About AR Subsystems

A subsystem is a platform-agnostic interface for surfacing different types of functionality and data. The AR-related subsystems are defined in this package and use the namespace UnityEngine.XR.ARSubsystems. This package only provides the interface for various subsystems. Implementations for these subsystems (called "providers") can typically be found in another package or plug-in.

This package provides interfaces for the following subsystems:

• Session
• Anchors
• Raycasting
• Camera
• Plane Detection
• Depth
• Image Tracking
• Face Tracking
• Environment Probes
• Object Tracking
• Body Tracking
• Occlusion
• Meshes

Installing AR Subsystems

ARSubsystems a dependency of platform-specific packages which make use of one or more of the subsystems defined within it. To use AR Subsystems, you must also install at least one of these platform-specific AR packages:

• ARCore XR Plugin
• ARKit XR Plugin
• Windows XR Plugin

Using AR Subsystems

All subsystems have the same lifecycle: they can be created, started, stopped, and destroyed. Each subsystem has a corresponding SubsystemDescriptor, which describes the capabilities of a particular provider. Use the SubsystemManager to enumerate the available subsystems of a particular type. Once you have a valid subsystem descriptor, you can Create() the subsystem. This is the only way to construct a valid subsystem.

Example: Picking a plane subsystem

This example iterates through all the XRPlaneSubsystemDescriptors to look for one which supports a particular feature, then creates it. You can only have one subsystem per platform.

XRPlaneSubsystem CreatePlaneSubsystem()
{
    // Get all available plane subsystems
    var descriptors = new List<XRPlaneSubsystemDescriptor>();
    SubsystemManager.GetSubsystemDescriptors(descriptors);

    // Find one that supports boundary vertices
    foreach (var descriptor in descriptors)
    {
        if (descriptor.supportsBoundaryVertices)
        {
            // Create this plane subsystem
            return descriptor.Create();
        }
    }

    return null;
}

Once created, you can Start and Stop the subsystem. The exact behavior of Start and Stop varies by subsystem, but generally corresponds to "start doing work" and "stop doing work". A subsystem can be started and stopped multiple times. To completely destroy the subsystem instance, call Destroy on the subsystem. It is not valid to access a subsystem after it has been destroyed.

var planeSubsystem = CreatePlaneSubsystem();
if (planeSubsystem != null)
{
    // Start plane detection
    planeSubsystem.Start();
}

// Some time later...
if (planeSubsystem != null)
{
    // Stop plane detection. This doesn't discard already detected planes.
    planeSubsystem.Stop();
}

// When shutting down the AR portion of the app:
if (planeSubsystem != null)
{
    planeSubsystem.Destroy();
    planeSubsystem = null;
}

Refer to the subsystem-specific documentation list above for more details about each subsystem this package provides.

Implementing an AR subsystem

If you are implementing one of the AR Subsystems in this package (for example, you are a hardware manufacturer for a new AR device), you need to implement a concrete instance of the relevant abstract base class this package provides. Those types are typically named XR<feature>Subsystem.

Each subsystem has a nested class called IProvider. This is the primary interface you'll need to implement for each subsystem you plan to support.

Tracking subsystems

A "tracking" subsystem is any subsystem that detects and tracks something in the physical environment (for example, plane tracking and image tracking). The thing that the tracking subsystem tracks is called a "trackable". For example, the plane subsystem detects planes, so a plane is a trackable.

Each tracking subsystem requires you to implement a method called GetChanges. This method retrieves data about the trackables it manages. Each trackable can be uniquely identified by a TrackableId, a 128-bit GUID (Globally Unique Identifier). A trackable can be added, updated, or removed. It's an error to update or remove a trackable that hasn't been added yet. Likewise, a trackable can't be removed without having been added, nor updated if it hasn't been added or was already removed.

GetChanges should report all added, updated, and removed trackables since the previous call to GetChanges. You should expect GetChanges to be called once per frame.

Technical details

Requirements

This version of AR Subsystems is compatible with the following versions of the Unity Editor:

• 2019.4
• 2020.3
• 2021.1
• 2021.2