Lutz Behnke
email: lutz.behnke@haw-hamburg.de

Sven Allers
email: sven.allers@haw-hamburg.de

Qi Wang

Christos Grecos

Kai Luck

Kai Von Luck

Avatar Density Based Client Assignment

Keywords:

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Managing the amount of processing, memory and network resources to provide the platform for any MMMOG follows the proven divide-and-conquer approach. Trying to scale up the number of players that can play the game concurrently a long list of bottlenecks have to be considered and overcome. The following work is part of a larger effort to to create a complete MMORPG system that a) will allow the creation of a single virtual environment with support for far more than 100k concurrent users; b) will provide a non-stop service with no planed down-time, c) will handle component failure (hard-and software) with minimal service degradation; d) will allow unparalleled level of flexibility of the contained environment.

Prior publications have targeted the partitioning of the actual server workload, the resultant storage requirements and the aggregate network bandwidth that is required for the communication between the components (see [START_REF] Behnke | QuP: Graceful Degradation in State Propagation for DVEs[END_REF]). The platform presented, QuP, will support a high number of concurrent players, as it allows large (> 20) clusters of servers to maintain a single, continuous game environment.

QuP will also support a high local Avatar Density (the number of avatars in a given space), as it allows the same spatial area to be maintained on more than one server. This may lead to the next bottleneck: sending environment updates to the client computer, as this connection is usually the weakest link. To provide a minimal reduction user experience while greatly reducing the amount of change messages needed, we introduced Dynamic Budget Based State Aggregation [START_REF] Behnke | Budget based dynamic state update aggregation[END_REF]. While the concept is a generalized one, will it mostly benefit from situations that are present in may MMOs, where crowds of people or blocks of military units are moving in a similar fashion. Their action can easily be described as a group, with massive reduction of required messages.

QuP and DyBuBSA both benefit from an optimal assignment of clients to servers in order to group information about avatars that are in spatial proximity to a small subset of the whole cluster, idealy to a single server. This is due to the pre-fetching and pre-processing nature of both techniques. Client Assignment (CA) is a NP-complete problem [START_REF] Chen | Locality aware dynamic load management for massively multiplayer games[END_REF], if encompassing a grouping problem it is even NP-hard [START_REF] Falkenauer | A New Representation and Operators for Genetic Algorithms Applied to Grouping Problems[END_REF].

Many existing solutions to CA in MMOGs are very application dependent. In many large-scale games, they range from the primitive, as in MMOBA-type (Massive Multiplayer Online Battle Arena) games like World-of-Tanks or Star-Craft, where very small groups of players (usually 4-40) are grouped into a large number of world instances. These instances only exist for a short time (usually less than 60 min) while a single battle lasts. MOBA CA usually follows a few computationally simple rules to group players of similar strength in order to optimize the entertainment value of the game.

In MMO-RPGs, the assignment is often done via a static shard selection (e.g. World of Warcraft). This selection may be changed by the user, if the vendor allows it. The selection usually is subject to capacity planing by the service provider. These shards, or replicated instances of the virtual world can than be distributed statically to pre-planed server instances.

When attempting to construct a MMOG that models a continuous or at least connected space, in which all users interact in a single, integrated, continuous environment, the CA is more difficult to pre-plan and requires constant monitoring and update while the game is running. To increase the number of concurrently supported players within the world, MMORPGs often used zoning to partition the game population together within the game world. Unfortunately this often fails to support scenarios, where the whole population gathers explicitly to partake in a single event (e.g. massive battles in EVE online [START_REF]CCP Manifest: A Weekend of Epic Destruction in EVE Online[END_REF])

We present a MMOG platform that is able to separate the spatial location of the avatar within the game environment from the Game Server (GS) to which it is assigned (e.g. QuP [START_REF] Behnke | QuP: Graceful Degradation in State Propagation for DVEs[END_REF] ).

Within it, large synergistic effects may be gained by grouping avatars in spatial proximity on the same server while avoiding overload situations before they develop. To minimize the required network traffic further, we introduce a method to reduce the average number of replicas of games objects on the GSs. We aim to achieve this goal by attempting to group avatars of similar Areas of Interest (AoI) on the smallest subset of servers as possible. To handle overload of a single server, QuP does allow for the same spatial area of the game to be represented by more than one server, albeit with a certain amount of additional network overhead.

Striking a balance between tight grouping and avoiding overload is the task of the Client Assignment Manager (CAM).

Related Work

A number of approaches to partitioning the client population prior to assigning them to servers have been discussed by commercial vendors as well as scientific publications. Most commercial vendors opt for primarily static or pre-computed assignment.

A wide range of work has been done on the subject of load distribution and load balancing. Most publications propose the partitioning of the game world and thus the avatar population. These partitions are then assigned to servers within the system. [START_REF] Ng | A multi-server architecture for distributed virtual walkthrough[END_REF] proposes to assign one region to one server, with the region being partitioned further into cells, which can be ceded to other servers in high load situations. The load information of each server are regularly submitted to a central load collector server. As [START_REF] Lau | Hybrid load balancing for online games[END_REF] observes, the cells are only moved to neighboring servers and global state is not considered, thus underutilized servers may be ignored since they are not adjacent to the server in overload. To order to address these shortcomings, [START_REF] Lau | Hybrid load balancing for online games[END_REF] proposes a hybrid approach in which the load is shed not only to neighbors, but appropriate servers are also chosen from a global set.

The work in [START_REF] Lui | An efficient partitioning algorithm for distributed virtual environment systems[END_REF] approaches load balancing via linear optimization, looking not only at server processing load, but the required communication overhead among the servers as well. The aim is to distribute the load evenly among servers while minimizing communication at the same time. They construct a graph based on the Area of Interest (AoI) of each avatar, connecting the graph when the AoIs overlap. [START_REF] Morillo | An ACS-based partitioning method for distributed virtual environment systems[END_REF] improve this by using an ant colony optimization algorithm, but also showed that there is a minimal size of avatar population and server cluster that limites the cost efficiency of more complex load balancing approaches. [START_REF] Chen | Locality aware dynamic load management for massively multiplayer games[END_REF] does not attempt to distribute load evenly, but rather to ensure that pre-defined levels of Quality of Service (QoS) are met by each server. These QoS include CPU utilization, network bandwidth consumption. They propose an appropriate partitioning and merging scheme.

Static partitioning is the state of the art of many commercial online games, but does not match the dynamics of an MMO-RPG. [START_REF] De | Dynamic microcell assignment for massively multiplayer online gaming[END_REF] investigate a number of algorithms and their fitness for approaching optimal distribution of clients to server. They show linear programming to be able to provide an optimal solution, but also that the required cost of computation is prohibitive for practical use. [START_REF] Deng | On Delay Adjustment for Dynamic Load Balancing in Distributed Virtual Environments[END_REF] show that all the other algorithms are costly as well. In [START_REF] Deng | Dynamic load balancing in distributed virtual environments using heat diffusion[END_REF] they propose a method that uses a simulated heat flow model, based solely on local information.

All above approaches assume that all client connections and thus avatars produce a similar load and thus partition the avatar population by partitioning the environment space. A different approach is taken by [START_REF] Morillo | An adaptive load balancing technique for distributed virtual environment systems[END_REF]. Here a server is focused on the avatars that are assigned to it. The scheme first sorts the avatars by the number of AoIs they are present in. It then repeatedly moves the most interesting avatar, until a number of iterations is reached.

