HoloField: Colocated Multiplayer Boilerplate

byBotao Amber Hu, Yuchen Zhang

HoloField: Colocated Multiplayer Boilerplate


This repository provides three multiplayer AR boilerplates, serving as foundations for developing your multiplayer AR projects.

Implementing a multiplayer AR project involves addressing two critical aspects: network communication and coordinate system synchronization. Network communication is essential for exchanging data across connected devices, and coordinate system synchronization ensures that virtual content is rendered consistently in the real-world locations across different devices, each with its own independent local coordinate system.

For networking, our boilerplates utilize Unity's Netcode for GameObjects in conjunction with Apple's Multipeer Connectivity transport package. Netcode for GameObjects is Unity's official solution for multiplayer games, while the MultipeerConnectivity framework, the technology behind AirDrop, enables easy and stable low-latency connections between nearby Apple devices. Beyond the MultipeerConnectivity framework, we also offer a local router transport solution using Unity Transport package. This allows client devices to connect to the host by entering the host's IP address within the local network.

There are primarily two types of AR localization mechanisms: the "cold start" approach and the "absolute coordinate" approach. The "cold start" approach involves the device initiating the AR session without prior knowledge of its surroundings, requiring it to simultaneously construct a virtual map and localize itself within it. In contrast, the "absolute coordinate" approach means the device begins with complete information about its environment, focusing only on relocalizing itself within a pre-constructed map. We have a specialized article that explains the concept and implementation of multiplayer AR in detail.

We offer three boilerplates: two for the "cold start" approach and another for the "absolute coordinate" approach. For "cold start" cases, we use the two methods in Image Tracking Relocalization package for coordinate system synchronization. For "absolute coordinate" cases, we employ the Immersal SDK, which enables users to pre-scan the environment and create point cloud map data on the cloud. This facilitates large-scale pre-scanned AR tracking.

Apart from differing in their approach to coordinate system synchronization, the three boilerplates share a majority of their codebase in other aspects.

These boilerplates are specifically designed for multiplayer AR experiences and require knowledge in networking programming, with a particular emphasis on Unity's Netcode for GameObjects. Multiplayer development fundamentally differs from single-player game development, necessitating a network-centric approach in both design and implementation. For those new to Netcode for GameObjects, we recommend familiarizing yourself with its documentation.

HoloKit is an optional tool that provides stereoscopic rendering capabilities, but you can build your mobile AR project without HoloKit using these samples.

Project Environment

Please be aware that these boilerplates are designed exclusively for iOS devices.

We have successfully tested them with the following software versions:

  • Unity 2022.3 LTS
  • Xcode 15.2

In theory, other versions of Unity and Xcode that are close to these should also be compatible. However, if you encounter any issues during the build process, feel free to raise an issue in the repository.

External Marker Relocalizaion Boilerplate

This boilerplate provides two network transport options: MultipeerConnectivity and a local router connection via the Unity Transport package. It employs an external marker relocalization technique from the Image Tracking Relocalization package from coordinate system synchronization. Each device independently relocalizes by scanning an external marker and resetting its coordinate origin to this marker.

The process is illustrated in the accompanying GIF: an iPhone tracks an external marker image. We enhance accuracy in image tracking by tracking multiple consecutive poses of a marker, minimizing the deviation that can occur with a single detection. Upon detecting a stable sequence, it computes the correct pose for this sequence and resets its coordinate origin accordingly.

To start, open the scene Assets/Scenes/Multiplayer AR Boilerplate_External Marker Relocalization.unity from project directory and build it for your iOS device. If you want to change the marker image, create your own XRReferenceImageLibrary asset for the ARTrackedImageManager component and add your marker image to the asset. The physical size of the marker must match the size set in the XRReferenceImageLibrary asset to ensure tracking accuracy. Familiarize yourself with the workflow and code structure before customizing it for your project.

For comprehensive details on the external marker relocalization method, refer to the README of the image tracking relocalization package.

Dynamically Rendered Marker Relocalization Boilerplate

This boilerplate utilizes MultipeerConnectivity for its network transport layer and employs a dynamically rendered marker relocalization method for coordinate system synchronization. Unlike external marker relocalization, which relies on a physical marker, this approach enables multiplayer AR experience in any location. Client devices track a marker displayed on the host device's screen, rather than an external marker. The synchronization process aligns the client devices' coordinate origins with that of the host. This ensures that virtual objects, assigned specific coordinates, appear at the same physical locations on both host the client devices.

The accompanying GIF demonstrates the process: two iPhones connect over a local network. The client device then scans the marker image displayed on the host's screen. After detecting a series of stable image poses, the client calculates the relative translation and rotation needed to align its local coordinate origin with the host's. Users need to verify the accuracy of this synchronization by checking if the alignment marker rendered in AR closely matches the host device's real-time physical location. Once synchronized, the HoloKit markers, intended to appear atop each connected device, will follow their respective devices as seen through the iPhone screen.

To get started, navigate to Assets/Scenes/Multiplayer AR Boilerplate_Image Tracking Relocalization.unity to open the scene, and then build it for your iOS devices. Familiarize yourself with the workflow and try to understand the code structure. You can then modify the code to tailor it to your project's needs.

For a detailed understanding of how the dynamically rendered marker re-localizatioin method functions, please refer to the README file of the image tracking relocalization package.

Immersal Boilerplate

This boilerplate offers two network transport layers: MultipeerConnectivity and a local router connection using the Unity Transport package. For coordinate system synchronization, it utilizes the Immersal SDK. Beyond these features, it provides the same functionality as the previous boilerplate.

