Scanning App

This is a port of the Apple sample Scanning and Detecting 3D Objects. The architecture and code design of the app follows that of the Swift app, with some exceptions discussed below.

This sample requires a device that supports ARKit's "object scanning" configuration (ARObjectScanningConfiguration). This configuration is processor-intensive and may not be supported or may not produce adequate tracking on older hardware. This configuration also "disables ARKit features not necessary for reference object scanning," and should only be used for, essentially, the use-case embodied in this app.

The app is used by developers to scan a real-world object to create an ARReferenceObject definition that can be saved to a file and used in end-user mixed-reality apps (for instance, recognizing a sculpture or factory-floor machine).

Usage

After ARKit initializes, a "ghost" bounding box will appear and try to fit around any object that is in the middle of the screen. Use pinch, pan, and two-finger pan to adjust the size of the bounding box until it is sufficient to encompass the scanned object. Points will appear within the bounding box, showing how well the object is being detected.

Defining the bounding box

When the bounding box fully encloses your object, press "Scan" to begin scanning. Move the camera around the bounding box. As the scan retrieves sufficient points, the corresponding area on the bounding box will be covered with a translucent panel.

Scanning

Once you've scanned the object from all sides horizontally and from the top, a set of axes will appear. Drag this with your finger to set the origin (within the bounding box) that you wish for your reference object. Press "Test" and a new AR session will begin, looking for your just-created ARReferenceObject. Move the object around, light it differently, etc. When the object is recognized, a bounding box will appear.

Testing

Press "Share" to save the ARReferenceObject to a file that can be used in your end-user apps!

Notes

Manual resource management in ARKit and SceneKit apps

In ARKit, it's crucial to manually manage resources. Not only does this allow high frame-rates, it actually is necessary to avoid a confusing "screen freeze." The ARKit framework is lazy about supplying a new camera frame (ARSession.CurrentFrame). Until the current ARFrame has had Dispose() called on it, ARKit will not supply a new frame! This has the appearance of the video "freezing" even though the rest of the app is responsive. The solution is to always access it with a using block (see ViewControllerSCNViewDelegate.Update in this example) or manually call Dispose() on it.

Beyond the ARSession.CurrentFrame, the major resource that needs to be managed carefully in this app are instances of PointCloud. This is a visualization of the raw feature points being sensed by ARKit. While this is not called every frame, it's called often enough that it would lead to significant memory consumption. You'll note that prior to the factory method PointCloud.CreateVisualization is called, the app takes care to call Dispose() on the current PointCloud.

Another complex use of Dispose() relates to notifications. The Apple design uses the Observer pattern to subscribe to various state changes (e.g., NSNotificationCenter.DefaultCenter.AddObserver(Scan.ScanningStateChangedNotificationName, ScanningStateChanged); ). You must unsubscribe from such observations while Dispose()ing your object or you will get a segfault when the system attempts to alert your now-deallocated object of the notification. The pattern is simple:

class MyClass : NSObject
{
    private NSObject notificationObservationHandle; 

    MyClass()
    { 
        notificationObservationHandle = NSNotificationCenter.DefaultCenter.AddObserver(notificationName, NotificationHandler);
    }

    private bool disposed = false;
    override protected void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                NSNotificationCenter.DefaultCenter.RemoveObserver(notificationObserverHandle);
            }
            disposed = true;
            base.Dispose();
        }
    }
}

Note that when implementing the Dispose pattern in Xamarin, you must set disposed = true prior to calling base.Dispose() in objects that are derived from NSObject. Xamarin's implementation of NSObject.Dispose() calls Dispose(true), which will cause the stack to overflow if disposed is not already set.

Related: There are more shared references within the Swift code than I'd prefer, so I tried to convert as many as possible into properties with private setters.

Fast, unsafe code to create PointCloud

Because the PointCloud is a short-lived object reflecting complex geometry (the raw feature points that ARKit is seeing within the target bounding box), I use unsafe code to create it directly from an NVector3 array rather than loop over each NVector3, allocate an SCNSphere for that point, add that sphere to the current SCNNode etc.