[9] states that regions do not reduce the inter-server communication to a sufficient degree. Instead they favor a simple load balancing scheme. They propose the use of a NAT-Server between server and client and the use of standard load balancing methods like round-robin. The proxy approach is also proposed by [START_REF] Quax | ALVIC-NG: state management and immersive communication for massively multiplayer online games and communities[END_REF] to isolate the region-to-server assignment from the client-to-server assignment.

Requirements

As this work is based on QuP platform for large scale MMOGs, the AoI of each avatar is represented by its view box, an Axis Aligned Bounding Box (AABB). The CAM should group the assignments to the game servers according to the spatial grouping of their avatars within the virtual environment. This is expected to allow a reduction of retrieved state and subsequent state update messages to a given server as multiple avatars will access the same pre-fetched object and environment data through a local lookup rather than costly remote query.

The CAM must also ensure that the CPU load on a given server is within acceptable parameters. As the load created by a single client connection can vary widely, this will require feedback from the GS. Available main memory and network bandwidth must be considered as well.

Based on the requirements, we derive the following cost function to govern the client assignment. The client will be assigned to the server that will return the minimal value from the cost function below:

cost(a, s) = w 0 * -g(a, s) + w 1 * Load(s) -w 2 * M emF ree(s) (1)
For an avatar a and the serve s the function g(a, s) will calculate the Grouping Factor (GF). The GF represents similarity of the AoI of the new avatar to the AoI of the avatars that are already present on a given server. It is assumed that most of the objects in the AoI of the other avatars have already been loaded and that further updates on those objects will be relevant to both the existing and the new avatars. The actual computation of the GF will be discussed in section 4.

Memory requirement is not given as the percentage used, but the amount of remaining memory instead, so that servers having large amounts of memory remaining are favored, regardless of their total amount of memory. w 0 , w 1 , w 2 are weights to control the priority of the potentially contradictory requirements of maximizing the grouping factor and not surpassing limits on CPU load and memory utilization. Network utilization is not considered as separate parameter here, as it directly relates of the grouping factor. More fine grained modeling on this may be an area for further study.

As the avatars move over time, the client assignment must be able to track validity of the calculation results over time. This may lead to the need for a reassignment of clients to new GS over time. Also any CAM solution should be able to scale with the MMOG to support a user population far in excess of 100k concurrent users, including the resultant churn, expected clustering of user log-in and number resultant number of GS.

Proposed Solution

We propose calculating the grouping factor, using a heat map. To construct the heat map, the virtual environment is partitioned into a set of blocks of equal size. This is similar to partitioning of the actual game environment into micro cells [START_REF] De | Dynamic microcell assignment for massively multiplayer online gaming[END_REF], but we use these blocks only for the grouping calculation. Due to the architecture of QuP, this is done in three dimensions. The diagrams below are restricted to two dimensions for the benefit of illustration only.

As the AoI of an avatar governs which objects in the game have to be prefetched to a server, we use the view boxes and their overlap to determine the grouping factor when considering a server for assignment.

0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 0 1 1 0 0 1 2 0 1 1 1 2 2 1 0 2 3 0 2 2 1 3 2 1 0 2 3 0 2 2 1 3 2 1 0 2 3 0 2 2 1 3 2 1 0 1 2 0 1 1 1 2 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Avatar A Avatar B
Avatar C

(a) Viewbox overlap and resultant heatmap for server A Each block holds a set of servers that serve at least one avatar whose view box overlaps with the block. A tally of the number of intersecting view boxes is kept, as shown in figure 1c In QuP and the LB all object coordinates are within a norm cube (0 <= x, y, z < 1). This cube is partitioned into n blocks per axis. For each block (identified by it coordinate between 0 and n-1) and each server, T (x, y, z, s), a tally of avatar view boxes that overlap this block is maintained. This allows the calculation of the grouping factor for each server in a block.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 1 1 1 1 1 2 0 0 2 2 2 2 1 1 1 2 0 0 3 2 3 3 1 1 1 2 0 0 3 2 3 3 1 1 1 2 0 0 3 2 3 3 1 1 1 2 0 0 2 2 2 2 1 1 1 1 0 0 1 1 1 1 1 1 (b) Heatmap for server B [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [{a,1}] [{a,1}] [ ] [{a,1}] [{a,1}] [ ] [{a,1}] [{a,1}] [ ] [ ] (
As each client is requesting a server assignment, the view box of the avatar is queried from the world state (maintained by the QuP servers). As the server is chosen, its tally is updated in each of the blocks that the clients view box intersects with. As a server is considered to cover the view box of an avatar even if the view box only intersects the spatial area covered by the server, this method forms an overestimating heuristic. It allows the computation of the GF used in equation 1.

CAM Components

The CAM is partitioned into two different components: the Load Balancer (LB) and the Server Monitor (SM). In our experiment we only used a single LB instance (for further scalability see section 7). One SM is assigned to each GS. See figure 2 for communication links and integration into the full system. The actual CAM components are shown in darker color.

As the central component, the LB holds the actual heat map and meta information on the known servers and the previously assigned clients. The Authentication Server queries the LB to receive a server assignment. The LB in turn maintain a lease on the client-server assignment to update the tallies for the heat map. This lease will be confirmed if an acknowledgement messages is received by the LB. This message will sent by the server once the client has connected to it.

The LB continuously monitors the servers for any messages send to it. This may either be heart beat messages by the server or responses to a client assignment requests. Should a server not have sent any message longer than a configurable threshold, it is considered dead and will be removed completely from the state of the LB. It may be reintroduced only by administrator command. If there are pending assignments that have not been confirmed yet, they are rolled back in the heat map as well.

The SM running in each GS will inform the LB of any successful client connections. As the avatars move through the environment, the SM will track which heat map regions are being covered and uncovered by the shifting view boxes of each avatar. The list of changes is reduced by applying inverse operations (e.g. the number of new avatars in a region is subtracted from the number of avatars that have left the region). The resulting difference list is sent to the LB in regular intervals.

Base Grouping Factor

The assignment algorithm has to weigh between placing an avatar on a server that already already covers a spatial region or placing it on a new server to avoid overloading the former server. There are two special cases to consider when selecting servers for the client: a) empty servers (i.e. without any clients) and b) server whose covered area does not intersect with the avatar view box. Empty servers will not only occur after starting the system, but can also result from avatars congregating on small spatial areas of the game, leaving others completely empty and thus the corresponding servers idle, as reassignment incrementally groups the avatars on fewer and fewer servers to optimize the grouping factor.

Both cases are similar, in that they result in a grouping factor of 0, as no view boxes overlap. The client assignment would only be controlled by the CPU and memory resource utilization values.

We propose to use a grouping factor for empty servers, that represents the state in an idealized game, containing only equidistant avatars that are evenly distributed among the servers. For this we calculate a Distance value (for a world in 3 dimensions) from an ideal field count to an average field count of the actual servers in the running game:

IdealF ieldCount = g 0 P 3 ServerCount + g 1 U sedF ields ServerCount
The heat map is split into precision P, given as the number of regions per dimensiong 0 , g 1 are weights so that (0 < g ≤ 1, g 0 + g 1 = 1). The weights represent the balance between two objectives: the even partitioning of the whole virtual environment among the servers and the even distribution of the regions already used among the servers. The optimal values are application specific.

