [START_REF] Jinjakam | Comparative Study of Virtual Sickness between a Single-screen and ree-screen from Parallax A ect[END_REF] found that the SSQ increases with an IPD of 2.0cm in comparison to 6.5cm. e IPD of 2.0cm is not adapted because normal IPD is between 5.0cm and 7.0cm [START_REF] Dodgson | Variation and extrema of human interpupillary distance[END_REF]. In our results, the absence of di erences between stereo condition and monocopic condition on global SSQ can be interpreted by the fact that the xed IPD of 6.3cm does not provoke negative e ects during virtual visits. Indeed, if it was not the case, the SSQ should increase when adding stereo.

Besides, we can note that used IPD does not bias size estimates, furnishing judgements, and habitability judgements because there was no signi cant di erence (no degradation) between ST and Mnot and Snot and MnoT in the judgements.

To con rm these interpretations, in future work, we will conduct an experiment with two conditions of stereo viewing, one with a xed IPD and the other with an adapted IPD.

CONCLUSION AND FUTURE WORKS

In this paper, we investigated the e ect of stereo and head tracking, with wall screen, on the estimation of distances, the presence and simulator sickness during virtual visits in a context of architectural project review. Distances were judged using a speci c questionnaire evaluating wall dimensions (size estimates), the possibility of speci c furnishing (furnishing judgements) and the leaving comfort (habitability judgements). Results revealed signi cant e ect of stereo when used with head tracking only on size estimates and habitability judgements. Otherwise, task di culty was higher when the two factors were removed. No signi cant in uence was found on presence and simulator sickness. In the present study, we have used a x inter-populary distance (IPD), for piratical reasons.

In the future we need to conduct an experiment to compare the e ect of a xed IPD and adapted IPD. Besides, we will conduct an experiment in real environment and compare results with those in the virtual environment.

2. 1 . 1

 11 Apparatus and stimuli. e experiment took place in the virtual reality platform In Virtuo in our laboratory. We have used a rear-projected wall screen (3 m x 2.25 m) with a 90 • horizontal eld

Figure 1 :

 1 Figure 1: e platform In Virtuo. An example of the virtual scene displayed on the large screen.

Figure 2 :

 2 Figure 2: Box plot representing mean of error gaps of size estimates for each experimental condition. Wall sizes were overestimated. Signi cant di erences were only between ST vs MT and SnoT vs MT.

  habitability judgements (p-value < 0.05). Pairwise comparisons have shown signi cant di erences between ST vs MT (p-value = 0.026, p-value = 0.001) and SnoT vs MT (p-value = 0.026, p-value = 0.015) for size estimates and habitability judgements respectively.

Figure 3 :

 3 Figure 3: Box plot representing mean of gaps of habitability for each experimental condition. Signi cant di erences were only between ST vs MT and SnoT vs MT.

Figure 4 :

 4 Figure 4: Box plot representing the mean of task di culty for each experimental condition. Task di culty was assessed as being average (means around 4). e task was more di cult in the condition MnoT.

Table 1 :

 1 1.651 as illustrated in table 1. Otherwise, the non-parametric test revealed signi cant in uence of stereo and head tracking on size estimates (p-value = 0.011) and All mean, p values showing the in uence of the stereo and head tracking on the dependent variables and PQ and SSQ.

	Conditions	SE	FJ	HJ	TD	PQ	SSQ
	ST	0.407	-0.923	-1.496 4.050 4.243 14.372
	MT	0.919	-0.404	0.522 4.068 4.244 8.889
	MnoT	0.672	-1.011	-0.431 4.300 4.290 10.097
	SnoT	0.436	-1.651	-1.044 3.400 4.469 12.288
	p-value	0.011	0.710	<0.05 0.016 0.576 0.144
	H(3)	11.079 F(3,36) = 2.670 10.23 9.356 5.407
			0.143