The PointCloud.CreateVisualization combines two techniques to create complex SCNGeometry as quickly as possible: first, the array is pinned and accessed as an NSData byte buffer. Second, a template SCNGeometryElement is created and the data buffer and the template are "zipped together" using SCNGeometry.Create(SCNGeometrySource[], SCNGeometryElement[]).

This is a good technique to have in your toolkit.

The threshold-aware gestures are nice

The gestures for moving and rotating the bounding box and origin only respond to touches that have either moved or rotated more than some threshold amount. The code in ThresholdPanGestureRecognizer, ThresholdPinchGestureRecognizer and ThresholdRotationGestureRecognizer should be quite reusable.

Matrix types and performance

In any 3D application, you're going to be dealing with 4x4 transformation matrices. In SceneKit, these are SCNMatrix4 objects.

The SCNNode.Transform property returns the SCNMatrix4 transform matrix for the SCNNode as backed by the row-major simdfloat4x4 type. So, for instance:

var node = new SCNNode { Position = new SCNVector3(2, 3, 4) };  
var xform = node.Transform;
Console.WriteLine(xform);
// Output is: "(1, 0, 0, 0)\n(0, 1, 0, 0)\n(0, 0, 1, 0)\n(2, 3, 4, 1)"

As you can see, the position is encoded in the bottom row's first 3 elements. This might be transposed (sideways) to your expectations. The functions SCNMatrix4 ToSCNMatrix4(this NMatrix4 self) and NMatrix4 ToNMatrix4(this SCNMatrix4 self) use the M{col}{row} fields to assure the values are proper, so that's fine, but there is a small performance cost to changing the representation, so avoid needless conversions.

Avoiding ghostBoundingBox Dispose() asynch defect

The original code contained an asynch bug. The ghost bounding box is removed if, during a frame update, the ARSceneView.SmartHitTest() (extension method defined in ARSCNView_Extensions.cs) returns null because no feature points are detected (this will happen if the camera is brought within 20cm of the bounding box). This is done in the result == null path at the beginning of ScannedObject.UpdateOrCreateGhostBoundingBox().

However, when the Scan object has called the asynchronous ARSession.CreateReferenceObject() the handler contains a reference to the ScannedObject.ghostBoundingBox. If, prior to the handler executing, the ghostBoundingBox has been removed as just described, the app throws ObjectDisposedException.

To avoid this, we added the ScannedObject.LockBoundingBoxForReferenceObjectCreation() method and the related busyCreatingReferenceObject field.

An alternate possibility would be to re-acquire the reference in the CreateReferenceObject handler (i.e., rather than pass in boundingBox make another call to ScannedObject.EitherBoundingBox) but I’m not sure if there are no consequences to the reference changing during the asynch period of CreateReferenceObject().

SimpleBox

The class SimpleBox is used to wrap .NET value objects in an NSObject that can be stored in iOS NSDictionary objects and used in the payload for NSNotification objects. It is helpful but should be used sparingly, as it will cause Ahead-of-Time compilation (AoT) to generate at compile-time the variations for all the different values of T.

Classes in the Controllers directory

The Swift code uses five files to organize the behavior of the single class ViewController. In C# these would be called "partial classes." I put the code in different classes, as this makes it easier to reason about the state and responsibilities of each individual class.

Minor structural differences

I occasionally refactored the Swift code where I thought it helped legibility or reduced repetition. Usually this was in the form of extracting a function.

One area where there is considerably variance from the Swift code is in the "Utility_Extensions" folder. Xamarin.iOS uses OpenTK vector and matrix classes for manipulating 3D objects and transforms. The OpenTK classes have different APIs and

A matter of internal debate was the clearest form for porting Swift guard statements that assign a local variable if the passed-in variable is not null. The options are:

  • Explicit check and branch :

    var someReference = passedInReference as TargetType;
    if (someReference == null) 
    {
        return; 
    }
    // ... use someReference
    
  • Pattern match:

    if (! passedInReference is TargetType someReference)
    {
        return;
    }
    // ... use someReference
    

I chose to use the former solely because the pattern matching form is still relatively new and therefore might be less familiar to some developers.

License

Xamarin port changes are released under the MIT license.

Author

Ported to Xamarin.iOS by Larry O'Brien