Florian Nouviale

Valérie Gouranton

Ronan Gaugne

Sonic interaction with a virtual orchestra of factory machinery

Keywords: Sound spatialization, audio-visual interaction, virtual environments H.5.2 [INFORMATION INTERFACES AND PRESENTATION (e.g., HCI)]: User Interfaces -Auditory (nonspeech) feedback, H.5.5 [INFORMATION INTERFACES AND PRESENTATION (e.g., HCI)]: Sound and Music Computing-Signal analysis, synthesis, and processing, H.5.5 [ (e.g., HCI)]: Multimedia Information Systems -Artificial, augmented, and virtual realities

The virtual orchestra of factory machinery.

INTRODUCTION

It has been proven that sound highly enhances the feeling of presence [START_REF] Lombard | At the Heart of It All: The Concept of Presence[END_REF] in virtual reality environments [START_REF] Taylor | RESound: interactive sound rendering for dynamic virtual environments[END_REF]. In particular, in immersive structures such as CAVE [START_REF] Cruz-Neira | The CAVE: audio visual experience automatic virtual environment[END_REF] or assimilated facilities, audio rendering is carefully integrated in order to improve the feeling of being-there for the user, and several technologies have been proposed and studied to implement a rich sound environment [START_REF] Kuhlen | A true spatial sound system for CAVE-like displays using four loudspeakers[END_REF] [START_REF] Gröhn | Localizing Sound Sources in a CAVE-Like Virtual Environment with Loudspeaker Array Reproduction[END_REF]. Nevertheless, relatively little work explore the possibility for the user to dynamically create a sound universe in an immersive environment. For example, in [START_REF] Berthaut | Myriam Desainte-Catherine Interacting with the 3D reactive widgets for musical performance[END_REF] Berthault et al. propose an immersive application for 3D musical interaction. However, the interface presented to the user is very rich, with complex interactive interfaces and is mostly dedicated to trained musicians.

In this paper, we present an immersive environment for 3D sound interaction that was designed for a public exhibition around the theme "Science and Music", the Journée Science et Musique (JSM). The immersive application was designed to be used in Immersia (http://www.irisa.fr/immersia), a large CAVE-like infrastructure of the IRISA-Inria laboratory, located in Rennes, France. The aims of the application are (i) to immerse users in an interactive visual and sound environment, (ii) to be easy to access for the general public, and (iii) to illustrate in an entertaining way the use of sounds in VR.

The virtual environment used in this application places two users in a factory with two machines, close to them, as presented in Figure 1. By using Wiimote controllers, they can trigger and control punctual sounds, animations, sound and visual loops, as well as the global tempo of the application.

Section 2 presents the context in which this application was created. Then, section 3 describes the application and the hardware used. Finally, before concluding, Section 4 presents implementation details.

CONTEXT OF THE APPLICATION

The Immersia platform

The immersive platform of the IRISA-Inria computer science laboratory is a large virtual-reality facility dedicated to real-time, multimodal (vision, sound, haptic, BCI) and immersive interaction. It hosts experiments involving interactive and collaborative virtualreality applications.

Images are rendered on four glass screens, a front one, two sides and a ground with an overall dimension of 9.6m wide, 3m deep and 3m high. Over 20 million pixels are displayed using a video projection system combining thirteen 3D HD Barco projectors. The tracking system is composed of 16 ART TRACK2 infrared cameras. Sound is spatially rendered using either a 10.2 sound Laurent S. R. Simon* system with speakers or a 5.1 headset system, controlled by the user's position.

Immersia is a key node of the FP7 European Project Visionair [START_REF] Kopecki | VISIONAIR: VISION Advanced Infrastructure for Research[END_REF] which goal is to create a European infrastructure that should be a unique, visible and attractive entry towards high-level visualization facilities for Virtual Reality, Scientific Visualization, Ultra High Definition, Augmented Reality and Virtual Services. These facilities, distributed across around twenty countries in Europe, are open and easily accessible to a wide set of research communities. Both physical access and virtual services are provided by the infrastructure. Full access to visualizationdedicated software is offered through call for projects, while physical access to high level platforms is partially accessible to other scientists, free of charge, based on the excellence of the project submitted. Immersia hosted three scientific projects from the beginning of Visionair, one in ergonomics, one in sport training, and one studying 3D sound perception in a large virtual environment.

The Science and Music day (JSM)

The JSM is an event of science popularization that occurs every year, during which a varied audience is invited to workshops, conferences and concerts on the theme of "Science and Music". The organizers attempt to keep a scope of activities as wide as possible, thus involving researchers, artists, engineers and technicians.

JSM has therefore become an event of discovery, meetings and discussions on the theme Science and Music.

More specifically, themes covered by the JSM include new interfaces for music interaction, room acoustics, music information retrieval, new medias, psychoacoustics, sound synthesis, 3D audio and virtual reality. In 2013, it included the Immersia workshop presented in this paper.

FACTORY CONCERT

Technical context and constraints

This project was conducted in a limited time. Consequently, it had to make use of existing spatialization tools instead of designing specific ones for Immersia. Ten Genelec 8040 and two Genelec subwoofers were available for use. They were linked to a Yamaha DME64N that performed the matrixing of the signals sent from the eight available outputs of the MOTU 896HD. Loudspeakers locations are shown on Figure 2. On this loudspeaker configuration, the two loudspeakers located at the center, front of the room, are connected to the same channel and placed 1.2m apart. Similarly, the two loudspeakers located at the center rear of the room, are connected to the same channel. It was not possible to replace them with a single loudspeaker, as the room is regularly used in dual mode, where each side of the room produces two independent images and sound workspaces. It was not possible either to use one different channel for each of the ten loudspeakers at the time of the project because of the limited number of free available inputs on the Yamaha DME64N. The screens of the immersive room are made out of glass, and the images are retro-projected on them. The loudspeakers could therefore not be placed at ear height but had to be fixed above the 3 meters high walls. This could cause the sounds to be perceived as coming from above when the visual objects are located at ears height. In addition, it also means that the environment of the Immersia platform is highly reflective (RT60 = 1.30 second measured from the slope of the impulse response curve taken between 0.1 and 0.5 seconds), causing numerous strong early and late reflections. This should increase the Apparent Source Width (ASW) and decrease the locatedness of the audio sources, loudspeakers in this case [8][9]. It can be hypothesized that increasing the ASW of the loudspeakers would consequently increase the ASW of the sound images produced by these loudspeakers.

The surface of Immersia implies that the distance between adjacent loudspeakers is at least 1.94m, which can create spatial aliasing, as will be discussed in section 4.2.1.

The audio processing was performed in MAX/MSP. Aside of this software, all the other audio tools used were freely available tools.

As described in section 2.1, the position of the main user was tracked and the visual environment display was modified according to his point of view. This means that the system can be optimal for a single user only. Sound reproduction could therefore be optimized only for this single user as well. The system is therefore suboptimal for any additional user.

User interaction and sound feedback

The system is designed to be controlled by two users, both of them wearing shutter glasses, but only one of them tracked for visual and sound computations.

The tracked user is immersed in the virtual environment and can navigate inside this environment by walking inside the surface of Immersia. He or she is provided with two Nintendo Wiimote controllers. The buttons of the first Wiimote can start and stop two pairs of visual and audio loops on the main machine and trigger two pairs of animations and audio sounds. Using this Wiimote can also change the tempo. By shaking the second Wiimote, the user can trigger a pair of sound and visual effect.

The second user however is not immersed in the virtual environment. This is especially true when he is far from the tracked user and when this tracked user is close to one wall, where the display of the environment on the screens could differ and can be deformed a lot. However, as the scene is containing few objects and the relative position of the virtual scene displayed compared to the tracking origin was fixed, it is easy to interpret the display of the screens.

This second user is also provided with two Wiimote controllers. The buttons of first controller allow controlling the travelling crane by triggering sounds and animations making the crane move on a horizontal plane and following a rectangle path around one of the machines. By shaking this Wiimote, the user can trigger a pair of a maracas-like sound and a visual effect on the secondary machine. The buttons of the second Wiimote controller trigger other sounds and animations of the crane, which will move on the vertical axis. The crane can thus reach 3 different heights. By shaking the second Wiimote controller, the user can also trigger a pair of an udu-like sound and a visual effect at the same position as the first controller.

The buttons of all the Wiimotes triggered sound and visual effects played at the same speed while the shaking encouraged the users to follow the global tempo given by the other effects.