Julien Casarin
email: julien.casarin@gfi.fr

Dominique Bechmann
email: bechmann@unistra.fr

Marilyn Keller
email: marilyn.keller@gfi.fr

A Unified Model for Interaction in 3D Environment

Keywords: Human-centered computing → Interaction design, • Computing methodologies → Graphics input devices, Mixed / augmented reality, Virtual reality, Model-Based Interactive System Development, Virtual Worlds, Input Techniques

: We propose a new model for designing VR, AR and MR applications independently of any device. Our model allows the user to visualize a virtual world and to interact with it on any kind of device. These pictures shows a user interacting with the same virtual object on a desktop, on a touchscreen and on an Oculus Rift HMD.

INTRODUCTION

The fast evolution of Virtual Reality (VR), Augmented Reality (AR) and Mixed Reality (MR) devices reduces significantly the lifetime of the applications running on those devices. On the other hand, there are major differences between devices (dimensions of the display, number of degrees of freedom of the interaction device). As a result, the portability of one application from one device to another often takes a long time and can be quite complicated. The existing approach consists in using game engines (Unity, Unreal Engine, ...) that enable the development of a single application for several types of devices. The combination of this approach with tools such as Middle VR [START_REF] Kuntz | MiddleVR a generic VR toolbox[END_REF] makes it possible to simplify the application development of VR, AR and MR by providing high-level tools. However, since the interaction has to be adapted to the specific features of each type of device, it is necessary to rework the application so that it supports a new technology. Thus, the appearance on the market of a new device leads to the need to update all the applications. We propose a new approach to increase significantly the portability of VR, AR and MR applications. This consists in considering devices as tools able to interact with any 3D environment. This approach requires the creation of a standard allowing the description of any environment for any device. If standards exist to describe objects of a 3D scene (OBJ, DAE, VRML, ...), there is no unified model so far, that allows to describe interactions of one or several users.

To create this model, we rely on the use of degrees of freedom to define an abstraction layer between the tasks to perform and the device. Manipulations, application control and selection are defined using a reduced set of interaction blocks. Those blocks are transmitted via a network to a device. Then the device interprets them and activates them according to the actions of the user.

After presenting the various interaction blocks that we used (section 3.3), we present our model implementation (section 4) , then we test it on two 3D environments samples with the three devices shown Figure 1 (section 5). Finally, we show the contribution of this method in terms of portability of the applications and compare it to the existing applications (section 6).

STATE OF THE ART 2.1 3D Environments

In the remainder of this article, we call 3D environments any virtual, augmented and mixed environment that includes virtual objects positioned in a three-dimensional space. A virtual environment is a fully computer-generated three-dimensional environment [START_REF] Bryson | Virtual Reality in Scientific Visualization[END_REF]. Augmented Reality allows virtual environments to be superimposed on the real world. We call augmented environment an area of real space on which a virtual environment is superimposed. In Augmented Reality applications, the interaction level between real and virtual objects is highly variable. It can range from graphical positioning of virtual objects on a real image [START_REF] Azuma | Recent Advances in Augmented Reality[END_REF][START_REF] Tumler | Mobile Augmented Reality in industrial applications: Approaches for solution of user-related issues[END_REF] to a realistic inclusion of virtual objects in the real world [START_REF] Koleva | Traversable Interfaces Between Real and Virtual Worlds[END_REF].

In [START_REF] Milgram | Augmented reality: a class of displays on the reality-virtuality continuum[END_REF], Milgram defines Mixed Reality as the continuum between reality and virtuality (Figure 2). Any environment including real elements and virtual elements can therefore be considered as a mixed environment.

In practice, environments are referred to as Mixed Reality environments when real objects and virtual objects interact (almost) as freely as if they would belong to the same physical space (subject to physical constraints such as gravity and collisions, occlusion, Bidirectional Data exchanges ...).

