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Runtime SDK Overview

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This section provides a high-level overview of the Object Anchors Runtime SDK, which is used to detect objects using an Object Anchors model. You'll gain an understanding of how an object is represented and what the various components are used for.

All of the types described below can be found in one of the following namespaces: Microsoft.Azure.ObjectAnchors, Microsoft.Azure.ObjectAnchors.Diagnostics, and Microsoft.Azure.ObjectAnchors.SpatialGraph.

Types

ObjectModel

An ObjectModel represents a physical object's geometry and encodes necessary parameters for detection and pose estimation. It must be created using the Object Anchors service. Then an application can load the generated model file using the Object Anchors API and query the mesh embedded in that model for visualization.

ObjectSearchArea

An ObjectSearchArea specifies the space to look for one or multiple objects. It's defined by a spatial graph node ID and spatial bounds in the coordinate system represented by the spatial graph node ID. The Object Anchors Runtime SDK supports four types of bounds, namely, field of view, bounding box, sphere, and a location.

AccountInformation

An AccountInformation stores the ID, Key and Domain for your Azure Object Anchors account.

ObjectAnchorsSession

An ObjectAnchorsSession represents an Azure Object Anchors session that is used to create ObjectObserver instances used to detect objects in the physical world.

ObjectObserver

An ObjectObserver loads object models, detects their instances, and reports 6-DoF poses of each instance in HoloLens coordinate system.

Although any object model or instance is created from an observer, their lifetimes are independent. An application can dispose an observer and continue to use the object model or instance.

ObjectQuery

An ObjectQuery tells an object observer how to find objects of a given model. It provides the following tunable parameters, whose default values can be retrieved from an object model.

MinSurfaceCoverage

The MinSurfaceCoverage property indicates the value to consider an instance as detected.

For each object candidate, an observer computes the ratio of overlapped surfaces between transformed object model and the scene, then it reports that candidate to application only when the coverage ratio is above a given threshold.

IsExpectedToBeStandingOnGroundPlane

The IsExpectedToBeStandingOnGroundPlane property indicates if the target object is expected to stand on the ground plane.

A ground plane is the lowest horizontal floor in the search area. It provides good constraint on the possible object poses. Turning on this flag will guide the observer to estimate the pose in a limited space and could improve the accuracy. This parameter will be ignored if the model isn't supposed to stand on the ground plane.

ExpectedMaxVerticalOrientationInDegrees

The ExpectedMaxVerticalOrientationInDegrees property indicates the expected maximum angle in degrees between up direction of an object instance and gravity.

This parameter provides another constraint on the up direction of an estimated pose. For example, if an object is up-right, this parameter can be 0. Object Anchors isn't supposed to detect objects that are different from the model. If a model is up-right, then it won't detect an instance laid side-down. A new model would be used for side-down layout. Same rule applies for articulation.

MaxScaleChange

The MaxScaleChange property indicates the maximum object scale change (within 0 ~ 1) with respect to spatial mapping. The estimated scale is applied to transformed object vertices centered at origin and axis-aligned. Estimated scales may not be the actual scale between a CAD model and its physical representation, but some values that allow the app to render an object model close to spatial mapping on the physical object.

SearchAreas

The SearchAreas property indicates an array of spatial bounds where to find object(s).

The observer will look for objects in the union space of all search areas specified in a query. In this release, we will return at most one object with highest confidence to reduce the latency.

ObjectInstance

An ObjectInstance represents a hypothetical position where an instance of a given model could be in the HoloLens coordinate system. Each instance comes with a SurfaceCoverage property to indicate how good the estimated pose is.

An instance is created by calling ObjectObserver.DetectAsync method, then updated automatically in the background when alive. An application can listen to the state changed event on a particular instance or change the tracking mode to pause/resume the update. An instance will automatically be removed from the observer when tracking is lost.

ObjectDiagnosticsSession

The ObjectDiagnosticSession records diagnostics and writes data to an archive.

A diagnostics archive includes the scene point cloud, observer's status, and information about the models. This information is useful to identify possible runtime issues. For more information, see the FAQ.

Runtime SDK usage and details

This section should provide you with the basics of how to use the Runtime SDK. It should give you enough context to browse through the sample applications to see how Object Anchors is used holistically.

Initialization

Applications need to call the ObjectObserver.IsSupported() API to determine if Object Anchors is supported on the device before using it. If the ObjectObserver.IsSupported() API returns false, check that the application has enabled the spatialPerception capability and\or upgrade to the latest HoloLens OS.

using Microsoft.Azure.ObjectAnchors;

if (!ObjectObserver.IsSupported())
{
    // Handle the error
}

// This call should grant the access we need.
ObjectObserverAccessStatus status = await ObjectObserver.RequestAccessAsync();
if (status != ObjectObserverAccessStatus.Allowed)
{
    // Handle the error
}

Next, the application creates an object observer and loads necessary models generated by the Object Anchors model conversion service.

using Microsoft.Azure.ObjectAnchors;

// Note that you need to provide the Id, Key and Domain for your Azure Object
// Anchors account.
Guid accountId = new Guid("[your account id]");
string accountKey = "[your account key]";
string accountDomain = "[your account domain]";

AccountInformation accountInformation = new AccountInformation(accountId, accountKey, accountDomain);
ObjectAnchorsSession session = new ObjectAnchorsSession(accountInformation);
ObjectObserver observer = session.CreateObjectObserver();

// Load a model into a byte array. The model could be a file, an embedded
// resource, or a network stream.
byte[] modelAsBytes;
ObjectModel model = await observer.LoadObjectModelAsync(modelAsBytes);

// Note that after a model is loaded, its vertices and normals are transformed
// into a centered coordinate system for the ease of computing the object pose.
// The rigid transform can be retrieved through the `OriginToCenterTransform`
// property.