From the ideal field count, a distance value can be computed.

Distance = 3 √ IdealF ieldCount -3 √ AvgF ieldCount 2
* 1 P As a large distance would be bad for grouping avatars, the grouping factor for empty servers is g(a, s) = -Distance All server that are neither empty nor have a server view box that overlaps with the avatar view box are considered in turn for assignment by setting the cost to the Manhattan Distance from the fields that contain the avatars view box. The position of a given non empty server is is calculated for each field it is assigned in.

Reassignment

After each avatar is assigned to a server, it is likely to move through the virtual environment. Thus the grouping factor of the servers will deteriorate over time. To counter this, our approach supports a protocol for client reassignment. Executing this protocol produces a small overhead. Reassignment may result on a negativ impact on the user experience, even if only for a limited time. Thus a certain amount of hysteresis is required to regulate the frequency of reassignments of a given client connection.

In order to track the quality of each assignment, as well as to monitor a user session, the LB maintains a (lightweight Erlang) monitor process for each connected client. In regular intervals, each process is activated in order to recalculate the server assigned to the client. Should the calculation result in a different server, the client is reassigned.

As the new avatar will shift the aggregate view box of the newly assigned server, the probability of assigning the same server on the recalculation increases. If needed, an additional weight against reassignment could easily be applied to the calculation.

The server will notify the LB when the client has disconnected. This will remove the monitoring process for the client session.

If the distance is greater than a pre-set threshold, a client reassignment process will be triggered. If the assignment results in a different server, the reconnection protocol is executed (see figure 3b).

The LB will also re-check the assignment of each avatar after a pre-set interval. This will slowly, but continuously, improve the assignment in the face of avatar movement. While this will regularly query the position of the avatar from the QuP subsystem, it will not suffer the very large bandwidth requirement of tracking each position change of each avatar.

A further method of initiating an assignment reevaluation will be triggered by the server directly. If it detects insufficient CPU, memory or bandwidth resources, it will send an emergency signal to the LB to trigger immediate assignment re-evaluation.

The reassignment protocol will notifiy the new server to start pre-fetching all object data required by the avatar. Then the client is informed by the old server of the need to switch. The message will include the contact details of the new server as well as a token string that allows the client to authenticate to the new server.

Experiments

We have implemented a prototype of each of the component's shown in figure 2. For the tests, the AS and the game clients were replaced by a test server in order to isolate the LB for measurements. Also, the LB was configured to contact a special QuP mock server in order to separate the time required for computing the server assignment from the time for the QuP look up. The mock was pre loaded with the position of the avatars, provided no redundancy and was co-located with the LB on the same maschine.

The test were done on three PC type virtual machines. The VM manager was configured to allow full CPU utilization during the test. The systems where running Erlang version OTP 18 on Ubuntu Linux.

Tests reflect the three types of avatar distribution introduced by [START_REF] Lui | An efficient partitioning algorithm for distributed virtual environment systems[END_REF]. We consider them sufficient representations of avatar behaviour in an MMOG. The distributions were adapted to three dimensions as supported by QuP. All avatars are place in a norm cube. Skewed distribution was set to 60% of avatars in a cube occupying the lower 0.3 of each dimension; clustered set to 75% of the avtars being in 10 evenly distributed groups; the remaining avatars were uniformly distributed in the whole environment.

Two test series with 10,000 and 100,000 avatars were set up. The heatmap was partitioned with a precision of P=10 blocks per dimension. The smaller scale Increasing P results will an cubic increase the memory requirements of the LB. We asume that P=10 will partition the space sufficiently to effectively distribute clients to a number of game servers in the order of 10 to 100. Any installation that outgrows this, would should have enough resources to support much higher values of P.

Results

Figure 3a, shows the 99.9th percentile for the time elapsed from a placement request being sent to reply given by the LB. It uses P=10, thus 1000 blocks. The measurement does not include the network communication to the QuP servers. We observe a constant time requirement once the data structures have been filled.

Figure 3 shows the distribution of the avatars to three servers in the smaller scale test environment for each of the three distributions.

Conclusion and Future Considerations

We have shown that, in combination with QuP [START_REF] Behnke | QuP: Graceful Degradation in State Propagation for DVEs[END_REF] and DyBuBSA [START_REF] Behnke | Budget based dynamic state update aggregation[END_REF], our approach to client assignment provides the needed capabilities to drastically scale Reply times for all distributions, P=10 Reassignment process up the number of supported concurrent users in an MMOG with a single continous game environment, or distributed virtual environment in general.

For a given set of servers, the load balancer will return an assignment in constant time, regardless of the number of avatars currently active. The only prerequisite is a sufficient amount of memory to maintain the heatmap, but this is constant and can be controlled by the number of blocks per dimension.

The quality of the assignment will improve the utilization rate of the prefetched data on the target server in the normal case. The worst case, if data in the LB has drifted too much, an less than optimal assignment is given, further increasing the drift distance. If a sufficient drift distance has been detected, a reassignment will be initiated for all avatars concerned. Sub-optimal assignments will only cause the CPU, memory and bandwidth resources of the server to be depleted at a lower number of client client connection compared to the optimal case. For a given number of expected supported client connections, this will either require more GS nodes to be started, or the maximum number of supported clients will be redcued. For the clients already connected, even in the face of sub-optimal assignment full quality service will be proved.

The reassignment process will minimize the impact on the user experience, but may create small interruptions as the network connection is switched from one server to another.

When scaling up the system, the number assignment requests and reassignment protocol messages will increase with the number of connected game servers and the number of clients connecting to the system concurrently. This will result in an increased network bandwidth requirement placed on the LB. Access to the data structure that holds the heatmap and its ancillary data may pose a bottleneck, since it is held on a single compute node and not replicated. It receives updates sent by the monitor processes of each server, as well as all the client assignment requests. More importantly, creates a single point of failure for the whole system with regards to connecting new users and the continuous improvement of the resource utilization for the already connected clients.

We intend to undertake further work on eliminating this source of system failure and to support even larger user bases. One possible method is to use a Distributed Hash Table (DHT) to maintain the heat map in a small set of servers, as the changes to the map are generally small and localized. The authentication server would contact the LBs in a round robin fashion. Thus the ASs can itself be duplicated and contacted by the clients in a round robin fashion themselves. The re-calculation processes and possible re-assignment processes can be separated onto further server nodes as well.

Fig. 1 :

 1 Fig. 1: View box overlap

Fig. 2 :

 2 Fig. 2: proposed Timadorus Architecture

Fig. 3 :

 3 Fig. 3: Avatar Grouping (P=10, 3 servers, 1000 avatars)


Oswald D Kothgassner

Helmut Hlavacs

Leon Beutl

Lisa M Glenk

Rupert Palme

Anna Felnhofer

Two Experimental Virtual Paradigms for Stress Research: Developing Avatar-Based Approaches for Interpersonal and Evaluative Stressors

Keywords: virtual reality, social stress, stress induction, stress research, Cyberball-Paradigm, Trier Social Stress Test (TSST)

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Given its role in the development of various diseases, social stress has been a subject of extensive research in past decades (c.f., [START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF]. The exposure to a psychosocial stressor is known to result in an acute physiological stress response, involving the activation of the Sympatho-Adrenal-Medullary (SAM) axis and the Hypothalamus-Pituitary-Adrenal (HPA) axis. The SAM-system causes an immediate increase in heart rate, whereas the HPA-axis activation entails a rise in blood and salivary cortisol levels [START_REF] Kudielka | Sex differences in HPA axis responses to stress: a review[END_REF].