One of the uses of the Immersia platform is for archeological studies, where old rooms or buildings can be visualized and researchers can physically walk and navigate in the reconstructed place. One could therefore also imagine this immersive system being used for simulation of concert rooms. Provided it is possible to reproduce any auditory scene sufficiently faithfully in the Immersia platform, architects could walk in the concert room they designed and listen to how it sounds. They could also compare different solutions they might have for a concert room. Following this idea, we simulated two different rooms for the scene of this project: in one case, the simulated room had a Reverberation Time (RT) of 1.2 second; in the other, it was a church that had a RT of 7 seconds. The change of room was both visible and audible and was triggered by the user's position: when the main user was in the left half of the Immersia platform, the simulated room was the low RT environment, and when the user was in the right half of the Immersia platform, the simulated room was the church (see Figure 3). The positions of the scene object did not change, only the room did. The sounds used for the scene were recorded in a room that had an RT60 of 380ms. In order for the experiment to remain entertaining and amusing, it was chosen to record beatbox sounds. Two percussive sounds taken from Logic Pro 9's drums virtual instruments were added later on. The beatbox sounds included two loops recorded at 120 Beats Per Minute.

IMPLEMENTATION

The system, as shown on Figure 4 is based on a Max/MSP patch for the sound computation and a Unity3D based application with the MiddleVR plugin for user inputs, distribution and visual rendering on the screens. MiddleVR launches one application per rendering node, configuring each one according to a previously defined configuration file, and synchronizes all the instances. It also acts as a VRPN client on one node (called server) that broadcasts the inputs to the other nodes (called clients) so that every node handles the inputs at the same frame. The ART Tracking software and Nintendo Wiimote controllers are configured as input devices on the VRPN server accessed by the MiddleVR server application. This server node application is also in charge of sending data to Max/MSP through a UDP socket to update the virtual objects and user positions and trigger sound effects and tempo changes depending on the users' Wiimotes inputs.

User interaction

To control the application, the two users are provided with two Nintendo Wiimotes each. This remote controller allows wireless interaction and presents buttons and sensors that part of the audience is already familiar with. Some of the buttons triggered events, resulting in sound and visual effects. The controls where defined as follows:

 The plus and minus buttons of the first Wiimote changes the tempo (i.e. the speed of the visual and sound effects, hence changing the pitch of the sounds) 

The directional pad of the first Wiimote can triggers punctual sound and visual effect. The A and up buttons enable or disable looping animations and sounds played at the global tempo 

The directional pad of the third Wiimote allows the second user to move the travelling crane (and therefore its sound source), triggering a sound and following a rectangle path on a horizontal plane 

The up and down buttons of the fourth Wiimote allows the user to move the travelling crane and its sound source up and down between three different heights. We used the accelerometer of some of the Wiimotes to provide additional controls to the users. Thus, when the up axis value of the accelerometer reaches a threshold (i.e. when the user shakes the device from up to down), a sound and a visual effect are triggered. The visual effects consist of a short time emission of particles. The Wiimote accelerometers values correspond to the following rule: a value of one represents the standard gravity, minus one means the accelerometer is upside down. A value outside the range [-1 ;1] indicates that the user is shaking the controller in this accelerometer's axis.

The main user's shutter glasses are equipped with ART markers providing the application with the position and orientation of the user's head. This information is necessary to MiddleVR to compute the point of view to display on each screen but was also used as an input to the Max patch.

When the tempo changes are triggered by the plus and minus buttons of the first Wiimote the sounds, visual loops and animations times are divided by 2 12 , resulting in a pitch change of a semitone.

Spatialized sound

Simulation of sound images' directions

In an immersive system, the notion of sweet spot is non-existent: subjects can move around freely. The sound reproduction system should therefore account for that. Since the position of the main user is tracked, in Immersia, binaural reproduction might seem like a good approach [START_REF] Lentz | Virtual Reality System with Integrated Sound Field Simulation and Reproduction[END_REF]. Binaural sound reproduction aims to simulate natural hearing by emulating the acoustic filters from a sound source to both of a listener's ears [START_REF] Blauert | Spatial Hearing -The Psychophysics of Human Sound Localization[END_REF]. Sound is then reproduced over headphones and headtracking was shown to improve the quality of the rendering [START_REF] Begault | Direct Comparison of the Impact of Head Tracking, Reverberation, and Individualized Head-Related Transfer Functions on the Spatial Perception of a Virtual Speech Source[END_REF]. However, during the JSM, many users would be there to see the demonstration at the same time. Although only one of them can visualize the 3D content, we wanted all of them to be able to listen to the music that would be created by the main users. Time was also a constraint for the demo; equipping all users with headphones and headtrackers would therefore have been an issue. For these reasons, binaural would have been an ill-adapted sound reproduction technique for this project.

Another common approach to sound reproduction in immersive systems is to use sound field reproduction methods. These methods, namely Ambisonics and Wave Field Synthesis (WFS), can reproduce a sound field in a limited area [START_REF] Rebillat | SMART-I2: Spatial multi-user audio-visual real-time interactive interface, A broadcast application context[END_REF][14] In an immersive system, it means that the users can walk around freely and still be able to localize any sounds of the auditory scene where they are supposed to. However, sound field reproduction techniques require loudspeakers to be numerous and placed close to each other. This is not possible in Immersia, as the surface to be covered by the loudspeaker system is large and the number of loudspeakers is small. According to [START_REF] Spors | Spatial Sampling Artifacts of Wave Field Synthesis for the Reproduction of Virtual Point Sources[END_REF], in WFS, the frequency fal above which spatial aliasing occurs is given by fal = c/(2d), where d is the distance between the acoustic centers of two adjacent loudspeakers. With a distance of at least 1.94m in Immersia, spatial aliasing would occur above 87 Hz. We considered using Ambisonics but did not have time to test it. Instead, the simple solution of Vector-Based Amplitude Panning was chosen [START_REF] Pulkki | Spatial Sound Reproduction with Directional Audio Coding[END_REF].

The concept of VBAP is a simplification of Ambisonics: the level of each loudspeaker is controlled in order to make sure the level is constant at the listening position whatever the direction of the sound source is. Gains are applied to channels and the width of the source can be controlled by making more than two channels active at the same time. When a wide source is generated, the levels of the loudspeakers are higher the closer they are to the desired simulated position of the sound source. In this project, channel gains for the direct sound were given by VBAP with a source spreading width set to the minimum (0 out of 100):: the perceived source width was altered by the room effects. Despite its similarity with Ambisonics, it does not reproduce sound field over an extended area and therefore can only reproduce correct direction of sound sources at a given position, the sweet spot.

VBAP assumes all loudspeakers are located at the same distance from the listener. In order to estimate the gain necessary on each channel for a given sound source position, it needs the direction of each loudspeaker as well as the desired direction of the sound image. In Immersia, all the loudspeakers are not at the same distance from the listener, wherever the listener is. The position of the loudspeakers in the room is known, and so is the position of the listener. The position of the loudspeakers relatively to the listener can therefore be estimated continuously. The directions of the loudspeakers are corrected in VBAP. Figure 5 shows an example of how the loudspeaker angle may change relatively to the user. For the room effect, we were limited to stereo reverberation processing: few multichannel reverberations exists for MAX/MSP, but they are developed either for 3-2 stereo systems (also known as 5.1 or ITU-R BSS.775-1 system) or convolution reverberation processors [START_REF] Harker | The HISSTools Impulse Response Toolbox: Convolution for the Masses[END_REF] Two-channel reverberation processors were therefore used for the room effects.

In room acoustics, early reflections and late reflections have very different perceptual effects: [START_REF] Barron | Spatial impression due to early lateral reflections in concert halls: The derivation of a physical measure[END_REF] [START_REF] Griesinger | The Psychoacoustics of Apparent Source Width, Spaciousness and Envelopment in Performance Spaces[END_REF] show that early reflections improve the spatial impressions, increase the Apparent Source Width (ASW), and are perceptually merged with the direct sound. Late reflections, however, increase the perception of Listener EnVelopment (LEV) [START_REF] Furuya | Arrival direction of late sound and listener envelopment[END_REF] and are directly perceived as a characteristic of the room. For geometrical reasons, early reflections tend to come more from directions close to the sound source than opposite the sound source, whereas late reflections are anisotropic. To simulate this property of room acoustics, direct sound, early reflections and late reflection were modeled and spatialized separately, as summarized on Figure 6.