The application creates a query to detect instances of that model within a space.

#if WINDOWS_UWP || DOTNETWINRT_PRESENT
#define SPATIALCOORDINATESYSTEM_API_PRESENT
#endif

using Microsoft.Azure.ObjectAnchors;
using Microsoft.Azure.ObjectAnchors.SpatialGraph;
using Microsoft.Azure.ObjectAnchors.Unity;
using UnityEngine;

// Get the coordinate system.
SpatialGraphCoordinateSystem? coordinateSystem = null;

#if SPATIALCOORDINATESYSTEM_API_PRESENT
SpatialCoordinateSystem worldOrigin = ObjectAnchorsWorldManager.WorldOrigin;
if (worldOrigin != null)
{
    coordinateSystem = await Task.Run(() => worldOrigin.TryToSpatialGraph());
}
#endif

if (!coordinateSystem.HasValue)
{
    Debug.LogError("no coordinate system?");
    return;
}

// Get the search area.
SpatialFieldOfView fieldOfView = new SpatialFieldOfView
{
    Position = Camera.main.transform.position.ToSystem(),
    Orientation = Camera.main.transform.rotation.ToSystem(),
    FarDistance = 4.0f, // Far distance in meters of object search frustum.
    HorizontalFieldOfViewInDegrees = 75.0f, // Horizontal field of view in
                                            // degrees of object search frustum.
    AspectRatio = 1.0f // Aspect ratio (horizontal / vertical) of object search
                       // frustum.
};

ObjectSearchArea searchArea = ObjectSearchArea.FromFieldOfView(coordinateSystem.Value, fieldOfView);

// Optionally change the parameters, otherwise use the default values embedded
// in the model.
ObjectQuery query = new ObjectQuery(model);
query.MinSurfaceCoverage = 0.2f;
query.ExpectedMaxVerticalOrientationInDegrees = 1.5f;
query.MaxScaleChange = 0.1f;
query.SearchAreas.Add(searchArea);

// Detection could take a while, so we run it in a background thread.
IReadOnlyList<ObjectInstance> detectedObjects = await observer.DetectAsync(query);

By default, each detected instance will be tracked automatically by the observer. We can optionally manipulate these instances by changing their tracking mode or listening to their state changed event.

using Microsoft.Azure.ObjectAnchors;

foreach (ObjectInstance instance in detectedObjects)
{
    // Supported modes:
    // "LowLatencyCoarsePosition"    - Consumes less CPU cycles thus fast to
    //                                 update the state.
    // "HighLatencyAccuratePosition" - Uses the device's camera and consumes more CPU
    //                                 cycles thus potentially taking longer
    //                                 time to update the state.
    // "Paused"                      - Stops to update the state until mode
    //                                 changed to low or high.
    instance.Mode = ObjectInstanceTrackingMode.LowLatencyCoarsePosition;

    // Listen to state changed event on this instance.
    instance.Changed += InstanceChangedHandler;

    // Optionally dispose an instance if not interested in it.
    // instance.Dispose();
}

In the state changed event, we can query the latest state or dispose an instance if it lost tracking.

using Microsoft.Azure.ObjectAnchors;

void InstanceChangedHandler(object sender, ObjectInstanceChangedEventArgs args)
{
    // Try to query the current instance state.
    ObjectInstanceState state = sender.TryGetCurrentState();

    if (state != null)
    {
        // Process latest state.
        // An object pose includes rotation and translation, applied in
        // the same order to the object model in the centered coordinate system.
    }
    else
    {
        // This object instance is lost for tracking, and will never be recovered.
        // The caller can detach the Changed event handler from this instance
        // and dispose it.
    }
}

Also, an application can optionally record one or multiple diagnostics sessions for offline debugging.

using Microsoft.Azure.ObjectAnchors;
using Microsoft.Azure.ObjectAnchors.Diagnostics;

string diagnosticsFolderPath = Windows.Storage.ApplicationData.Current.TemporaryFolder.Path;
const uint maxSessionSizeInMegaBytes = uint.MaxValue;

// Recording starts on the creation of a diagnostics session.
ObjectDiagnosticsSession diagnostics = new ObjectDiagnosticsSession(observer, maxSessionSizeInMegaBytes);

// Wait for the observer to do a job.

// Application can report some **pseudo ground-truth** pose for an instance
// acquired from other means.
diagnostics.ReportActualInstanceLocation(instance, coordinateSystem, Vector3.Zero, Quaternion.Identity);

// Close a session and write the diagnostics into an archive at specified location.
await diagnostics.CloseAsync(System.IO.Path.Combine(diagnosticsFolderPath, "diagnostics.zip"));

Finally we release resources by disposing all objects.

using Microsoft.Azure.ObjectAnchors;

foreach (ObjectInstance instance in activeInstances)
{
    instance.Changed -= InstanceChangedHandler;
    instance.Dispose();
}

model.Dispose();
observer.Dispose();