Generally, two kinds of social stressors may be distinguished: social-evaluative threats versus interpersonal (rejection) stressors (c.f., [START_REF] Dickerson | Acute stressors and cortisol responses: a theoretical integration and synthesis of laboratory research[END_REF]. The first usually occurs in a performance context such as public speeches or jobinterviews. It involves being potentially exposed to negative evaluations and even failure. Studies show that cortisol reactivity is particularly large and recovery rates are low when confronted with social evaluative threats, especially if they are perceived as uncontrollable [START_REF] Dickerson | Acute stressors and cortisol responses: a theoretical integration and synthesis of laboratory research[END_REF]. In everyday situations, however, these performance based social evaluations are not as common. More frequently, individuals may be exposed to (quite subtle) forms of social rejection. These may involve disapproval and criticism by peers or even being ignored by others. It has been shown, that social exclusion is particularly threatening for fundamental social needs like belonging and self-esteem [START_REF] Williams | Ostracism[END_REF]. Also, a repeated exposure may lead to cardiovascular diseases [START_REF] Kemp | Depression, comorbid anxiety disorders, and heart rate variability in physically healthy, unmedicated patients: implications for cardiovascular risk[END_REF] and depression [START_REF] Mcewen | Glucocorticoids, depression, and mood disorders: structural remodeling in the brain[END_REF].

Experimental Paradigms in Social Stress Research

In light of its pathogenic significance, it is not surprising that in recent years research on social stress has gained in numbers (c.f., [START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF]. Traditionally, paradigms such as arithmetic tasks or public speaking scenarios [START_REF] Dickerson | Acute stressors and cortisol responses: a theoretical integration and synthesis of laboratory research[END_REF]) have been used to induce stress in laboratory settings.

One of the most widely applied experimental paradigms for social stress induction is the Cyberball-Game [START_REF] Williams | Ostracism[END_REF]. It represents a computer-based ball-tossing game, in which the participant is excluded from the game by two computer-animated players. As such, it is a perfect example of an interpersonal rejection stressor. Past studies have demonstrated the paradigm's effectiveness in evoking a number of stress related responses (e.g., [START_REF] Eisenberger | Does rejection hurt? An FMRI study of social exclusion[END_REF][START_REF] Geniole | State, not trait, neuroendocrine function predicts costly reactive aggression in men after social exclusion and inclusion[END_REF][START_REF] Moor | The Heartbrake of Social Rejection Heart Rate Deceleration in Response to Unexpected Peer Rejection[END_REF][START_REF] Wesselmann | Dial a feeling: Detecting moderation of affect decline during ostracism[END_REF][START_REF] Williams | Ostracism[END_REF][START_REF] Zwolinski | Psychological and neuroendocrine reactivity to ostracism[END_REF]. However, the task has also been criticized for its lack of mundane realism [START_REF] Parsons | Virtual Reality for Enhanced Ecological Validity and Experimental Control in the Clinical, Affective and Social Neurosciences[END_REF]. The abstract stimulus bears only minimal resemblance to face-to-face social interactions and is, thus, regarded as limited in its ecological validity.

Another commonly used protocol for social stress induction is the Trier Social Stress Test (TSST; [START_REF] Kirschbaum | The 'Trier Social Stress Test'-a tool for investigating psychobiological stress responses in a laboratory setting[END_REF]. It requires a participant to prepare a five minute speech which s/he then delivers in front of two to three confederate actors; also, the participant performs a five minute arithmetic task. The TSST constitutes a typical social evaluative threat which is mostly perceived as uncontrollable because of the actors' neutral facial expressions and lack of predictability and which results in significant levels of stress (Kudielka et al., 2004). Yet, the drawback of this method is that it requires the actors to hold their reactions constant in each trial. It is a prerequisite which is difficult to achieve across studies and may, thus, limit inter-and intra-individual comparisons [START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF].

Virtual Reality based solutions for stress research

A viable solution for these methodological problems is proposed by immersive virtual environments (IVEs). On the one hand, IVEs may provide a standardization of stimuli by introducing computer-controlled characters which reduce human error and circumstantial variations [START_REF] Bohil | Virtual reality in neuroscience research and therapy[END_REF]. On the other hand, they allow for a real-time, dynamic interaction which approximates real-life social encounters [START_REF] Parsons | Virtual Reality for Enhanced Ecological Validity and Experimental Control in the Clinical, Affective and Social Neurosciences[END_REF]. As such, IVEs constitute a loophole for the intricate problem of combining ecological validity and experimental rigor.

However, the precondition for IVEs to be considered ecologically valid is that they evoke reactions in users which are comparable to real-life situations. A phenomenon which has extensively been discussed to be responsible for the success of IVEs is the so called sense of presence. Presence may be defined as a sense of being there in the IVE. It is seen as a basis for a concordance between an emotion experienced in an IVE and one experienced in a comparable physical environment [START_REF] Slater | A note on presence terminology[END_REF]. Yet, the debate regarding the nature of the relationship between presence and emotions is still ongoing (e.g. [START_REF] Diemer | The impact of perception and presence on emotional reactions: a review of research in virtual reality[END_REF].

For both paradigms (Cyberball and TSST), attempts have been made to develop virtual counterparts. So far, three studies introduce a virtual Cyberball-Game and present preliminary results on its usefulness (i.e., [START_REF] Kassner | Virtually Ostracized: Studying Ostracism in Immersive Virtual Environments[END_REF]Kothgassner et al., 2014;[START_REF] Segovia | Virtual imposters: Responses to avatars that do not look like their controllers[END_REF]. However, [START_REF] Kassner | Virtually Ostracized: Studying Ostracism in Immersive Virtual Environments[END_REF] did not use physiological markers to assess stress reactivity, and [START_REF] Kothgassner | Does social exclusion hurt virtually like it hurts in real life? The role of agency and social presence in the perception and experience of social exclusion. Challenging Presence[END_REF] only assessed the SAM axis reactivity. Given that cortisol is particularly sensitive to social stressors [START_REF] Dickerson | Acute stressors and cortisol responses: a theoretical integration and synthesis of laboratory research[END_REF], the generalizability of both studies is limited and further empirical support is needed.

Similarly, virtual TSST versions have been introduced by different research groups (e.g. [START_REF] Delahaye | Learning aptitude, spatial orientation and cognitive flexibility tested in a virtual labyrinth after virtual stress induction[END_REF][START_REF] Fich | Can architectural design alter the physiological reaction to psychosocial stress? A virtual TSST experiment[END_REF][START_REF] Kelly | Psychosocial stress evoked by a virtual audience: relation to neuroendocrine activity[END_REF][START_REF] Jönsson | Cardiovascular and cortisol reactivity and habituation to a virtual reality version of the Trier Social Stress Test: A pilot study[END_REF][START_REF] Montero-López | A virtual reality approach to the Trier Social Stress Test: Contrasting two distinct protocols[END_REF][START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF]. However, the virtual TSSTs used in these studies differ with regards to content. While [START_REF] Fich | Can architectural design alter the physiological reaction to psychosocial stress? A virtual TSST experiment[END_REF] and [START_REF] Jönsson | Cardiovascular and cortisol reactivity and habituation to a virtual reality version of the Trier Social Stress Test: A pilot study[END_REF] used three virtual interviewers each, other authors used five [START_REF] Kelly | Psychosocial stress evoked by a virtual audience: relation to neuroendocrine activity[END_REF] or even 80 avatars [START_REF] Delahaye | Learning aptitude, spatial orientation and cognitive flexibility tested in a virtual labyrinth after virtual stress induction[END_REF] instead of the formerly proposed two to three interviewers. Also, these studies tend to alter the experimental setup by including an audience that is 'restless' rather than neutral (e.g. [START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF]. Hence, it is possible, that these alterations confounded the observed stress reactions.