Early reflections

In order to produce early reflections for eight independent channels, four stereo reverberation processes were used for each sound image, the direct and late reflections components being removed by adjusting the settings of the reverberation process. Using the same settings for each stereo process would have caused the four stereo room effects to be strongly correlated. According to [START_REF] Blauert | Spatial Hearing -The Psychophysics of Human Sound Localization[END_REF], correlated sounds lead to a single perceived sound image, which is the opposite of what the early reflection processes are supposed to achieve here. The settings of each reverberation process were therefore altered. For the low RT room, the room size of each stereo reverberation process varied between 16 and 16.3 (no unit), while for the high RT room, the room size parameters varied between 143 and 143.3.

As explained above, early reflections are not anisotropic. This was simulated in Immersia by estimating channel gains in VBAP with a large source spreading width (50) and the same direction as the original sound source. For better spatial impression, early reflections should have come from each side of the direct sound, but not from the same direction as the direct sound.

Late reflections

The late reflections are supposed to be anisotropic. The four reverberation processors were therefore uncorrelated in a similar way to the reverberation processors of the early reflections and then played to the same level on each of the channels, contrarily to early reflections. Since the source distance was not taken into account for the late reflections (in a diffuse field, the sound level is theoretically constant across space [19]), a single set of reverberation processors was used for all the sound images.

CONCLUSION

The Immersia JSM event fulfilled its three initial goals ((i) to immerse users in an interactive visual and sound environment, (ii) to be easy to access for the general public, and (iii) to illustrate in an entertaining way the use of sounds in VR.). The feeling of immersion, associated with intuitive interactions with the visual and sound environment provided the visitors an entertaining illustration of spatialized sounds in VR to which the public gave satisfied and encouraging feedbacks.

On the scientific side, this work addresses the implementation of real-time room acoustics tools in large immersive systems through loudspeakers.

One drawback of the application was the single valid point of view in the immersive structure, while two persons were interacting at the same time. There exist several solutions to bypass this problem, like using a combination of HMD, or putting the two users in two different CAVE-like systems. Some immersive structures offer several valid point of view at the same time by combining different stereoscopic technologies [START_REF] Martin | A reconfigurable architecture for multimodal and collaborative interactions in Virtual Environments[END_REF].

In future works, a more formal evaluation of the application could be performed on the perception of the sound sources localisation.

Figure 2 :

 2 Figure 2: Location of the loudspeakers in the Immersia virtual reality room

Figure 3 :

 3 Figure 3: Factory / Church environment

Figure 4 :

 4 Figure 4: Hardware Architecture of the system

Figure 5 :

 5 Figure 5: Modification of the loudspeaker angles when the listener moves. This figure shows two different possible listener positions in a simple loudspeaker configuration. It shows that the loudspeaker angles change in relation to the position of the listener.

Figure 6 :

 6 Figure 6: Routing of the audio signal in MAX/MSP

ACKNOWLEDGEMENTS

The authors wish to thank Astrée Rossetti for her work on the implementation, and Jules Espiau for his kind support. They also want to thank all the volunteers and visitors of the event.


F Grani

S Serafin

F Argelaguet

V Gouranton

M Badawi

R Gaugne
email: ronan.gaugne@irisa.frfernando.argelaguetsanz

A L Ècuyer

Audio-Visual Attractors for Capturing Attention to the Screens When Walking in CAVE Systems

Keywords: Audio user interfaces, head-related transfer function, 3-D audio, virtual auditory space, navigation, walking, virtual reality, multimodality, virtual environments

In four-sided CAVE-like VR systems, the absence of the rear wall has been shown to decrease the level of immersion and can introduce breaks in presence. In this paper it is investigated to which extent user's attention can be driven by visual and auditory stimuli in a four-sided CAVE-like system.

An experiment was conducted in order to analyze how user attention is diverted while physically walking in a virtual environment, when audio and/or visual attractors are present. The foursided CAVE used in the experiment allowed to walk up to 9m in straight line. An additional key feature in the experiment is the fact that auditory feedback was delivered through binaural audio rendering techniques via non-personalized head related transfer functions (HRTFs). The audio rendering was dependent on the user's head position and orientation, enabling localized sound rendering.

The experiment analyzed how different "attractors" (audio and/or visual, static or dynamic) modify the user's attention. The results of the conducted experiment show that audio-visual attractors are the most efficient attractors in order to keep the user's attention toward the inside of the CAVE. The knowledge gathered in the experiment can provide guidelines to the design of virtual attractors in order to keep the attention of the user and avoid the "missing wall".

INTRODUCTION

In a four-sided CAVE environment, the absence of the rear wall is prone to decrease the user immersion and introduce breaks in presence during a virtual experience [START_REF] Slater | A virtual presence counter[END_REF]. Although real walking is often the most appropriate navigation interface in virtual environments, the walking area is restricted by the physical laboratory space. Several solutions have been proposed to cope with the limitations of a laboratory's physical space, such as redirected walking [START_REF] Bruder | Arch-explore: A natural user interface for immersive architectural walkthroughs[END_REF], redirection based on change blindness [START_REF] Suma | Leveraging change blindness for redirection in virtual environments[END_REF], redirected walking in place [START_REF] Razzaque | Redirected walking in place[END_REF][START_REF] Razzaque | Redirected walking[END_REF] and natural metaphors [START_REF] Cirio | The magic barrier tape: a novel metaphor for infinite navigation in virtual worlds with a restricted walking workspace[END_REF].

In this paper, we investigate a different approach, the usage of virtual attractors in order to capture users' attention away from the missing wall. While studies in auditory perception and cognition have demonstrated the importance of sound to capture users' attention in real environments [START_REF] Fritz | Auditory attention-focusing the searchlight on sound[END_REF][START_REF] Shinn-Cunningham | Object-based auditory and visual attention[END_REF] there is still a lack of research in Virtual Environments. An exception to this lack of studies in audio-visual attractors in VE is the work presented in [START_REF] Peck | Evaluation of reorientation techniques and distractors for walking in large virtual environments[END_REF][START_REF] Peck | Improved redirection with distractors: A large-scale-real-walking locomotion interface and its effect on navigation in virtual environments[END_REF], where reorientation techniques based on the concept of visual and auditory distractors were proposed. Results showed that an audio distractor did not produce as high a feeling of presence as a natural Figure 1: A user walking in the wide four-sided CAVE towards one of the short sides. The attractor (virtual crow) is placed on the front screen of the CAVE in order to divert the users' attention. audio-visual distractor. However, it produced an higher feeling of presence than an unnatural distractor without audio.

In this paper, we present an experiment whose goal was to investigate whether the use of auditory and visual attractors captures users' attention, in such a way that they do not notice the missing wall in a four-sided CAVE system. We combined several audio and visual attractors and analyzed the user behavior in several conditions. The results showed that the user behavior while physically walking can be modified by the addition of audio-visual attractors inside the virtual environment.

EXPERIMENT

The goal of the experiment was to explore how users' attention is driven by visual and auditory stimuli in a four-sided CAVE. In the experiment, we employed visual and auditory attractors and measured to which extend users' attention was modified by the presence of such attractors while walking in a virtual environment. The experiment was conducted in a wide four-sided Virtual Environment which enabled users to physically walk along a path of 9 meters (see Figure 1).

Two groups of attractors were considered: "bad" and "good" attractors. "Good" attractors are virtual objects placed in the projected space opposite to the missing wall: these can be either visual, audio-visual, or auditory stimuli. In the contrary, "bad" attractors those placed outside the CAVE by the missing wall side; these kind of attractors can only be auditory. In order to have the same configuration on both the directions of walking, "good" and "bad" attractors were placed in a symmetrical configuration with respect to the longitudinal axis of the CAVE (coincident with the path to be walked by users) to explore how users' behavior was modified. See

Apparatus and Participants

The experiment was run in a four-sided CAVE with retro-projected glass screens. The dimensions of the CAVE are: Main screen: 9.6m x 3.1m (12.5 MPixels); lateral screens: 3m x 3.1m (2.3 MPixels each); horizontal acrylic ground: 9.6m x 2.9m (3.6 MPixels). Users' head position and orientation was tracked by 16 ART Tracking infrared cameras, using reflective markers placed on the 3D glasses. Finally, audio rendering was delivered using Sennheiser HD600 full-sized circumaural open-air headphones and a wearable radio receiver (to avoid cabling issues in the walking task).

Thirteen subjects participated in the experiment (4 Female, 9 Male). Aged between 21 and 59 (x = 31.3; σ = 9.7). Each participant was able to finish the experiment in less than 45 minutes.