To operate this boilerplate, first select the transport layer at the screen's bottom UI. If "AirDrop" (MultipeerConnectivity) is chosen, the connection process initiates automatically. Conversely, if "Router" (Unity Transport) is selected, manually input the host's IP address, shown on the host devices's screen, into the client's input field.

Coordinate system synchronization is executed through the Immersal SDK's functionalities. Unlike the image tracking relocalization method, the Immersal SDK doesn't alter the AR session's coordinate origin but transforms the ARSpace object within the scene. keeping the camera pose unchanged. Thus, we have to put all objects, which we want to synchronize through the network, under the ARSpace object. Consequently, synchronizing the scene's game objects' poses isn't as straightforward. For example, to align a game object, instead of using its world coordinate, we need to calculate its relative pose from the ARSpace. The script PlayerPoseSynchronizer_Immersal shows how player poses are synchronized in this context. Similar coordinate transformations are required when spawning objects in the scene. For comprehensive information on how ARSpace operates, please refer to the Immersal documentation.

To begin, navigate to Assets/Scenes/Multiplayer AR Boilerplate_Immersal.unity. You'll need to scan, upload, and download your Immersal map via the Immersal Mapper App and the Immersal Developer Portal. If scanning an Immersal map is unfamiliar to you, the Immersal documentation provides a detailed guide. Once the map file is downloaded, import it into the Unity project and place it in the MapFile slot of the ARMap script, as demonstrated below.

After building the project onto your iOS devices, successful relocalization by the Immersal SDK will enable the HoloKit markers to be rendered accurately atop each connected devices.

Which Network Transport You Should Use

  • MultipeerConnectivity

MultipeerConnectivity is typically the best choice for most applications. It allows nearby iOS devices to connect automatically, leveraging Bluetooth and WiFi capabilities for a peer-to-peer local network. This feature is advantageous as it does not require WiFi or cellular data, enabling device connection in extreme locations like deserts or snow-covered mountains. Additionally, it offers relatively low latency, approximately near 30ms, as it bypasses the need to route messages through a remote server. This mechanism is ideal for the inherently on-site nature of AR experiences.

However, MultipeerConnectivity may not be suitable in two specific scenarios:

  1. Distance Limitations: If devices are likely to be more than 10 meters apart, connection stability can become an issue.
  2. Interference in High-Tech Environments: In areas crowded with digital devices, such as exhibition booths surrounded by multiple routers, wireless signals may be disrupted.

Apart from these two situations, MultipeerConnectivity is highly recommended as the transport layer of choice.

  • Local Router Connection

This method contrasts with MultipeerConnectivity in that it requires the presence of a router and manual input of the IP address. However, like MultipeerConnectivity, it also achieves low latency by facilitating local message transfer.

A prime scenario for employing a local router connection is in exhibition AR projects, where stable connections with multiple devices are crucial. A typical setup might involve a Mac or PC serving as the host, with client iPhones connected to it. Utilizing a Mac or PC as the host provides a robust computational core to manage operations. Additionally, this arrangement allows developers to monitor all connected clients and intervene in real-time, ensuring an optimized AR experience.

  • Unity Relay

While most AR projects do not require remote message transfers, Unity Relay service can be a viable option for your project. Unlike the local router connection method, where a router acts as an intermediary server, Unity Relay provides a remote server to handle message transfers between devices.

Transitioning from a local router connection to using Unity Relay with Unity Transport is straightforward. To get started, we recommend the following YouTube tutorials:

Which Coordinate System Synchronization Method You Should Use

The selection of a coordinate system synchronization method should align with your project's unique requirements. If your project involves a "cold start" scenario, then two methods from the image tracking relocalization package are appropriate. Conversely, for projects requiring "absolute coordinate", the Immersal SDK is the better option.

To elaborate, "cold start" is preferable for AR projects targeting a general audience. This approach is ideal because it doesn't assume that the majority of users will visit a specific location to engage with your AR game. The primary benefit of "cold start" AR is its ability to be experienced anywhere, making it widely accessible. The selection between external marker and dynamically rendered marker relocalization methods should be based on your specific environment and project needs. The external marker method offers a more straightforward setup, as it operates independently, unlike the dynamically rendered approach that requires all client devices to scan the marker on the host's screen. Additionally, the external marker method is more error-tolerant. AR tracking often becomes inaccurate, and an external marker allows for easier and more frequent relocalization during a session without interrupting the host device, in contrast to using a host device-displayed marker. Therefore, for projects involving more than three devices with an accessible external marker, the external marker approach is typically more advantageous. Conversely, for projects with only two or three devices where an external marker is not feasible, the dynamically rendered marker method is preferable.

If your AR project is tailored for specific events, such as exhibitions or museum displays, the "absolute coordinate" approach is recommended. Utilizing a pre-scanned AR map provides more accurate and robust tracking compared to "cold start" setups. This method enables AR experiences in larger spaces, surpassing the room-scale limitations typical of "cold start" projects.

For a comprehensive understanding of multiplayer AR, including its concept and implementation, we recommend reading this specialized article on the subject.

Questions and Feedback

Designing and implementing a multiplayer AR experience can be a complex endeavor. If you have any questions or suggestions regarding this repository, please don't hesitate to raise an issue. We're here to help and value your input!

For more direct communication, join our community on Discord, or feel free to email us at dev@holoi.com or yuchenz27@outlook.com. We look forward to your contributions and are excited to see what you create. Thank you!