Objective

In sum, social stress research is in need of ecologically valid stress induction scenarios. As traditional paradigms suffer from a number of disadvantages, IVE based counterparts are increasingly considered as valuable alternatives. This study set out to create virtual analogs for the two most commonly used and most well researched social stress paradigms, the Cyberball-Game and the TSST protocol. The objective was to provide support for their effectiveness using a multimodal assessment of stress responses (including subjective and objective markers). Also, by covering two complementary social stressors (social-evaluative threats vs. rejection stressors), more far-fetched conclusions about social stress reactivity in IVEs as well as generalizations to other paradigms were supported.

Methods

Two experimental psychological studies were conducted to explore the virtual paradigms' effectiveness in evoking social stress. Study 1 evaluated a virtual Cyberball-Game and Study 2 involved a virtual TSST. In accordance with the Declaration of Helsinki, subjects signed an informed consent and received course credits.

Study 1 (Cyberball)

Participants. Eight female (4 each group) German speaking undergraduates with an age ranging from 22 to 25 years participated in the current study. Measures. To assess physiological stress reactions, salivary cortisol samples were taken via cotton swabs (Salivette®, Sarstedt, Wiener Neudorf, Austria). Participants were instructed to put the swabs into their cheek pouch and wait for 80 seconds to ensure sufficient saturation of the cotton with saliva. Prior to further processing, the samples were stored at -20°C. Sample analyses were centrifuged at room temperature (at 3000g for 15 min) and then analyzed according to a sensitive cortisol enzyme immunoassay (see [START_REF] Palme | Measurement of cortisol metabolites in feces of sheep as a parameter of cortisol concentration in blood[END_REF]. A total of 10 µl of a 1:10 clear saliva dilution were used and all samples were assayed in duplicates. To assess the virtual experience, a single item from the iGroup Presence Questionnaire (IPQ, [START_REF] Schubert | The experience of presence: Factor analytic insights[END_REF] was used ("In the computer generated world I had a sense of 'being there'). Additionally, a single question ("Are you stressed?") served as an indicator for the subjective stress response. Another single item assessed the participants' perception of exclusion during the study ("Do you feel rejected and excluded?") on a visual analogue scale (VAS, 10 cm length). Procedure. Upon their arrival at the VR-lab, participants were randomly assigned to one of two conditions (1) inclusion vs. (2) exclusion. They were asked to fill out a demographic screening and the VAS stress question before being donned the headmounted-display (Sony HMZ-T1, Sony, Japan) with an external head-tracking unit (TrackIR 5, NaturalPoint, US). A wireless control unit (F710, Logitech, Switzerland) allowed tossing the ball. The left key had to be pressed to toss the ball to the left player and vice versa. Participants in the exclusion condition only received 4 ball tosses (duration: 45-60s) before being excluded from the game with no further tosses until the end of the 5-minute interval. The remaining participants received a preprogrammed amount of ball tosses (30 %). Afterwards, participants were asked to fill out the remaining psychometric measures (IPQ and VAS stress question).

Cortisol samples were collected at 4 time points: (1) 20 min before the IVE exposure, (2) immediately before exposure, (3) 20 mins and (4) 35 min post exposure. The overall procedure lasted approx. 80 min; all experiments were conducted in the afternoon (1:30-3:30 p.m.) to account for diurnal variation in cortisol release.

Study 2 (TSST)

Participants. Five German speaking students (3 females, 2 males) with an age ranging from 24 to 26 years were assessed via the VR-TSST. Measures. Salivary cortisol samples also served as an indicator for social stress in this experiment. The extraction method as well as subsequent analyses did not differ from Study 1; the time points for sample collection are described below. Again, the IPQ item "sense of being there" [START_REF] Schubert | The experience of presence: Factor analytic insights[END_REF] and the VAS stress question assessed the subjective experience. Procedure. Prior to exposure, participants filled out the pre-experimental questionnaires (demographics, VAS stress). Subsequently, they were informed about the job interview and a preparation time of 5 min was provided. Analogous to study 1, the same HMD and head tracking unit were used. At the start of the simulation, the two virtual interviewers (1 female, 1 male) entered the room, sat down and asked the participant why s/he was the best candidate for the job. Like in the original TSST, participants passed a job-interview in the first half of the task (5 min), for the remaining 5 minutes, they conducted an arithmetic task (subtract 13 from 1022). The avatars retained neutral expressions throughout the interview; their answers included instructions such as "This is incorrect. Please start over from 1022." After the procedure, participants answered the IPQ item and the single stress-question; 20 min post procedure, subjective stress was measured again. Cortisol samples were collected at 6 time points: (1) 20 min prior to stressor onset, (2) 10 min, (3) 20 min, (4) 25 min, (5) 40 min and (6) 60 min post exposure. The overall procedure lasted 80 min.

Technical implementation

The applications use the open source render engine Ogre3D for real-time rendering of the scenes. The source code is written in C++ using the express version of Visual Studio. OpenAL, the well-known audio library was used for playing the sounds. All objects and animations in the scenes were created using Blender3D.

Options Cyberball

1. Mood: The park can be rendered in 5 different 'moods': (1) happy, (2) anger, (3) boredom, (4) sadness, (5) fear. The moods are masked by numbers to prevent the users from seeing the chosen option. Happy will render the park with a clear sky and pleasant surroundings. Anger is the same as happy, but there will be loud construction sound playing and construction vehicles will be visible. Sadness renders the park with a dark cloudy sky and rain. Boredom renders a concrete floor surrounded by concrete walls. Lastly, fear renders the park scenario at night. For this study, the default setting 'happy' with a sunny park was used. 2. People: If this option is selected, other virtual characters (i.e., agents) will walk around the park, yet interacting with them is not possible for the user. 3. Ballplayers: This option is the core of the Cyberball-scenario (figure 1). It will render two additional virtual humans (1 male, 1 female) which will engage the user in a short football game. In the 'inclusion condition' the players keep playing with the user, in the 'exclusion condition', however, the avatars continue to play only with each other and further exclude the user from the game.

Options TSST 1. Interviewer: This setting allows specification of the interviewers' gender. 2. Speaker: This option specifies the interviewer who does the talking.

3. Emotions: Sets the interviewers' facial expressions to the emotion they are supposed to express. By default neutral is selected, other options are happy or hostile. 4. Ally: If this option is selected, another person (an ally) will participate in the interview. Again, the gender can be chosen. The ally is seated directly next to the user and s/he will answer the questions if the user fails to do so. 5. Helper: This option will make the ally more supportive, encouraging the user with some predefined phrases (i.e., "You are doing great").