Visual rendering

The environment chosen for the experiment was a natural VE without salient virtual objects (e.g. tree, rocks). This was made in order to limit salient features which might have driven the attention of the user. As users were instructed to walk along the CAVE, the path the users had to follow was encoded in the ground texture of the environment (gravel trail). For safety reasons, a virtual fence was introduced in order to avoid the user to run through the walls of the CAVE and also to provide additional cues regarding the pathfollowing task (clearly defining the limits). See Figure 1.

Unity3D and Blender were used to design both the visual environment and the visual attractors while MiddleVR was used to drive the CAVE rendering, synchronization and the tracking system.

Auditory rendering

Binaural audio rendering was performed in real-time on a MacBook Pro running MaxMSP 6 and High Order Ambisonic Library 1.2 released from CICM -Maison des Sciences de l'Homme Paris Nord [START_REF] Colafrancesco | La bibliotheque hoa, bilan et perspectives[END_REF]. This library first encodes the soundfield in High Order Ambisonic to render it then to different output possibilities (different speakers layouts, headphones). The hoa.binaural object contained in the library virtualizes ambisonic soundfield using HRTF to perform FIR filtering of the incoming signal. HRTFs are extracted from the CIPIC HRTF database [START_REF] Algazi | The cipic hrtf database[END_REF].

The relative coordinates of all the sounding objects (with respect to the users' head position and orientation) are sent from Unity3D to MaxMSP as raw data packages via UDP to achieve a minimum latency in the binaural audio rendering. The sound materials used to create the natural audio landscape as well as the animal sounds were Creative Commons sources gathered from the Freesound.org archive.

Procedure

The task to be performed by the user was simple, walk along a straight path, parallel to the main wall of the CAVE. The path was clearly visible for the user as it was represented on the floor as a gravel trail. After explaining the task to the participant, the examiner asked him to step inside the CAVE and place himself in one of the sides. Users were then instructed to walk straight without stopping.

Participants started each trial from the Safe Zone (see Figure 2) and walked straight to the Safe zone on the other side of the CAVE. When entering the Attractor Zone, the current virtual attractor was enabled (according to the current condition). Once the user covered the Attractor Zone and reached the second Trigger Zone, the attractor was deactivated. Finally, when reaching the end of the path in the Safe Zone participants had to perform a u-turn and repeat the task until the end of the experiment. The end of experiment was indicated by a virtual stop sign which appears in the center of the CAVE.

Design and Hypotheses

The experiment covers different configurations of virtual attractors. In this study, we are considering the Stimuli (Baseline, Auditory, Visual and Auditory), the Position of the stimuli (inside/outside the CAVE) and the Relationship between the stimuli and the user (fixed position, or following the user). For practical reasons, we constrained the number of levels for the Relationship factor. We only considered two fixed attractors (Goose and Crow) and two moving attractors (Mosquito and Snake). Figure 2 depicts the position of the attractors with respect the user and the CAVE, note that the position is symmetrical with respect to the path. Furthermore, as environmental factors (physical environment) or user factors (eye dominance, handedness) can influence the user behavior when performing the task, we considered the Direction of the walk as an additional factor with two levels: Left to Right (L2R) and Right to Left (R2L).

Regarding the factors Type and Position due to obvious conflicts (e.g. when the attractor is placed outside the CAVE it can not be visible) we merged them in a new factor Condition with the levels: Baseline (no attractors), Audio Inside (AI), Audio Outside (AO), Audio + Visual (AV). This lead to factorial design of 4x4x2 (Condition x Attractor x Direction) resulting in 32 combinations. To decrease the data variability we considered 3 repetitions for each combination, leading with a total of 96 trials presented in random order. In addition we included 4 training trials (with no attractors) were added at the beginning of the experiment to ensure that the participants understood the procedure.

The dependent variables of the experiment were (1) the mean orientation of the users' head while performing each trial and (2) the mean speed of the user per trial. For the analysis, only orientation data recordings inside the Attractor Zone area are considered (see Figure 2).

In the experiment we wanted to explore how the different stimuli influenced the user behavior. Our hypotheses considering the design of the experiment were: H1 The mean users' orientation for the "Baseline" condition will be zero. H2 Users will be more attracted towards the attractor when the attractor is visible and audible. Figure 3: Average mean orientation of users' head along the path for each attractor. X-axis is centimeters, Y-axis degrees. Positive angles means that users is looking towards the Main Wall. We observe the immediate reaction of the user when entering the Attractor Zone (150cm).

It is interesting to notice users' behavior for the mosquito in the Audio Outside condition. First, users tried to locate the mosquito in the Main Wall and then shifted their attention towards the "missing wall". This effect is also visible for the other attractors although it is weaker.

H3 The mean head orientation will be higher when the attractors are inside the CAVE. H4 The mean head orientation will not be dependent on the Direction. H5 The mean head orientation will be higher for moving attractors (Mosquito, Snake).

At the end of the experiment participants had to fill a questionnaire in order to gather subjective information about the level of attractiveness of each attractor considering each type of feedback (7-Likert scale questionnaire).

Results

The analysis of the mean speed did not showed any significant differences among the different factors. In average the mean user speed was of 0.95m/s 2 with a σ = 0.16m/s 2 .

Mean Orientation

In order to analyze the head orientation data, we computed the mean orientation for each trial. As a convention, if the orientation is positive, the user is looking towards the inside of the CAVE. A value of 0 is that the head orientation is parallel to the main screen. We first checked the validity of H1. For this purpose we performed a t-test checking whether the mean head orientation for the trials in which no attractor was present was equal to zero. The t-test rejected the NULL hypothesis (p = 0.001), thus we have to reject H1. The mean head orientation is different than zero. Participants were attracted to the main wall of the CAVE although any attractor was active.

We conducted an analysis of variance (ANOVA) to analyze the effect of Condition and Direction in the mean head orientation. For the post-hoc tests we used the Bonferroni method with an (α = 95%), only significant pairwise comparisons are discussed (all p < 0.05). The Anderson-Darling normality test showed that the data was normally distributed (p < 0.001). The two-way ANOVA of the mean orientation of the user versus the Condition and Direction showed two main effects: (1) there is a significant difference among Conditions (F(3, 36) = 154.75; p < 0.001) and ( 2) there is a significant difference among Directions (F(1, 12) = 97.30; p < 0.001). Post-hoc tests showed that among the different Condition the AV condition was the one driving more the users' attention (thus accepting H2) followed by the AI condition. The AO and the Baseline conditions were the conditions driving less the users' attention (there is no significant difference among them), thus accepting H3. Post-hoc tests among the two levels of Direction showed a significant mean difference among the two levels, R2L (x = 20.887; σ = 21.931) and L2R (x = 11.070; σ = 21.730). Users performing the task walking from the right side of the CAVE towards the left side, showed a higher mean head orientation, thus rejecting H4.

In order to analyze the effect of the attractors (Bird, Goose, Mosquito and Snake), we performed a second analysis removing all the trials for the Baseline condition. The two-way ANOVA of the mean orientation versus Condition and Attractor showed two main effects: (1) a significant effect on Condition (F(2, 24) = 128.66; p < 0.001) which is consistent with the previous analysis (same pairwise differences) and ( 2) a the significant effect on Attractor (F(3, 36) = 3.39; p < 0.05). Post-hoc pairwise tests for the four levels of Attractor showed only a significant pairwise difference between the mosquito (x = 22.88; σ = 28.37) and the crow (x = 17.60; σ = 19.82) attractors.

Questionnaires

A post-experimental questionnaire was given to the subjects, asking the degree to which they noticed the role of animal sounds to capture attention in all three conditions (see Figure 4).

A two-way ANOVA of attractiveness vs Condition and Attractor, showed two main effects on Condition (F(2, 24) = 14.51; p < 0.001) and Attractor (F(3, 36) = 6.07; p < 0.001). In addition, it Figure 4: Level of attractiveness (subjective questionnaires) for each attractor. Plot shows a clear increase of attractiveness for the crow and the goose when visible and audible at the same time. also showed an interaction effect between both factors (F(6, 72) = 2.89; p < 0.05). Post-hoc analysis showed that attractors combining sound and visual feedback had a level of attractiveness significantly higher (p < 0.05). Also, regarding the different attractors, the Crow also presented a significantly higher level of attractiveness (p < 0.05), thus rejecting H5. The interaction effect was related to the fact that the attention captured by the attractors inside the field of view of the user (goose and crow) is greatly increased when visible and audible at the same time.