Manipulation in 3D environment

In 1994, the work of R.J.K. Jacob & al. [START_REF] Robert | Integrality and Separability of Input Devices[END_REF] showed that the device, whose control structure is the closest to the perceptual structure of a task, is the most suitable for performing this task. Therefore, it seems that the degrees of freedom composing the perceptual structure of a task can be used to describe it generically. In more recent work, M. Veit & al showed that users instinctively decompose some complex tasks in order to simplify them [START_REF] Veit | Influence of Degrees of Freedom's Manipulation on Performances During Orientation Tasks in Virtual Reality Environments[END_REF]. Moreover, this decomposition may vary during the task according to the desired speed of execution and/or the precision [START_REF] Veit | Dynamic Decomposition and Integration of Degrees of Freedom for 3-D Positioning[END_REF]. Finally, it appears that a separation of certain degrees of freedom in the design of the interaction leads to better performances of the user if this decomposition is consistant with how the user would simplify the task by himself [START_REF] Veit | Influence of Degrees of Freedom's Manipulation on Performances During Orientation Tasks in Virtual Reality Environments[END_REF][START_REF] Veit | An Experimental Analysis of the Impact of Touch Screen Interaction Techniques for 3-D Positioning Tasks[END_REF]. Therefore, we need to add to our generic tasks description, information on how the degrees of freedom are preferentially integrated or separated by the user.

OUR MODEL

The aim of our work is to restrict the porting of a set of 3D environments on a new device to the development of a single dedicated application.

For this purpose, we totally separate the development of the 3D environment from the user device. So the 3D environments developed with our model run on a separated computer or on a server. As these environments are described generically, it is possible to develop a dedicated application, for a given device, able to display all 3D environment. The portability the of 3D environments is clearly improved because the classic approach requires the development n*m applications when our approach requires the development n+m applications (with n the number of devices and m the number of environments). In the same way, it is sufficient to develop a unique application (or client) to support a new device on all the environments and each new environment will be automatically supported by all the devices.

To achieve our model, we propose the use of an abstraction layer in order to design the interaction independently of the nature of the device. This abstraction layer consist in a set of interaction blocks common to all devices. The instantiation and the composition of these blocks makes it possible to script the interaction of the user with the environment.

Because of the separation between the 3D environments and the devices, the devices have to instantiate the interaction blocks transmitted by the environment in order to generate the user interface allowing the interaction with the environment.

Overall functioning

The overall functioning of our system is illustrated in Figure 3. A 3D scene is transmitted in real time to one or several devices through a communication network. A copy of the scene is then created and synchronized on each device. The same network allows the transmission of a set of basic interaction blocks to the device. These blocks constitute a set of standard objects allowing the description of the actions that the device may perform on the environment. Each device is able to perform a projection of these blocks on itself. This operation consists in generating dynamically the user interface making it possible to carry out the described actions with the device. Menus and parametrization tools are created on the fly to allow the setting of the environment and the selection of geometric tasks. The transmitted tasks are then projected on the degrees of freedom of the device. Finally, the interaction blocks also indicate which objects can be targeted and/or selected by the user. When implementing a client for a device, the interaction blocks projection is optimized according to the device properties (dimensions of the display, number of degrees of freedom of the interaction device, ...).

Once the projection on the device is completed, the interactions of the user with the device triggers the activation of some blocks. This activation is requested by the device through the network already used for the transmission of the scene and the interaction blocks. The environment reacts then by modifying the scene and/or the list of interactions given to the user. For example pressing a button could make an object of the scene bigger while an other could activate the rotation of an object and thus add the rotations along the x, y, and z axis to the interaction list

Basic interaction blocks

We define the interaction blocks as a finite set of instantiable and parametrizable objects (Figure 5). These objects describe the interactions with a 3D environment. Each VR, AR or MR device must be capable of interpreting and activating all of these blocks. The real-time evolution of the blocks allocated to the users, according to their actions, defines the interactive scenario associated with the 3D environment. We have chosen our interaction blocks according to the following criteria. First of all, the blocks must be interpretable by any type of device. Then, the composition and the parametrization of the blocks must make it possible to design any type of interaction with a 3D environment. It must also be possible to give information on the perceptual structure of the tasks so as to guide the projection on the device. Finally, the number of blocks must remain limited to facilitate the development of the interpreter for a given device.

