How to get true Orientation/Rotation value after a Transform applied?

Question

In my code, I am currently loading an Entity using the .loadAsync(named: String) method, then adding to my existing AnchorEntity. As a test, I am then rotating my Entity 90°, and would like to determine how to then get the current angle of rotation.

The long-term intent is that I am going to allow users to rotate a model, but want to limit the rotation to a certain degree (I.E., the user can rotate the pitch of the model to 90° or -90°, but no further than that). Without being able to know the current angle of rotation for the Entity, I am unsure what logic I could use to limit this.

Entity.loadAsync(named: "myModel.usdz")
    .receive(on: RunLoop.main)
    .sink { completion in
        // ...
    } receiveValue: { [weak self] entity in

        guard let self = self else { return }
        self.objectAnchor.addChild(entity)
        scene.addAnchor(objectAnchor)
        
        let transform = Transform(pitch: .pi / 2,
                                    yaw: .zero,
                                   roll: .zero)
        
        entity.setOrientation(transform.rotation,
                                  relativeTo: nil)
        
        print(entity.orientation)

        // Sample Output: simd_quatf(real: 0.7071069, 
        //                           imag: SIMD3<Float>(0.7071067, 0.0, 0.0))
    }
    .store(in: &subscriptions)

I would have expected entity.orientation to give me something like 90.0 or 1.57 (.pi / 2), but unsure how I can get the current rotation of the Entity in a form that would align with expected angles.

Answer 1

simd_quatf

In RealityKit 2.0, when retrieving simd_quatf structure's values from .orientation and .transform.rotation instance properties, the default initializer brings real (scalar) and imaginary (vector) parts of a Quaternion:

public init(real: Float, imag: SIMD3<Float>)

Hamilton's quaternion expression looks like this:

In your case, the value 0.707 corresponds to a rotation angle of 45 degrees. If you set both real (a) and imag.x (bi) to 0.707, you'll get a total rotation angle of 90 degrees for X axis.

It's easy to check:

let quaternion = simd_quatf(real: 0.707, imag: [0.707, 0, 0])   // 90 degrees
entity.orientation = quaternion

To check what "readable" model's orientation is, use regular parameters from another initializer:

public init(angle: Float, axis: SIMD3<Float>)

print(entity.orientation.angle)         // 1.5707964
print(entity.orientation.axis)          // SIMD3<Float>(0.99999994, 0.0, 0.0)