Discussion

The results showed that the addition of attractors in the virtual environment is able to modify the user behavior when physically walking. The results of the mean head orientation while walking (see Figure 3) showed that the effect is stronger when the virtual attractor was easily localizable by the user (Audio + Visual attractor). Also, we have to notice that even in the absence of attractors users are slightly attracted towards the inside of the CAVE. If we observe the behavior of the user along the path, we can clearly observe how the user orientation is modified along the trajectory for all attractors and also the fact that when the attractor is visible and audible, they are able to keep the attention of the user during a longer period of time. In addition, we can observe the clear difference between the Audio Inside and Audio Outside conditions. "Good" attractors are able to drive the users' attention towards the inside of the cave while the "bad" attractors, in which the audio source was placed outside the cave, are not able to modify the user's behavior. This result is also supported by the subjective questionnaires.

Another interesting (and unexpected) result was the fact that a difference in behavior appeared according the direction of the walk. A possible explanation for this behavior could be also the presence, by the missing wall, of other kinds of "Attractors" such as physical stimuli present in the lab space (hardware systems, a door, the operator).

Finally, the higher level of attention towards the crow in the questionnaires contradicts the analysis of the mean orientation and our hypothesis. However, several facts can explain the lower mean orientation (1) the Crow was the attractor closest to the user (2) when the Crow appears, it is already inside the field of view of the participants and (3) it was the easiest attractor to locate. In Figure 3 we can observe how the changes of head orientation are smaller while approaching the Crow (from 150cm to 300cm) compared to other attractors which resulted in stronger orientation changes.

CONCLUSIONS

In this work, we have explored how user attention is modified when virtual attractors are included in the virtual environment. Among the different auditory and visual stimuli analyzed, attractors combining visual and auditory cues provided the best results. From the results obtained, we can affirm that audio-visual attractors are able to drive the user attention towards the inside of a CAVE-like environment. Another important result is that attractors (audio sources) placed outside the CAVE are not able to modify the user behavior (as there are no significant differences compared with the Baseline condition) and that for our particular setup the direction of the walk influences the user's behavior. While the first and second results matched our hypotheses, the third is unexpected. Possible explanations for this unexpected result are users' eye-dominance or the physical stimuli present in the lab. Both factors could have biased the experiment. Additional experiments need to be performed to confirm our hypotheses.

The results we are presenting here could be promising for a wide range of applications as the audio-visual attractors could be employed to attract users' attention and could be adapted for its use to a variety of applications and tasks. For example, results could be applied to avoid users getting too close to the walls in a CAVE-like environment and in general for redirection and navigation purposes in games, architecture exploring, virtual prototyping and many other applications.

As future directions, we want to extend the analysis to explore a wider range of attractors (e.g. virtual avatars, task-specific attractors), analyze how user attention can be diverted when the user is performing a task involving a higher mental workload or how the approach can be extended to other VR setups. Finally, the ultimate goal could be the design an algorithm capable of generating automatically attractors when detecting that the user is about to look towards an unwanted direction.

Figure 2 :

 2 Figure 2: Schematic top view of the experiment layout indicating the CAVE walls, the testing path to be walked by the subjects, and the positions of the different Attractors.

Figure 2 .

 2 Figure 2.


Rozenn Bouville Berthelot

Thomas Lopez

Florian Nouviale

Valérie Érie Gouranton

Bruno Arnaldi

Rozenn Bouville

An interaction abstraction model for seamless avatar exchange in CVET

Keywords: [Computing methodologies], Virtual reality-[Software and its engineering], Virtual worlds training simulations-

INTRODUCTION

Training in real life hardly ever involve a single person. Indeed, a trainee is helped by a trainer and assisted by teammates in case of a teamwork training. Therefore, virtual humans are an essential feature for CVET. The challenging point with virtual humans in CVET is how to make users and virtual humans collaborate and interact as naturally as possible. Our objective is to provide a CVET that proposes intuitive and seamless ways to collaborate and interact with autonomous virtual humans. Besides we intend to enhance the collaboration between trainers and trainees to make training sessions more efficient and more interactive. Finally, depending on the requirements of the training, a variety of hardware devices is available for a user to interact with a virtual environment. Thus, we think that the solution we propose must run on many set-ups.

Hereafter, the term actor refers to both a user and a virtual human. During a CVET session, users are learning a team procedure. To complete the collaborative task, the actors involved in the training must be aware of the role of each teammate and understand what they are doing at any moment of the procedure. Thus, it is essential to them to have access to the knowledge acquired in the course of the procedure. Moreover, in order to synchronize their tasks, they need to exchange information. Besides, it can also be interesting for actors to exchange the control of their embodiments in order to change their role and access to the knowledge associated to this new role. Based on these observations, we believe that CVET must provide a common interface to both type of actors in order to ease the interaction, the knowledge management and enable exchange. To further illustrate this claim let us consider a scenario in the industrial training field. For example, trainees that are learning how to operate a car as shown in Fig. 1. During collaborative tasks, replacing the engine from the car for instance, at least two teammates must synchronize their tasks to achieve the collaborative goal. In real-life situation, the teammates would use dialog to share information and advices. By providing a common interface for both type of actors, the communication of information and even dialog is possible between actors whatever they are controlled by a user or by a virtual human. Otherwise, during real-life training, the trainer is able to replace one trainee to help him/her or to show him/her the correct manner to proceed. By allowing actors to exchange their roles, this situation can be simulated in the CVET.

In the remainder of this paper, we present in section 2, a literature review on collaboration between virtual humans and users in CVET as well as related work on exchange of avatars. In section 3, we introduce our interaction abstraction model allowing avatar exchange called the Shell then in section 4, we explain our exchange protocol based on the Shell. Section 5 depicts our implementation of the model of the Shell, the different set-ups used to run our sample application and some observations on the distraction of the user. The following section, section 6, explains the improvement for collaboration induced by the Shell. Finally, the section 7 concludes our contribution.

RELATED WORK

In this section, we will first present how virtual humans for CVET are represented in the literature and how users interact and collaborate with them. We will then discuss the exchange of avatars in CVET and how it improves the collaboration and the interaction between users and virtual humans.

Virtual Humans in CVET

In every CVET, the purpose of virtual humans is to interact and collaborate with users or with other virtual humans in order to complete a task. Nevertheless, they do not always have the same role. In the CVET literature, three primary roles assumed by autonomous humans have been identified [START_REF] Sycara | Literature review of teamwork models[END_REF] :

• as a personal assistant assigned to a single trainee to take care of basic tasks and help him,

• as a team assistant assuring the communication between users of the simulation, helping them to coordinate their actions and to better focus on the tasks requirements,

• as an equal team member operating as an autonomous entity that performs a collaborative procedure alongside users and other virtual humans.

Besides, whatever their role, they shared similar attributes: a knowledge base containing information on the procedure, a control module to interact within the world and, in some cases, a decision making module.

In order to represent virtual humans, different models have been proposed. Some proposes to split their representations in two parts: the body and the mind [START_REF] Kenny | Building interactive virtual humans for training environments[END_REF][START_REF] Lee | Thoughts on fml: Behavior generation in the virtual human communication architecture[END_REF]. The body can act within the world and perceives events whereas the mind can take decisions and influences the emotive state of the actor. Here, the mind part does not handle the knowledge base associated to the actor. These kinds of models were exclusively designed for virtual humans, they are not shared by users.

On the contrary, in the Interactive Storytelling field, a unified interface for both real and virtual actors is often proposed [START_REF] Machado | Tell me a story[END_REF][START_REF] Machado | Is the wolf angry or... just hungry[END_REF]. An actor can thus be controlled by either a human or by the system. Here, a memory concerning the actor's part and its perceptions is attached to the actor. It allows directing the actor or to help a user to better fit its part. In this model, actors are considered omniscient and the memory associated to an actor is only used when the actor is controlled by a virtual human. In case a user is controlling the actor, this memory is not accessible but used by the system to supply the pedagogy, giving pieces of advice to better stick to his/her role. However, even if the interface is shared by both real and virtual actors, this method does not propose to exchange the control of a character during the simulation.