Manipulation.

The existing approach consists in directly linking the degrees of freedom of a 3D object to the degrees of freedom of an input device, while our approach consists in associating a generic motion to a task in a 3D space. This movement, described with interaction blocks that we name Dof and Dof Integration, is projected by the device on its own degrees of freedom. Figure 4 shows how the translation of a cube on a table can be achieved by composing a Dof Integration block with two Dof blocks and how these blocks can be projected on a gamepad.

The Dof object defines a generic movement with one degree of freedom. This degree of freedom can be either a translation or a rotation around an axis. The motion described is either continuous or discrete. This degree of freedom is defined relative to a reference object in the scene. If no reference object is defined, the user space is used as the default reference.

The Dof Integration object allows to combine several degrees of freedom. For example, this block can be used to define a translation not along an axis (1 Dof) but in a plane (2 Dofs) like in Figure 4. Thus, the interaction block Dof Integration allows to define a movement with several degrees of freedom. The device on which these interaction blocks will be instantiated must then integrate these degrees of freedom if its structure allows it. If the device is not able to integrate the set of degrees of freedom as defined by the Dof Integration block, they remain separated. In this case, a menu is generated to let the user select alternately the separated degrees of freedom.

Figure 4: Sample of interaction blocs usage. Two Dof blocks and a Dof Integration block are associated in order to describe the planar translation of a virtual cube on a table with a two degrees of freedom generic movement. These blocks are transmitted to a device equipped with a joystick. This device interprets the interaction blocks and projects them on the joystick by associating the described generic movement with a two degrees of freedom stick. This association allows to activate the Dof Interaction bricks when moving the stick and thus move the cube.

Selection & Targeting.

The way to target or to select a 3D object differs greatly from one device to another. Indeed, even if the use of ray casting is the most widespread technique, other techniques exist. On the other hand, the way of manipulating the casted ray totally depends on the structure of the device. Therefore, it seems inappropriate to constraint how the targeting and the selection are carried out by the device, so we introduce two basic interactions to indicate that a 3D object can be targeted or selected.

The Hover object tells the device if a virtual object can be targeted by the user. This block can be activated to indicate the beginning and the end of the targeting and makes it possible to create a visual feedback showing to the user that he can interact with the object.

The Pick object allows to associate an event to the selection of a virtual object by the user.

Application control.

The application control gathers all the interfaces available to the user to set up the application and/or select tools: Menus, buttons, text input, . . . Since the 3D environment is created separately from any device, the application control have to be described to the devices with some specific interactions blocks. We propose to describe, in a generic way, the means of parametrization by two interaction blocks:

The Action object allows the launch of an event. This object can have a list of parameters as argument. An Action without parameters typically corresponds to a button in the final interface. An action with parameters corresponds to an input form.

The Direct Input object allows to modify in real time a parameter of the application or a 3D object (numerical value, options selection, text field, . . . ).

If the two previously described blocks are sufficient to describe all the elements necessary for the application control, they do not always make it possible to generate an optimal user interface. Indeed, the arrangement and the grouping of the similar elements of the interface are essential for ergonomics. We introduce then a last interaction block called Interaction List which guides the creation of menus and submenus during the generation of the user interface. The Interaction List block is used to gather several interaction blocks that have the same semantic value or close uses. This aims to cluster elements which must eventually be grouped in the user interface.

IMPLEMENTATION 4.1 Shared Virtual Environment Platform (SVEP)

To validate our model, we implemented a Web platform able to host simultaneously multiple 3D environments. This platform was developed with the Node.js technology which allows us to quickly create 3D scenes using the Three.js library. A 3D environment hosted on the platform is made of a script, jointly defining a 3D scene and interaction blocks, but also resource files corresponding to 3D models and textures. Finally, Socket.io was used to create a WebSocket allowing real-time communication between a 3D environment and the visualization devices. All the technological choices for the platform were made to validate the model quickly. A future use of game engines such as Unity or Unreal Engine is considered in order to enrich the developed 3D environments and increase performances. The global architecture of our implementation is presented in Figure 6.