Results

The results for the stress reactivity in both studies are presented in figure 2.

Study 1 (Cyberball)

The repeated measures ANOVA showed a significant increase of salivary cortisol concentration (F(18,3)=6.262; p<0.004), especially in the excluded group (F(18,3)=4.069; p=0.023). There is a distinct interaction effect, displaying that excluded participants showed significant increases in saliva cortisol (+51 % to baseline), whereas included participants had a lower increase after the Cyberball game (+22 % to baseline). Moreover, there is a significant increase in self-reported stress levels (F(5,2)=12.298; p<0.001), but only excluded participants rated the Cyberball game as stressful (F(5,2)=10.821; p=0.002) and stated, that they felt excluded (F(8,1)=28.612; p=0.002). Table 1 shows a null finding regarding the relationship between the sense of being there and cortisol stress reactivity as well as the self-reported measure after social exclusion and inclusion.

Study 2 (TSST)

A repeated measures ANOVA with a subsequent Greenhous-Geisser correction depicts a significant stress response in salivary cortisol (F(7.345,1.836)=5.247; p=0.040). Both the speech and math task seem to evoke a stress response in salivary cortisol concentration of 68 % for the speech task and 83 % for the math task respectively compared to the baseline sample measure. Furthermore, participants had significant stress ratings post exposure (F(4,2)=7.051; p=0.017; ƞ²=0.23). Again, table 1 shows no correlation between presence and cortisol or the subjective stress response.

Discussion

In light of the rather limited ecological validity of paradigms traditionally used for social stress induction, the current paper set out to introduce virtual analogues of two popular, well researched paradigms, the Cyberball-Game [START_REF] Williams | Ostracism[END_REF] 1. Correlations between sense of being there and stress measures* * No significance within an Alpha of 5 % # Measured during stressor; self-report stress is measured by the single-item question immediately after the stressor (VAS); salivary cortisol values point out mean salivary cortisol levels during speech and math task or the saliva sample immediately after the Cyberball stressor Trier Social Stress Test (TSST, [START_REF] Kirschbaum | The 'Trier Social Stress Test'-a tool for investigating psychobiological stress responses in a laboratory setting[END_REF]. The present findings suggest that both the virtual Cyberball and the virtual TSST may be used as effective tools for social stress induction. All TSST-participants as well as those excluded from the Cyberball-Game showed significant increases in salivary cortisol during exposure to the stressor and reported to feel stressed. These observations are largely in line with prior research (e.g., [START_REF] Jönsson | Cardiovascular and cortisol reactivity and habituation to a virtual reality version of the Trier Social Stress Test: A pilot study[END_REF][START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF], and the current effect sizes are comparable to those achieved with the original tasks (Beekman, Stock, & Marcus, 2015;Kudielka et al., 2004).

The findings on the virtual Cyberball-task particularly constitute a valuable addition to the body of data on cortisol responses to social exclusion. To date, there is no consensus on whether the experience of social exclusion under laboratory settings results in an increase in cortisol (c.f. [START_REF] Dickerson | Cortisol responses to social exclusion In[END_REF]. Prior studies using the original Cyberball-Game (i.e., Beekman, Stock, & Marcus, 2015;[START_REF] Zöller | No effect of social exclusion on salivary cortisol secretion in women in a randomized controlled study[END_REF] produced conflicting findings. A more ecologically valid virtual scenario, however, may lead to different results than the minimalistic original. Thus, the present study offers first evidence that virtual social threatssimilar to real life-threatsmay result in a considerable HPA axis activity. Certainly, a drawback of this study is the fact that possible gender differences in stress reactivity could not be considered as the sample consisted only of women. In order to substantiate the claim that cortisol increases are larger in women when confronted with social rejection, whereas men react more to achievement stressors [START_REF] Stroud | Sex differences in stress responses: social rejection versus achievement stress[END_REF], future VR research is challenged to balance participants' gender.

Another issue that was not considered in the current paper was the question of whether participants believed to interact with an avatar (i.e., a human-controlled character) or an agent (i.e., a computer algorithm). The Threshold Model of Social Influence [START_REF] Blascovich | Immersive virtual environment technology as a methodological tool for social psychology[END_REF] assumes that the degree of agency attributed to the virtual other influences the interaction. Other authors, however, have succeeded in demonstrating that it does not matter whether the Cyberball-players are controlled by a human or a computer (e.g., [START_REF] Kothgassner | Does social exclusion hurt virtually like it hurts in real life? The role of agency and social presence in the perception and experience of social exclusion. Challenging Presence[END_REF]. In contrast to the virtual Cyberball and to the best of our knowledge, agency was not yet evaluated in studies on virtual TSST versions. Hence, additional research is needed to further substantiate the claim that there is no difference in stress responses to avatars or agents for both virtual tasks.

A further concern is the validity of the findings. Generally, IVE stress induction paradigms have been found to produce medium to large effect sizes, approximating those of real-life-scenarios (c.f., [START_REF] Parsons | Virtual Reality for Enhanced Ecological Validity and Experimental Control in the Clinical, Affective and Social Neurosciences[END_REF]. Even though the present results are in line with those achieved with the original TSST (e.g., Kudielka et al., 2004), a direct comparison of the virtual and real TSST would be the method of choice to establish validity. Also, comparing the virtual Cyberball to a face-to-face version would allow for a more thorough examination of its ecological validity. A first attempt [START_REF] Kothgassner | Does social exclusion hurt virtually like it hurts in real life? The role of agency and social presence in the perception and experience of social exclusion. Challenging Presence[END_REF] has shown that virtual exclusion may result in similar stress responses (heart rate) as real exclusion, but results on cortisol reactivity are still missing.

Finally, presence was not correlated with the salivary cortisol response or subjective stress in the current study. This result corresponds with a virtual TSST study [START_REF] Ruiz | Adaptation of The Trier Social Stress Test To Virtual Reality: psycho-physiological and neuroendocrine modulation[END_REF] which found no connection between the experience of presence and cortisol. Similarly, another study [START_REF] Felnhofer | Afraid to be there? Evaluating the relation between presence, self-reported anxiety, and heart rate in a virtual public speaking task[END_REF] found no correlation between presence, subjective anxiety and heart rate. Hence, rather than directly increasing the intensity of the emotional reaction, presence seems to serve as a precondition for an emotion or stress reaction to occur at all in IVEs. This study supports these findings and extends them to yet another indicator of stress -salivary cortisol. Future research should, however, complement questionnaire measures of presence with a multimodal approach (i.e., using behavioral markers) to shed more light on this issue. Also, the assessment of presence could be supplemented by another related yet distinct construct which could proof to be useful especially in avatar based IVEs: social presence (c.f., [START_REF] Biocca | The networked minds measure of social presence: Pilot test of the factor structure and concurrent validity[END_REF].

Limitations and Conclusion

The present results once again strongly encourage the use of IVEs for diverse research purposes. Apart from traditional laboratory-based stress research, the introduced virtual counterparts may well be applied especially in fMRI studies. While being scanned, participants are usually confined to an enclosed space with little to no interaction. By creating Internet based shared IVEs, within which participants located kilometers apart may interact with each other, scientist may study neural responses simultaneously and under more ecologically valid circumstances [START_REF] Bohil | Virtual reality in neuroscience research and therapy[END_REF]. For instance, the Cyberball-Game could consist of three participants actually playing with each other, and the TSST could make use of an extra functionan ally who sits right next to the interviewee and supports him/her.