For the purpose of our project, we focus on the situation where virtual humans interact with other actors in CVET as full-fledged team members and take part in the procedure i.e they are equal team members. This is the most complex and challenging case. They are able to perform tasks, interact with the objects in the environment and, to some extent, communicate with other teammates. In most CVET, virtual humans are able to perform their task independently [START_REF] Badler | Virtual humans for validating maintenance procedures[END_REF][START_REF] Rickel | Virtual Humans for Team Training in Virtual Reality[END_REF]. They are generally able to play different roles such as collaborators, instructors or assistants that help the trainee. They can also replace missing team members needed for a training. In the collaborative version of STEVE [START_REF] Rickel | Task-oriented collaboration with embodied agents in virtual worlds[END_REF][START_REF] Rickel | Extending virtual humans to support team training in virtual reality[END_REF], virtual humans play the double role of collaborator and instructor. They play their part in the procedure, but a specific human is assigned as tutor to help the trainee. In the fire training simulation of Dugdale [START_REF] Dugdale | Emergency fire incident training in a virtual world[END_REF], a group of trainees deals with a fire situation in a hotel. Various characters interact in the training and each one of them can be controlled by either a real user or an autonomous human. Unfortunately, interactions between team members and particularly between virtual humans and users are limited. They perform parallel tasks, working towards the team's shared goal but cannot neither interact collaboratively on a same object nor exchange their roles during the simulation. Nevertheless, some platforms allow collaborative interactions between teammates. This is the case of the Generic Virtual Training (GVT) [START_REF] Gerbaud | GVT: a platform to create virtual environments for procedural training[END_REF][START_REF] Mollet | Storytelling in virtual reality for training[END_REF] in its collaborative version. The MASCARET model [START_REF] Buche | An expert system manipulating knowledge to help human learners into virtual environment[END_REF] also potentially handle collaborative interaction [START_REF] Chevaillier | Semantic modeling of Virtual Environments using MASCARET[END_REF].

Exchange of Avatars in CVET

The exchange of avatars, also termed role switching or role swapping, is a powerful feature to improve interaction and collaboration in CVET. It gives to users, the ability of taking control over another avatar to see and act in the environment from another actor's perspective. In the context of a team training, this feature has a real pedagogical interest but it also increases the training sessions efficiency by giving the possibility to learn several roles with less training. For instance, a trainer can switch his/her role with a trainee so that he/she can show a specific manipulation or gesture as he/she would have done in real life. In the Intelligent Robots field, it has been studied for several years [START_REF] Martinson | Learning to role-switch in multi-robot systems[END_REF][START_REF] Monacelli | A reactive assistive role switching for interaction management in cooperative tasks[END_REF] . It is used to improve the achievement of collaborative tasks by giving the possibility of switching the role assigned to robots between leader and follower to speed up a task and to resolve conflicts. In the Virtual Reality domain, Evrard et al. [START_REF] Evrard | Homotopy switching model for dyad haptic interaction in physical collaborative tasks[END_REF] also take advantage of the possibility of switching between a leader and a follower role to improve the collaboration of users involved in a dyad haptic interaction for a physical collaborative task. More recently, role exchanges have also taken place in the context of human computer haptic collaboration [START_REF] Oguz | Haptic negotiation and role exchange for collaboration in virtual environments[END_REF]. The aforementioned work provides haptic feedback to the user through a force negotiation mechanism, in order to help him/her achieve a dynamic task. Furthermore, the system is able to detect when the user needs help and, if necessary, switches from an assistive role to an equal-control role to complete the task.

In the context of CVET, a few works proposes an exchange of avatars. For example, Raij et al. [START_REF] Raij | Ipsviz: An after-action review tool for human-virtual human experiences[END_REF] present after-action reviewing tools using role exchange to enhance experiences between users and virtual humans. Yet, the exchange is a promising and challenging trend for this area because it allows new usages for both designers and users of CVET. However, outside the scope of CVET, it is interesting to note that several studies have demonstrated the importance of the link between users and their avatar [START_REF] Maselli | The building blocks of the full body ownership illusion[END_REF][START_REF] Lenggenhager | Video ergo sum: manipulating bodily self-consciousness[END_REF][START_REF] Petkova | If i were you: perceptual illusion of body swapping[END_REF]. For instance, Slater et al. [START_REF] Slater | First person experience of body transfer in virtual reality[END_REF] conducted a perception study about the experience of body transfer in a virtual environment. In this study, the user point of view is modified during the experimentation allowing the user to watch the same scene sequentially from different perspectives, from an external viewpoint to a view embodied in the main character's eyes. This study shows that there are differences in the users' reactions depending on their point of view, in other word, their role in the virtual environment. These works demonstrate that an exchange of avatars is not a simple exchange of embodiment but that it induces users to act differently.

Synthesis

To sum up, the interactions between virtual humans and users are usually restricted to their participation in parallel tasks working towards the shared goal of the team. Their contribution mainly consists in advancing the procedure on their own, with limited interactions with others. We feel these are strong limitations to collaboration between autonomous humans and users in CVET. Besides, having an effective collaboration with virtual humans, is not only a promising feature for the realism of CVET but it will also increase performance and cognition. Our interaction abstraction model, by answering this issue, also enables an interesting feature: the exchange of avatars. The exchange of avatars is a promising feature for CVET even though only a few studies exists for this domain. Yet, it has been used in several fields for many years and it is still commonly used, even in video games like in the last version of Grand Theft Auto (GTA 5) 1 . Furthermore, it has been demonstrated that its usage induces interesting mechanism in users' reaction for the purpose of CVET.

THE SHELL

This section presents our interaction abstraction model called the Shell. The Shell aims at simplifying the collaboration between virtual humans and users by providing them a common interface.

Motivation

Collaboration between virtual humans and users is of great interest for CVET. This type of cooperation not only requires compatible means of representing the world and the tasks for virtual humans and users, but also that these representations can be interpreted and used by actors of both nature [START_REF] Bradshaw | Toward a deliberative and reactive agent architecture for augmented cognition[END_REF]. Thus, humans need to be able to understand the collaborative task from the virtual human's perspective [START_REF] Christoffersen | How to make automated systems team players[END_REF], and vice versa. In order to meet these requirements, we create the concept of the Shell [START_REF] Luna | The Perceptive Puppet: Seamless Embodiment Exchange Between Real and Virtual Humans in Virtual Environments for Training[END_REF], an entity containing a knowledge base and which controls can be exchanged between virtual humans and real users. The Shell acts as a gateway unifying the actions and knowledge of both type of actors through different modules.

Concept

The Shell is an abstraction level for the design of avatars that is generic whether the avatar is controlled by a user or by a virtual agent. It wraps common features for both users and virtual humans to allow seamless interaction between a user and his/her avatar as well as between a virtual agent and its avatar. The Shell holds three main modules as shown in Fig. 2 : the interaction module, the acquisition module and the knowledge module.

The interaction module handles interaction capabilities by dealing with the inputs from both virtual humans and users and by relaying it to the avatar which acts consequently in the virtual world. It allows collaborative interactions between several avatars such as collaborative manipulation of an object.

The acquisition module gathered information from the virtual world during the training session. Information is acquired through a perception process and is transmitted to the knowledge module. The perception modes used to acquire new knowledge could be of different nature: vision or dialogue.

The knowledge module contains at once basic information that is known a priori and information transmitted by the acquisition module.

The information managed by the knowledge module is of four types: task, ego, team and world. We refer to task information, knowledge that is known a priori, such as pieces of knowledge concerning the procedure to be trained or objects in the world. In the context of CVET, this information is crucial. The actor needs to know the series of tasks required to complete the procedure (i.e. the scenario) and whether a task is individual or collaborative in order to synchronize its actions with the team. The other three types of information (ego, team and world) are managed jointly by the knowledge module with the acquisition module. In fact, the acquisition module fills the knowledge module with these types of information. Ego refers to information about the avatar: internal resource states, task-specific skills and knowledge, interaction skills, internal states or collaborative profile. It is filled using an introspection process. Team refers to information about the collaborators and their actions like the role of each teammates or the completion of their current task. World refers to knowledge concerning the surroundings and the virtual world in order to locate objects or tools needed by the procedure. For instance, the last known position, the type, the properties or the interactive capacities of an object. These last two types of knowledge are filled using multi-modal perception i.e. through vision and dialogue.

The Shell concept is especially relevant in CVET with mixed agent/human teams. In this context, each actor has a role to play in the collaborative task whatever its real or virtual nature. The Shell wraps all the features needed to the completion of the task without considering whether the actor is controlled by a user or a virtual agent. The Shell, indeed, allows to pool the management of all the processes that are common to both type of actors. Moreover, using this representation, the knowledge base of a Shell can be accessed by both type of actors to help their decision process or update its own knowledge base.

THE EXCHANGE

This section presents how the model of the Shell enables a seamless role exchange between actors in CVET. Moreover, we discuss here how to make exchange of avatar efficient. Users must indeed understand it and, therefore, an appropriate representation of the exchange must be provided.