SVEP Unity 3D client

In order to speed up the implementation of a client for a given device, we have developed a package for the Unity 3D game engine. This package makes it possible to disregard the network connection with the SVEP platform during the development of a client. Moreover, it allows to pool the architecture of all clients using Unity (Figure 6). A SVEP client is made of 5 modules:

4.2.1 SVEP Core.
The SVEP Core module is in charge of the communication between the clients and the hosted 3D environments (Figure 7). It also deserializes messages sent by SVEP into standard objects and it controls the other modules.

RESULTS

Tested devices

In order to evaluate the portability of 3D environments developed according to our model, we implemented three clients (Figure 1) for the SVEP platform and could verify that the development of a single application allows the portage of all the 3D environments on a new device. The main stages of the implementation are summarized below and in Table 1.

5.1.1 Scene. Since the computing power and the graphic capabilities are similar, all the tested devices share the same implementation of the Scene module. [START_REF] Kato | A registration method based on texture tracking using ARToolKit[END_REF] to perform, in real time, the tracking with markers using the ARToolKit.

Menu & Application control.

For devices 1 and 2, the user interface grouping the Menu module and the application control generated by the Player module is a traditional 2D interface superimposed to the 3D scene. However, the Oculus Rift (device 3) does not allow this type of interface, so a three-dimensional interface has been designed for this device. In order to evaluate our model, we developed a 3D environment allowing a user to manipulate a virtual object (Figure 8) and published it on the SVEP platform. Our objective was to verify that our model allows the manipulation of a virtual object by any device, that a reaction to the manipulations of the user can be dynamically generated, and finally, that it is possible to update in real time the interactions available to the user so that we can script his or her exchanges with the environment. This second mode gives to the user a set of interactions allowing him to configure the degrees of freedom of the manipulation mode. Figure 10 shows the menu generated by device 1 (Table 1) for configuration mode. The configuration makes it possible to choose if manipulations are continuous or discrete. In the case of a discrete rotation, the rotation pitch is configurable. The translations along the X and Y axes can be integrated (Figure 8) or separated (Figure 9). Finally, selecting the object results in returning to the manipulation mode. If the implementation of the environment presented in Section 5.2 allowed to validate the basic principles of our model, this single test is not sufficient to evaluate the relevance of the model for the scripting of complex interactions. We present below a complementary environment showing how the definition of a theoretical movement by interaction blocks makes it possible to script the simultaneous manipulation of several objects. With this test, we wanted to ensure that the Dof and Dof Integration interaction blocks are sufficient to create, not only virtual objects translations and rotations,but also complex manipulations like scaling, multiple selection or event triggering. The developed 3D environment shows that it is possible to associate such a complex manipulation to a generic translation or rotation move created with the Dof Integration and some Dof interaction blocks. The environment is made of a stack of plates. The goal of the user is to sort the plates according to their size. The only authorized operation is to return the top of the stack (Figure 12).

Environment 1 : Virtual object manipulation & feedbacks

One Dof selection.

A block with one degree of freedom is used in association with a vertical movement. A movement from top to bottom or from bottom to top allows to add or remove a plate from the selection (Figure 13).

Using a discrete Dof allows to define the amplitude of the movement used to add or remove a plate from the selection. The possibility of selecting the plates in several goes allows the user to correct his selection.

Full task.

