How to access the native GlobalCoordinateSystem pointer with OpenXR

[Audenrith]

Before Unity 2019.3, one has been able to access the native pointer to the world SpatialCoordinateSystem via UnityEngine.XR.WSA.WorldManager.GetNativeISpatialCoordinateSystemPtr().
Switching to Unity 2019 one is able to access the same pointer via UnityEngine.XR.WindowsMR.WindowsMREnvironment.OriginSpatialCoordinateSystem using the Windows Mixed Reality XR Plugin.
Can you describe the way of accessing the native SpatialCoordinateSystem using OpenXR?

There are some issues which describe how to access a SpatialCoordinateSystem as a Unity/C# Object (e.g. in the Unity forum or in this repository) with OpenXR, but the solutions do not describe how to get the native pointer (IntPtr) (Allocating a GCHandle from the C# object obviously can not be used as a Windows::Perception::Spatial::ISpatialCoordinateSystem^ in c++).

Is there a way on accessing the native pointer of the origin SpatialCoordinateSystem ?

In c++ we are currently casting the pointer via:

reinterpret_cast<Windows::Perception::Spatial::ISpatialCoordinateSystem ^>(unitySpatialCoordinateSystemIntPtr);

and use it for getting the transform to a specific frame:

unitySpatialCoordinateSystem->TryGetTransformTo(frameCoordinateSystem);

[Audenrith]

According to this comment one can access the "sceneCoordinateSystem" via:

using namespace Microsoft.MixedReality.OpenXR;
SpatialCoordinateSystem origin = PerceptionInterop.GetSceneCoordinateSystem(UnityEngine.Pose.identity) as SpatialCoordinateSystem;

But this does not answer the question on how to get the underlying native pointer. I tried several approaches which all resulted in some error:

The following errors occur:

var handle = GCHandle.Alloc(origin , GCHandleType.Pinned); // <-- Error
IntPtr coordinateSystemNative = (IntPtr)(handle);

ArgumentException: Object contains non-primitive or non-blittable data.

var handle = GCHandle.Alloc(origin);
IntPtr coordinateSystemNative = (IntPtr)(handle);
// Error in assigning the variable in c++

Access violation reading location

__pUnknown->__abi_AddRef(); in __abi_winrt_ptr_assign(void** __ppTargetArg, const volatile ::Platform::Object^ __objArg)

var handle = GCHandle.Alloc(origin);
IntPtr coordinateSystemNative = handle.AddrOfPinnedObject(); // <-- Error

InvalidOperationException: Handle is not pinned.

IntPtr coordinateSystemNative = Marshal.GetIUnknownForObject(origin);
// Error in using the variable in c++
// sceneCoordinateSystem->TryGetTransformTo(frameCoordinateSystem);

Access violation executing location.

Can you tell me, how is the correct way to get this object to a Windows::Perception::Spatial::ISpatialCoordinateSystem^ in c++?

[keveleigh]

The result from PerceptionInterop.GetSceneCoordinateSystem(UnityEngine.Pose.identity) is an object that was marshalled from an IUnknown. If you're mainly interested in passing that object back to C++, you should be able to pass it back in as-is.

You'll want to use [MarshalAs(UnmanagedType.IUnknown)] object origin as the C# interop parameter, and then receive it in C++ as an IUnknown* and you should be able to QI it to ISpatialCoordinateSystem from there.

[Audenrith]

Thank you for the clarification.

I managed to get the object to C++ and statically cast it to an IUnknown* (we have to go through a C interface) but since the ISpatialCoordinateSystem requires a ref pointer (an ordinary pointer to a C++/CX mapping ref class or interface class is not allowed), I didn't manage to perform the QueryInterface.

IUnknown* unityObject;
Windows::Perception::Spatial::ISpatialCoordinateSystem^ coordinateSystem;
unityObject->QueryInterface(&coordinateSystem); // Error: no instance of overloaded function matches the argument list
unityObject->Release();

What do I miss here?

[Audenrith]

We didn't managed to correctly receive the pointer in c++. After trying different approaches it seems that the pointer we are receiving will reduce the reference count and will be deleted before we can use it. I should mention, that we require to store the pointer permanently (as long as the program is running in Unity) and use it in a separate thread.

Additionally, we aren't able to use c++ 17 and clr so using a native GCHandle object in c++ is probably also not possible.