Everyday social encounters involve a broad array of cues such as verbal and nonverbal reactions, joint attention, theory of mind as well as representation of self and others [START_REF] Parsons | Virtual Reality for Enhanced Ecological Validity and Experimental Control in the Clinical, Affective and Social Neurosciences[END_REF]. Even though IVEs have come a long way to approximating real-life settings, they are still challenged to convincingly implement mechanisms of social interactions. In this study, basic avatar models were used with minimal facial and behavioral variability; spontaneous reactions were limited and the user was not able to freely engage the avatars. Yet, still, our participants responded with considerable stress to the virtual character. The Media Equation Concept [START_REF] Nass | Machines and mindlessness: Social responses to computers[END_REF] may provide an explanation as to why image fidelity is not a key issue for a social response to computer generated images of others. This theory suggests that humans are evolutionarily prepared to react socially to even very minimal social cues (e.g. the impression of a face). This is alleviation for IVE programmers, as even low quality IVEs may trigger a substantial emotional and behavioral reaction in users as long as they are present. A study [START_REF] Kothgassner | Salivary cortisol and cardiovascular reactivity to a public speaking task in a virtual and real-life environment[END_REF] assessing heart rate and cortisol responses to a public speech task in three groups (real audience, virtual audience, controls) further supports this assumption: it shows that stress responses in IVEs may solely be attributed to the confrontation with the social stimulus and not to the VR apparatus or other factors. Hence, the paradigms introduced here may also be used for therapeutic purposes, e.g. learning how to control one's stress level when confronted with social cues which are unpredictable and uncontrollable.

In sum, this study provided strong support for the implementation of virtual paradigms for social stress induction. The virtual analogues may on the one hand overcome the lack of ecological validity stemming from the minimalistic and abstract material; on the other hand, they may ensure an appropriate degree of control in order to enhance internal validity by providing interactions with no inter-subject variations.

Figure 1 .

 1 Figure 1. The virtual Cyberball-Paradigm (left) and virtual TSST (right)

Figure 2 .

 2 Figure 2. Saliva cortisol and self-reported stress response.

Acknowledgements

We would like to thank Anna-Katherina Heinzle, Ellen Gerlach, Farina Flick and Johanna Rudyk for their support in the acquisition of parts of the data.


Stefan Liszio
email: stefan.liszio@uni-due.de

Maic Masuch
email: maic.masuch@uni-due.de

K Alt

N Bloch

F Born

J Feuerbach

T Günter

H Häuser

F Münch

A Schröder

P Sykownik

E Meinike

Designing Shared Virtual Reality Gaming Experiences in Local Multi-Platform Games

Keywords: Virtual Reality Gaming, Multiplayer, Immersion, Presence, Game Design, Player Experience, Social Interaction

Designing multiplayer virtual reality games is a challenging task since immersion is easily destroyed by real world influences. However, providing fun and social virtual reality experiences is inevitable for establishing virtual reality gaming as a convincing new medium. We propose a design approach to integrate social interactions into the game design while retaining immersion, and present design methods to implement this approach. Furthermore, we describe the game design of a collaborative local multi-player/platform virtual reality game to demonstrate the application and effectiveness of our methods.

1 Make or Break: Social Interaction vs. Immersion Virtual Reality (VR) games are able to create unique realistic experiences in fictional worlds to an extent that is out of reach for traditional gaming systems. One of the main contributing factors for a compelling virtual reality gaming experience (VRGX) is the system's ability to shift the user's perceptive and cognitive attention from the real to the virtual world to elicit sensory [START_REF] Slater | A virtual presence counter[END_REF] and cognitive [START_REF] Sherman | Understanding virtual reality: Interface, application, and design[END_REF] immersion. Hence, any action or event happening in the real world might destroy this illusion. This holds for any interaction with other social entities, which make the real world become salient again. This is why VR is often perceived as being a solitary technology, which isolates a single individual in an artificial environment [START_REF] Crecente | Nintendo's Fils-Aime: Current state of VR isn't fun[END_REF]. On the other hand, social interactions while gaming are considered a key factor for joyful gaming experiences [START_REF] Lazzaro | Why we play: Affect and the fun of games: Designing emotions for games, entertainment interfaces, and interactive products[END_REF]. Some authors are of the opinion that there is a contradiction between social interactions and cognitive immersion [START_REF] Sweetser | Gameflow: a model for evaluating player enjoyment in games[END_REF]. In contrast, Cairns et al. [START_REF] Cairns | Who but not where: The effect of social play on immersion in digital games[END_REF] state that without social interaction some games would not be fun at all. According to the authors, only such interactions which do not happen within the game's context are a potentially disruptive influence on cognitive immersion. This argument is supported by a study on the disruptive factors on presence (i.e. feeling of actually being in the virtual world [START_REF] Heeter | Being there: The subjective experience of presence[END_REF]) by Slater and Steed [START_REF] Slater | A virtual presence counter[END_REF]. One of the most reported reasons for breaks in presence was the hearing of background noises such as people talking. Then again some participants mentioned the wish to share their experience as a cause for the perceived break in presence. In line with these findings, it was found that players experience higher levels of cognitive immersion when playing with others [START_REF] Cairns | Who but not where: The effect of social play on immersion in digital games[END_REF]. To solve this issue, the other players must become part of the game in such way, that any interaction becomes an action intrinsic to the gameplay. We assume that cognitive immersion in digital games can be preserved and become a shared experience, if all interactions between players are expressed through game mechanics. Taking this as a design paradigm, we are able to design multiplayer VR games, which provide a rich social VRGX to all players without cutting down immersion.

Expanding the Game Space

Following the design paradigm set up in this paper, social interaction has to be integrated as a core mechanic of the game, meaning that it is essential for the game design to work. If game related social interaction takes place beyond the virtual game world, as this is the case in local multiplayer games, it is necessary to push the game's boundaries [START_REF] Salen | Rules of play: game design fundamentals[END_REF], [START_REF] Fullerton | Game design workshop: A playcentric approach to creating innovative games[END_REF] beyond the mere virtual game world. We call this approach the expansion of the game space . The systematic expansion of the game space is comparable to the basic idea of pervasive gaming and mixed reality gaming. These genres expand the magic circle of play, either socially, spatially or temporally [START_REF] Montola | Exploring the edge of the magic circle: Defining pervasive games[END_REF] to combine natural world interactions with digital game elements [START_REF] Cheok | Touch-space: Mixed reality game space based on ubiquitous, tangible, and social computing[END_REF]. Including player interactions in the physical world as a game mechanic makes the game happen in both worlds: the virtual and the real world. Hence, the game space is stretched out to allow all players to enter the game world, whether they access it using a head-mounted display (HMD) or any other gaming device. Applying this idea to VR gaming implicates that not only what happens in VR is part of the game, but also all related actions and events happening outside. It is possible that multiple players become part of the game and share the same gaming experience facilitated by the cognitive immersion stimulated by the game [START_REF] Cairns | Who but not where: The effect of social play on immersion in digital games[END_REF]. A shared cognitive immersion is the foundation of live action role-playing game or even traditional pen and paper role-playing games. The latter game genre demonstrates that sensory immersion is not crucial, although it can facilitate and enrich the experience (e.g. costumes, props). With regard to (partly) digital games, alternate reality games show that it is possible to create engaging games, which emerge to the physical world, blurring the line between reality and fiction. Popular examples are the location-based mobile games Ingress, and the recently in 2016 released Pokémon GO developed by Niantic Inc. The success of these games proves that highly immersive experiences depend more on factors like storytelling, and social interaction than on the technological characteristics of the medium itself.

