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Design

Shared spatial anchors

Overview

Multiplayer experiences can have fully remote, local, or a mix of user types. Shared spatial anchors supports local multiplayer and sharing of the activity space. Each additional user/avatar becomes a dynamic obstacle that users try to avoid, regardless of physical presence. Compared to a single user in a space, local multiplayer increases risk, as multiple users have restricted visibility of their surroundings and each other (see Limited field of view (FoV) in General Considerations).

Shared spatial anchor Health and Safety (H&S) Sample Apps for Unity and Unreal are designed to help developers implement warning features for multiplayer experiences.

Directional passthrough cues users to nearby local multiplayer users in their periphery that are blocked by the HMD’s restricted FoV.
Segmented passthrough applies a cutout to nearby local multiplayer users.
- It is wider than the avatar to account for pose mis-estimation and includes the lower body. See Avatar mapping for potential use cases.
Note: Enabling Depth API within passthrough in tandem with scene can mitigate user challenges in shared activity spaces (see also Occlusions with virtual content).

Crowded experiences create safety risks – it is important to consider the number of users in the same activity space.

The risk of contact increases with more users in the activity space.
Larger Activity Spaces and experiences that allow for dispersion can help reduce risk.
Restricting free space to move around or instructing users to follow the same path while physically present with others can increase the risk of contact.
Typical home living spaces are not suitable for more than two local multiplayer users to engage in active movement and maintain good dispersion.

Avoid encouraging rapid or unexpected movements during local multiplayer as these behaviors increase risk of injury and property damage.

Image showing risk levels for local multiplayer playing seated tabletop or dynamic games

Figure 1: Left: local multiplayer in a seated tabletop game is lower risk because all users are stationary and within the FoV (blue region). Right: local multiplayer in a dynamic target chasing game is higher risk because users are moving and can be blocked from view by the HMD (red region).

Highly active experiences that include engaging with other content in the environment pose a higher risk than adversarial experiences.
- Other users are more likely to be within the FoV during adversarial gaming because they are the focus of attention; hence, users can better maintain appropriate distance.
Active experiences pose a greater risk when users make fast arm movements in close proximity to each other.

Users should be cautioned when entering or leaving the same activity space as another user.

If a user leaves a local multiplayer session, the shared experience should be paused for the remaining users to allow for safe exit and re-entry.
Unplanned exits (e.g. HMD connection failure) can also increase risk for remaining users.

How to show the local user

A user may have an avatar overlaid or a passthrough cutout. In both cases, this allows the user to be seen and not blocked from view by content behind them.

Mapping both the size and position of the avatar to the user is the safest representation.

Making the avatar appear larger or smaller than the user makes it difficult to interact or avoid contact with local multiplayer users
Avoid using the controller input for navigation. Decoupling the position of the avatar from the user’s position (e.g., through teleportation) poses a safety risk and creates a poor experience.
- For seated experiences, teleporting or 1:1 movement should be used with appropriate spacing between users.
- For ambulatory gaming, local multiplayer users should ‘travel’ together maintaining their physical spatial relations in the virtual environment.

Image showing how the local user is displayed to other users

Figure 2: If spatial mapping is not 1:1, a mechanism should be enabled to warn users of the physical position of other local multiplayer users using directional and segmented passthrough. The shared spatial anchors health and safety sample apps for Unity and Unreal provide examples of proximity warnings.

If tracking is lost during asynchronous joining/leaving, the shared experience should be paused for the remaining co-located users to decrease the risk of injury
For immersive virtual environments, passthrough can be used to mitigate risk when desynchronization occurs.
- Transitioning to 100% passthrough can help the users to identify when the virtual avatar is desynchronized; however, this is only for avatars in the FoV.
Re-spawning presents a unique challenge for the user (see note in Virtual Environment):
- For the user(s) continuing play, the risk of contact increases when another user’s avatar disappears.

Virtual environment

The virtual environment is the virtual content that surrounds the user. This includes passthrough primary mixed reality experiences and fully immersive experiences. Shared virtual environments provide a consistent reference frame between users, allowing for potential movement of others to be inferred.

Image showing how virtual content appears to different users

Figure 3: Non-shared virtual environments between local multiplayer users may occur when users join/leave the app or are placed in separate virtual environments (e.g. re-spawning, 1:1x movement through virtual space).

In some cases, users may not see local multiplayers because:
- They are occluded by their virtual environment.
- They are responding to different visual input.
- They have poor awareness of the position and existence of fellow co-located users, increasing risk of contact.
Re-spawning presents a uniques challenge
- For the user that is respawning, partial passthrough (e.g., 50% alpha) showing the physical space with virtual landmarks can reduce contacts.

Figure 4: The physical position of the user is not always a 1:1 match with their virtual avatar. This can lead to users inaccurately estimating where other players are located in the activity space. Passthrough can be used to show the physical space and other users to reduce the likelihood of desynchronized mapping. For the user(s) continuing play, see note in Avatar mapping.

Virtual objects can block others from view and user’s can cut through virtual objects, this combination increases the risks of co-located play. Objects in the space restrict the space to move around (i.e., reduces dispersion of users).

The larger the objects are, the more likely they are to block the other players from view
Restricting the free space to move around by having users walk along similar paths, along corridors, or between objects can increase the risk of contact

Next steps

For more health and safety guidelines, see the following:

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