Motivation

Based on the Shell architecture, we have defined an exchange protocol that supports role switching between two actors. The exchange protocol gives to users the possibility of taking control over another avatar to see and act in the environment from a new perspective. This exchange can result from different reasons: control of multiple avatars at the same time, making an avatar perform a specific action, or accessing some knowledge it possesses but also resolving conflicts and deadlock situations or assisting another actor. In fact, the exchange of avatar enables new usages for both designers and users of CVET. Designers can create more complex scenarios than those generally used to enrich user experience. Training scenarios could indeed integrate mandatory exchanges to force users to assimilate various perspectives of the same procedure. Conversely, embodied in a new avatar, the trainee can feel and act as if he had a different role in a procedure at a crucial step.

Concept

We define an exchange protocol that allows two actors to exchange the avatar they are controlling. It basically consists in a Shell exchange between the actors. As the Shell provides a similar abstraction for the interactions and the knowledge, taking control of a new Shell gives users and virtual humans access to all the elements needed to resume the procedure started by the previous owner of the Shell.

Figure 3 shows the sequence of messages exchanged between two users and their shell in order to perform an exchange of avatars. First, User A ask for an exchange with the avatar of User B The shell associated to User A (Shell 1 ) transmits the message to the shell associated to User B (Shell 2 ). User B receives and accepts the demand. Finally, the shells proceed to the exchange and notify their new controller: Shell 1 notifies User B and Shell 2 2 notifies User A .

This exchange protocol presents various assets. First, the two actors involved in the role switching do not have to physically move or even to navigate in the virtual world. They directly take control of the avatar previously controlled by another actor. Second, this protocol handles a critical key element in the role switching as it keeps consistent the knowledge representation associated to an actor. Therefore, the exchange can be carried out at any time of a procedure without disrupting the training. After an exchange, a user has access to the knowledge associated to his/her new embodiment. It may differ from its previous knowledge so that it improves his/her own knowledge.

Evaluation of the Exchange of Avatars

While working on the exchange protocol, a question emerges: "How to represent an exchange to make it understandable by users ?"

We highlight three criteria that an exchange feature has to meet in order to be usable by users. First, users must comprehend they are involved in an exchange process. Second, they must understand they are now embodied in a new avatar. Third, they must be able to locate themselves in the virtual environment before and after the exchange happened. Otherwise, they also need to deal with various metadata associated with their new avatars such as new capabilities, different knowledge on the environment or on simulation history and so on. To meet these requirements, we have worked on the visual representation of an exchange of avatars. We call exchange metaphor, a specific configuration of parameters used to represent an exchange (animation, color, duration, ...). Although exchanges of avatars have often been used in the literature (see section 2.2), the method of representing such an exchange has never been evaluated.

Therefore, we have conducted a perception study to evaluate different exchange metaphors in order to assess the relevant criteria that need to be taken into account in the design of exchange metaphors. Our study revealed several results that have been published in [START_REF] Lopez | Exchange of avatars: Toward a better perception and understanding[END_REF]. First, the study indicates that different parameters should be taken into account to define an exchange metaphor depending on whether the user is involved or not in the exchange. If a user performs an exchange, the other users indeed needs to be aware of it. Nevertheless, the exchange metaphor used for a user witnessing the exchange should not be the same as for a user triggering it. Second, different metaphors should be proposed depending on the level of familiarity of the end-user with Virtual Environment. The study indeed shows that users that are familiar with VR want efficient and quick metaphors whereas novice users need explicit and comprehensible metaphors. Eventually, metaphors must be adapted to the targeted application. User-friendly metaphors, despite being well-appreciated by users, often lead to a longer task accomplishment duration, even though no difference has been found in terms of either ease of use or exchange awareness. This study answered our initial question and enabled us to improve the representation of exchange of avatars in our sample application. We present in this section our implementation of the Shell concept and explain how we handle the exchange of avatar. Our contributions has been implemented in a sample CVET application that is available on several platforms.

IMPLEMENTATION AND RESULTS

Implementation

According to our model, the implementation of the Shell gathers the control, the acquisition and the knowledge attached to an avatar in one single entity. Thus, its architecture, illustrated in Fig. 2, is composed of three parts following the conceptual analysis of section 3.2 :

1. the interaction module that allows the Shell to interact with the environment, 2. the acquisition module that allows the entity to assimilate new knowledge, 3. the knowledge database that stores and classifies the knowledge acquired by the Shell.

At runtime, the acquisition module captures and transmits new information in order to update the knowledge database. Knowledge is classified in the database depending on its type as defined in the previous section (3.2): ego, task, team and world. Our implementation enables new knowledge to be acquired through both vision and dialogue but it can easily be extended to other perception channels. Various data is attached to the objects of the virtual world such as their types, their locations and their properties. Each time the object is perceived by the avatar (i.e each time the object is in its field of view), the data associated to the object is transmitted to the acquisition module. This data is updated each time the object is perceived. Besides, an inference system allows to acquire information about the other Shell when perceived by an avatar. For example, a Shell can retrieve information about the action performed by another Shell. Moreover, our application includes speech recognition and synthesis to enable users and virtual humans to exchange information about the current task. Information exchanges through dialog is also transmitted to the acquisition module. Both virtual humans and users have access the knowledge database of their associated Shell. On the one side, autonomous virtual humans are based on a BDI architecture [START_REF] Mascardi | Languages for programming BDI-style agents: an overview[END_REF][START_REF] Bunt | Multifunctionality in dialogue[END_REF] and uses the knowledge of their Shell for decision-making and for conversation using an Information-State based approach. On the other side, users have access to the knowledge of the Shell through a graphical user interface. The knowledge displayed by the user interface complete their expertise during the training.

The Shell has been implemented in a CVET using the GVT platform 2 and Unity 3 . We use a test scenario resulting from a concrete CVET application defined by an industrial partner. This scenario describes a maintenance procedure in a plastics manufacture that consists in the replacement of a mold in a plastic injection molding machine (see fig. 4). This specific intervention requires a precise coordination of tasks between two operators : a setter and a machine operator. The use of an autonomous virtual human allows the learners to execute the learning procedure even if there is no other user available for the training session. To begin, each learner chooses one of the two roles. When ready, he/she is given control of the Shell corresponding to his/her role and executes his/her part of the training scenario. If the other role has not been provided, a virtual human takes it and executes its part of the procedure. At the beginning of the training, each actor has the same knowledge 2 Generic Virtual Training -http://www.gvt-nexter.fr/ 3 http://unity3d.com/ but their role differs. While running the procedure, they acquire new knowledge using perception channel i.e. dialogue and vision. For example, an actor can ask its teammate what is the next action he/she is going to perform and improve his/her knowledge.

We have also implemented our exchange protocol in this application. In order to perform an exchange, the user selects an actor in its surrounding. If the selected actor is controlled by a virtual human, the exchange is automatically accepted. Then, immediately, the user is moved in its new embodiment, changing its point of view. Otherwise, if the selected actor is controlled by another user, the later must accept the exchange to make it effective as explain in section 4.2. If the actor he wants to exchange with is not in his/her field of view, a graphical user interface allows him/hers to select one of the role required by the procedure.

Immersion of the User in CVET

Most serious games and virtual environments for training are designed as classical computer softwares. We do think, however, that the immersion of the user is a crucial element that needs to be taken into account in the assimilation of a scenario. Indeed, the more a training situation will be close to real-life condition, the easier it will be for a trainee to adapt the acquired knowledge in the real world. Based on this idea, our CVET application has been designed for multiple immersive platforms. As a result, a trainee can now practice the example scenario presented in 5.1 on the following setups:

1. a classical desktop computer (Fig. 5 Table 1: Set-ups tested for our CVET application and ranking from 4 (less adequate) to 1 (more appropriate) regarding three criteria: the immersion, the affordability and the freedom of movement.

Razor Hydra 5 to follow the hands' movements of the user (Fig. 5(c)), 4. in a large 4-sided immersive room with stereoscopic rendering and motion tracking to follow the user's actions (Fig. 5(d)).

These set-ups, however, do not provide to the user the same feedbacks and are not accessible in the same conditions to the trainees. Table 1 provides an overview of these set-ups and proposes a ranking of them, from 1st to 4th, regarding three criteria : the immersion degree, the affordability for a single user (i.e. financial needs) and the freedom of movement for the user. On the one hand, the immersive room provides a better immersion as well as a better freedom to the user, however, it is a very expensive set-up which is not easily available for virtual training. On the other hand, desktop station are really common and a rather cheap equipment. Nonetheless, they do not provide an immersive training as the interaction using keyboard and mouse does not reproduce real movements and nor interactions. During our demonstrations, we observe that the use of a screen and a video-projector with a tracking system, or the use of an HMD with another tracking device might be appropriate solutions to obtain a good equilibrium between immersion, affordability and freedom of movements.