A second discrete degree of freedom is used to trigger the reversal of the selection by associating it to an horizontal movement. The integration of these two Dof allows the user to select and flip the top of the stack in a single motion (Figure 14). By construction, the portage on a new device of all the environments developed according to our model does not depend on the number of environments, but depends on the difficulty to implement the projection of interaction blocks on the device. The use of our model thus allows an increasing portability gain when the number of environments increases. On the basis of the tests presented in Section 5, it seems that the composition and the dynamic evolution of interaction blocks (Section 3.2) allow to script and describe the interactions with a 3D environment in a generic way. Moreover, it appears that this model naturally allows collaboration between different types of devices.

However, additional work is needed to establish good practices for scripting in a shared environment. Indeed, it is likely that visual feedbacks such as the one proposed in Figure 11, generate in some cases, cognitive overloads for the other users. These visual feedbacks could, on the other hand, be beneficial for solving known problems such as the join attention issue presented in [START_REF] Schnier | Collaboration in Augmented Reality: How to establish coordination and joint attention?[END_REF].

CONCLUSIONS

The main objective of this article was to present a generic model of the interaction in 3D environment allowing to totally ignore the nature of the user device during the development of the environment. To do this, we have shown that the degrees of freedom can be used as an abstraction layer common to all devices to define manipulation of 3D objects. Finally, after presenting our implementation of the model, we have shown how it can be concretely used for the scripting in 3D environment.

Moreover, this model seems suitable for the creation of collaborative environments. The possibilities offered by facilitated collaboration between complementary devices open up new perspectives, particularly in the area of collaborative design. However, complementary work establishing good scripting practices with the model is to be foreseen before any rigorous evaluation of its contribution in the field.

Figure 2 :

 2 Figure 2: Milgram continuum 1995. [7]

Figure 3 :

 3 Figure 3: Main model

Figure 5 :

 5 Figure 5: Basic interaction blocks (UML)

Figure 6 :

 6 Figure 6: SVEP

Figure 7 :

 7 Figure 7: Communication between a SVEP Client and a 3D environment

4. 2 . 2

 22 Scene. The Scene module dynamically loads the virtual scene of the 3D environment and synchronizes it in real time. The Scene module must extend the Abstract class Scene of the Unity package. The use of an abstract class makes it possible to restrict the developments for a given client to the implementation of a loading strategy adapted to the device capabilities. Several devices can therefore use the same implementation of this module if they have similar performances (RAM, graphic memory, quality of the network connection).

4. 2 . 3

 23 Player. The Player module is responsible for the dynamic projection of the interaction blocks on the device. If an abstract class of the Unity package allows to pool the interfaces with the SEVP Client module, it is necessary to specifically implement the projection of different interaction blocks on the device.

4. 2 . 4

 24 Menu. The Menu module allows the user to navigate between the 3D environments available through the SVEP platform.

4. 2 . 5

 25 Tracking System. The Tracking System module (or TS) is only implemented for the Augmented and Mixed Reality devices. It is responsible for the dynamic positioning of virtual objects based on the real objects of the environment (markers, GPS position . . . ).

Figure 8 :

 8 Figure 8: Integration of the degrees of freedom

Figure 9 :

 9 Figure 9: Separation of the degrees of freedom

Figure 10 :

 10 Figure 10: Manipulation configuration

Figure 11 :

 11 Figure 11: Feedbacks

5. 3

 3 Environment 2: Simultaneous manipulation of several objects

Figure 12 :

 12 Figure 12: Sort the stack !

Figure 13 : 1

 131 Figure 13: 1 Dof selection

Table 1 :

 1 Tested devices Tracking System. Only the device 2 is capable of displaying a 3D environment in Augmented Reality. It is therefore the only device implementing the Tracking System module. The module makes it possible like in

	Device	1	2	3
	Display	Smart TV	Pc touchscreen	Oculus rift HMD
	Navigation	Mouse & keyboard	ARToolKit	Oculus rift IR tracking
	Tracking System	.	ARToolKit & Camera	.
	Interaction System	Mouse & keyboard	Touchscreen	Oculus rift remote
	Max Integrated Dofs	2	2	1+1
	UI	2D overlay	2D overlay	2D in world position