How can I get the yaw, pitch, roll of an ARAnchor in absolute terms?

Question

I've been trying to figure this out for a few days now.

Given an ARKit-based app where I track a user's face, how can I get the face's rotation in absolute terms, from its anchor?

I can get the transform of the ARAnchor, which is a simd_matrix4x4. There's a lot of info on how to get the position out of that matrix (it's the 3rd column), but nothing on the rotation!

I want to be able to control a 3D object outside of the app, by passing YAW, PITCH and ROLL.

The latest I thing I tried actually works somewhat:

let arFrame = session.currentFrame!
guard let faceAnchor = arFrame.anchors[0] as? ARFaceAnchor else { return }

let faceMatrix = SCNMatrix4.init(faceAnchor.transform)
let node = SCNNode()

node.transform = faceMatrix
let rotation = node.worldOrientation

rotation.x .y and .z have values I could use, but as I move my phone the values change. For instance, if I turn 180˚ and keep looking at the phone, the values change wildly based on the position of the phone.

I tried changing the world alignment in the ARConfiguration, but that didn't make a difference.

Am I reading the wrong parameters? This should have been a lot easier!

You need to calculate the eulerAngle for pitch, yaw and roll. There's near no documentation about it, but give a look here — Andrew21111
While that does give me the values, they're still not "absolute". Meaning, if I rotate myself and the phone (spin on a chair while looking straight at the phone for instance), the rotation values change drastically. — Andre
Did you try to convert the coordinates into another coordinate system with node.convert ? — Andrew21111
Yes. I tried to do node.transform = node.convertTransform(node.transform, from: sceneView.scene.rootNode) but it was still affected by camera movements. Did I do it wrong? — Andre

Andre Andre · Accepted Answer · 2018-11-22T15:43:35

I've figured it out...

Once you have the face anchor, some calculations need to happen with its transform matrix, and the camera's transform.

Like this:

let arFrame = session.currentFrame!
guard let faceAnchor = arFrame.anchors[0] as? ARFaceAnchor else { return }

let projectionMatrix = arFrame.camera.projectionMatrix(for: .portrait, viewportSize: self.sceneView.bounds.size, zNear: 0.001, zFar: 1000)
let viewMatrix = arFrame.camera.viewMatrix(for: .portrait)

let projectionViewMatrix = simd_mul(projectionMatrix, viewMatrix)
let modelMatrix = faceAnchor.transform
let mvpMatrix = simd_mul(projectionViewMatrix, modelMatrix)

// This allows me to just get a .x .y .z rotation from the matrix, without having to do crazy calculations
let newFaceMatrix = SCNMatrix4.init(mvpMatrix)
let faceNode = SCNNode()
faceNode.transform = newFaceMatrix
let rotation = vector_float3(faceNode.worldOrientation.x, faceNode.worldOrientation.y, faceNode.worldOrientation.z)

rotation.x .y and .z will return the face's pitch, yaw, roll (respectively)

I'm adding a small multiplier and inverting 2 of the axis, so it ends up like this:

yaw = -rotation.y*3
pitch = -rotation.x*3
roll = rotation.z*1.5

Phew!

How can I get the yaw, pitch, roll of an ARAnchor in absolute terms?

2 Answers