Besides, to reinforce the immersion of the user, and in addition to the use of different set-ups, we also explored two other features. First, we increase the interaction level of the user by adding a communication module to our CVET. This communication module handles natural communication between users and virtual humans. To do so, we are using speech recognition to capture the dialogue of the user. For virtual humans, we are using the architecture presented in section 5.1 which allows them to generate communicative behaviors toward the user. Second, we added a module to handle a physically-simulated environment. Indeed, most CVET are using precomputed animation during a training scenario. Using physics allows us to render real reactions of objects from the world instead of scripted and predefined reactions. Thus, when coupling this physics with a tracking of the user's gestures, he/she is able to interact easily and naturally with all the virtual training environment which enhances his/her user experience.

Exchange of Avatars and Attention of the User

It appears to us while working on our exchange protocol that this feature, despite being useful, could distract the user and make him/her less focused on the training. For that reason, we try to evaluate whether some metaphors are less distracting than another. Thus, the evaluation presented in section 4.3 has been interested in assessing the distraction of the user while an exchange of avatars happened.

The first experiment compares 3 exchange metaphors for users that are only witnessing the exchange, they do not take part in it. The 3 metaphors used are illustrated in fig. 6: • flickering avatar (see 6(a)): each time an exchange happened, the two involved avatars flickered from a color to white, 5 http://www.razerzone.com/fr-fr/gaming-controllers/razer-hydra/

• ghost translation (see 6(b)): each time an exchange happened, two "ghosts" appear that translate from one avatar to another,

• popup notification (see 6(c)): each time an exchange happened, a popup with the picture of the involved avatar appears to notify the user that their control has been exchanged.

We had 54 participants that has been divided in 3 groups. Each one of them uses one of the 3 metaphors presented above. They were informed that several exchanges of avatar will happen during the session and they were asked to run a series of task while the exchange happened. To the question: "Does the metaphor distract you from the task ?", our analysis suggests that no metaphor was distracting the participant from the task to execute than others. This suggests that, concerning exchanges that are witnessed by a user, the visual representation is of few consequences on the distraction of the user.

The second experiment compares 5 metaphors for users that are triggering the exchange. The used metaphors combined two visual stimuli: camera moves and animated feedback. Camera moves can be whether a straight linear motion between the two involved avatars (camera move 1) or a camera motion that goes up smoothly from the first avatar to an external point of view then comes back down to the second avatar (camera move 2). The animated feedback is, in turn, a combination of the flickering avatar and the ghost translation described in the first experiment. Our five metaphors are defined this way:

• Metaphor 1: camera move 1 with animated feedback,

• Metaphor 2: camera move 1 without animated feedback,

• Metaphor 3: camera move 2 with animated feedback,

• Metaphor 4: camera move 2 without animated feedback,

• Metaphor 5: no camera move and no animated feedback i.e. direct teleportation.

We had 52 participants. They were asked to compare two of our 5 metaphors at a time and therefore performed 10 comparisons. At the end of the experiment, for each metaphor, they were asked to note the affirmation: "The metaphor allows remaining attentive to the environment." from 1 to 7 (1: not agree at all, 7: totally agree). The result is illustrated in Fig. 7. The results obtained suggest that the metaphor 1 was perceived as more distracting than the metaphors 2, 3 and 4. Nevertheless, this conclusion must be caught with caution for two main reasons. First, the conclusion is not really obvious if we refer to the Fig. 7. We can indeed notice that our statistical analysis could not compare the metaphor 5 with the others although its repartition is exactly the same as for the metaphor 3. Second, the experimental condition can not be applied to every virtual environment. The virtual environment used was indeed a small room and the user was always facing the avatar he/she exchanged with (see Fig. 8). Therefore, the conditions of our experiment are too specifics to be applied to every CVET. We believe that further studies must be conducted in order to answer whether the attention of the user is disturbed by the exchange he/she is triggering.

HOW DOES THE SHELL IMPROVE COLLABORATION IN CVET ?

This section discusses how the contributions presented above raise new ways to collaborate in CVET. Most of the Shell improvements on collaboration between users and virtual humans are induced by the exchange feature it allowed.

Collaboration Between a Trainer and the Trainees

Using the exchange protocol, the trainer has several possible ways to collaborate and interact with trainees. He/she can :

• take control of any actor involved in the collaborative task with the trainee. In this way, he/she can interact with the trainee to help and guide him through dialogue. On the contrary, he/she is able to provoke unexpected situations by making mistakes or giving wrong information to the trainee on pedagogical purposes.

• take control of the trainee himself. In this way, he/she can induce the user to see the situation from a different perspective at a crucial step. He/she can also take the user's role to show him/her a specific step that the trainee failed to complete as he would have done in real life.

• let a trainee complete a procedure, started by the trainer, in order to evaluate the trainee's capabilities.

Collaboration Between Trainees and Real or Virtual Teammates

Thanks to our exchange protocol, a trainee can exchange his/her role in the procedure at any time and as often as desired. This can help him to collaborate more efficiently with his/her real or virtual teammates by:

• improving his/her virtual training experience. For instance, if he/she feels comfortable with one step of a procedure, then the trainee can choose another role. Thus, he/she remains focus and learns from this new perspective to better collaborate during another training session.

• improving his/her procedure comprehension. For example, during a surgical procedure, a nurse can exchange his/her role with another nurse at a specific step to integrate his/her point of view in order to improve her/his own reaction in real situation.

• reinforcing the team spirit. In fact, the exchange of avatars allows the trainee to develop more efficiently his/her cognitive empathy and thus, the team cohesion. It has indeed been demonstrated by Seers et al. in [START_REF] Seers | Team-member exchange quality: A new construct for rolemaking research[END_REF][START_REF] Seers | Team-member exchange under team and traditional management: A naturally occurring quasiexperiment[END_REF] that the quality of lateral (within-group) relationships is as important as that of vertical relationships (with hierarchical supervisors).

CONCLUSION

This paper presents an interaction abstraction model for the control of avatars in CVET that is shared by both users and autonomous virtual humans. This abstraction level, called the Shell, allows pooling the main process of avatar control: the interaction response and the knowledge management. Moreover, the Shell enables us to define an exchange protocol that allows users of CVET to exchange their role seamlessly during a training session. This exchange feature preserved all the data associated to a role. It ensures that the new owner of a role has access to every data required to resume the task initiate by the previous owner. We show that the exchange is a promising feature for CVET to improve collaboration and interaction. Nonetheless, it raises several questions that must be answered such as how much the exchange distracts the user and how to minimize its impact. We have conducted preliminary studies that tends to indicate that the representation of the exchange is of few consequences on the distraction of the user. Nevertheless, work remains to be done to clearly answer this question. Besides, several other questions must be studied to make the exchange feature more usable by users. For example, how to adapt the exchange metaphor to all kind of render devices from a portable device to a fully immersive set up ? Future works will also focus on proposing different uses of the Shell. For instance, it would be interesting to have a simultaneous access on a Shell by two or more actors. An actor could then follows a user as a passive controller, monitor his/her actions or take over a part of the procedure when needed. This control sharing could also allows a trainer to fine-tune a manipulation performed by a trainee.

Figure 1 :

 1 Figure 1: A sample CVET to learn how to operate a car.

Figure 2 :

 2 Figure 2: The shell: an abstraction level for the control of an avatar that is generic whether the avatar is controlled by a user or by a virtual agent.

Figure 3 :

 3 Figure 3: Thanks to the shell abstraction, a user can exchange seamlessly his/her avatar on-the-fly while preserving the data collected by its new avatar.

Figure 4 :

 4 Figure 4: Our test scenario running in an immersive room.

Figure 5 :

 5 Figure 5: The different platforms running our sample CVET.

  (a)), 2. a portable screen combined with a projector and the tracking of the user's motions (Fig. 5(b)), 3. an HMD set-up using the Occulus Rift 4 combined with a Desktop station Screen + movements tracking HMD + Hands movements Immersive room Immersion

Figure 6 :

 6 Figure 6: Illustration of the metaphors used in our first perception study.

Figure 7 :

 7 Figure 7: Our experiment shows that when the user is triggering the exchange, metaphors with multiple visual stimuli are a bit more distracting than more simple metaphors.

Figure 8 :

 8 Figure 8: The environment used for our second perception study.

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ACKNOWLEDGEMENTS

This work was supported by the French Research National Agency project named CORVETTE (ANR-10-CONTINT-CORD-012) and the French Unique Interdepartmental Funds SIFORAS (FUI 11).