Design Methods

The following methods and techniques help designers of VR games to implement our approach of the expansion of the game space.

Unification. Unification is a game design technique where in a first step a fitting and homogeneous theme for a game is identified [START_REF] Schell | The Art of Game Design: A Book of Lenses[END_REF]. In the second step, every detail of the game is designed along this theme. Thus, all elements of a game from interface to visuals and music must reinforce the game's theme. If possible, even the surrounding real world environment should be arranged and shaped accordingly. For VR games this is a promising approach since today's VR systems do not exhibit a level of sensory immersion high enough to make the necessary hardware components completely "invisible" to the user (i.e. "illusion of non-mediation" [START_REF] Lombard | Defining presence[END_REF]). If the designer manages to find an explanation for these deficiencies which fits the game's theme and story, they may even facilitate the theme. Thus, instead of trying to make the player believe there is no VR system, explain its existence with the game's theme. It is then possible to strengthen cognitive immersion and to compensate lacks in sensory immersion.

Storytelling. Good storytelling establishes an emotional bonding with the virtual world and its inhabitants and involves the player in the unfolding story [START_REF] Fullerton | Game design workshop: A playcentric approach to creating innovative games[END_REF]. Storytelling evokes cognitive immersion on the player, because it gives meaning to the actions and events happening in the virtual world. For VR games, storytelling is important, because the main characteristic of VR games over traditional games is that the player can find herself literally in the middle of the story. Furthermore, it has been shown, that the ideas and emotions transported by the story are equally important for the experience of presence like the VR system's technical qualities [START_REF] Baños | Immersion and emotion: their impact on the sense of presence[END_REF]. Storytelling stimulates the human imagination while the individual properties of the transporting medium become subordinated. Because VR entertainment is a quite unexplored new medium yet, designers have to learn how to craft intriguing stories tailored for VR. Storytelling supports the unification of the game's theme by involving the player mentally and emotionally. Stimulated Communication. Verbal communication is a common form of social interaction found in many games. Provoking game-related chat while the game is actually running can reciprocally amplify unification. Thus, the storytelling should provide a common vocabulary to the players. Then again, if all player communication is consistent with the theme, this supports the occurrence of a unified or homogeneous gaming experience. A recent example for a local multiplayer (VR) game, which stimulates verbal communication between the players is the award-winning Keep Talking and Nobody Explodes developed and published by Steel Crate Games in 2015. The VR player has to defuse a bomb with the instructions given by a second player, who is outside the VR and cannot see the bomb. Real-world communication is the main mechanic in this game, therefore it is impossible to leave it out without breaking the game.

Player Roles and Mutual Dependencies. Assigning identifiable roles to certain players allows the game designer to equip each individual player with special abilities, tasks and objectives, information about the current game state, and views on the virtual game world. This is a prerequisite to establish mutual dependencies between all players. Depending on the structure of interaction between the players and the game system [START_REF] Fullerton | Game design workshop: A playcentric approach to creating innovative games[END_REF] this asymmetric distribution of abilities and information can force players either to collaborate or to compete. The result are complex and dynamic game-related interactions, which make the shared gaming experience unique. Giving players the opportunity to choose roles that fit their individual play styles enhances the gaming experience and strengthens the player's involvement.

Combination of Multiple Platforms. Using diverse game controllers and displays can support the game's theme and establishing player roles. Further, to date it is unlikely to find more than one HMD per household. Hence, game concepts which consider one player wearing a HMD and others participating in the game with alternative hardware are preferable. This technological setup can be used to implement the aforementioned design elements mutual dependencies and player roles. However, complexity and implementation efforts might increase. The advantage of private input and output devices lies in the possibility to provide each player with individual abilities, views on the game world, or an asymmetric information distribution [START_REF] Emmerich | Defining second screen gaming: Exploration of new design patterns[END_REF].

Social VR Game Case Study: Lunar Escape

As a case study, we designed the collocated multiplatform VR game Lunar Escape, using the design methods presented before. In this collaborative game, one player wears an Oculus Rift DK2, while two others play on tablet PCs. Verbal communication between all players is essential to achieve the goal. The players have to find all parts of their broken space ship on a foreign planet to fix it.

Player Roles and Game Mechanics

Lunar Escape is playable with three active players. Each player has to choose a distinct role, which is associated with unique abilities, tasks, and perspectives on the game world, as well as a certain input and output device.

Mech-Operator. The Mech is a robot remotely controlled by the VR player by using a Razer Hydra motion and orientation detection game controller. This special controller was integrated to provide a natural and intuitive VR interface and to support the game's futuristic theme. Any movement of the player's arms is mapped to the arms of the Mech (Figure 1). The Mech is armed with shields and different weapons to defend against enemies. It can carry things and use a tool to repair the shipwreck. The Mech-Operator is not able to change weapons or tools on her own. Therefore, she has to communicate with the Copilot. Additionally, shields, walking, and firing needs energy, which has to be produced and managed by the Copilot. The Mech-Operator's main objective is to find and collect all the wreckage to repair the space ship. The player has no other information about the game state (i.e. energy status, navigational information).

Copilot. The Copilot manages the internal systems of the Mech using a tablet PC. He is not able to interact with the virtual world directly. However, the Copilot has the most information about the current game state, such as energy and ammunition level, available Scout drones, as well as a minimap of the virtual world, which is updated with new information gathered by the Scout. The player has to provide necessary information to her teammates by direct communication. A major task of the Copilot is to select the weapons or tools as called by the Mech-Operator. Further, the Copilot has to gain energy from minerals for the Mech to walk and shoot, or for repairs. To do this, she has to repeat increasingly difficult visual patterns displayed on the tablet PC in time (Figure 1).

Scout. The Scout steers an unmanned reconnaissance drone to discover the planet. The player uses a tablet PC as input and output device and has an isometric view on the drone and the world below (Figure 1). The drone is controlled by tilting the tablet, e.g. by tilting the tablet PC forward, the drone moves in the same direction. The player can use a grappler to collect minerals from the ground. In addition, the Scout can launch fireworks to distract enemies from itself or the Mech. Further, the Scout has to explore and discover the planet in search of the lost wreckage and enemy hordes. As soon as the Scout spotted a part of the ship, the position is shown on the Copilot's minimap.

Conclusion

With the design of the social multi-platform VR game Lunar Escape, we have proven our design approach to include social interaction in the real and the virtual world as an intrinsic game design feature. Playtest sessions were characterized by vivid game-related communication and deeply focused, engaged play. We assume that the players experienced high levels of immersion and social presence and an overall positive shared VRGX. Encouraged by these results, we believe that the expansion of the game space allows designers to create compelling VR games that are social, immersive, and fun.

Fig. 1 .

 1 Fig. 1. (A) View of the Mech-Operator with tools for carrying things selected. (B) The Copilot interacts with a GUI on a tablet PC to gain energy from minerals. (C) The scout views the drone he's steering and has to collect the green minerals.