How to calculate distance from the centering point between two eyes to the camera using ARCore + SceneKit?

Question

I'm using ARCore + SceneKit (Swift language) to calculate the distance from the centering point between two eyes to the camera.

I determine the coordinates of the camera:

let cameraPos = sceneView.pointOfView?.position

The coordinates of the left eye and right eye:

let buffer = face.mesh.vertices
let left   = buffer[LF]
let right  = buffer[RT]

NOTE:

LF and RT is defined base on: https://github.com/ManuelTS/augmentedFaceMeshIndices

• LF = 159 is the index that contain the Vector3 condinate of the Left eye

• RT = 386 is the index that contain the Vector3 condinate of the Right eye

Compute the centering point (in SCNVector3):

let center = SCNVector3(x: (left.x - right.x) * 0.5,
                        y: (left.y - right.y) * 0.5,
                        z: (left.z - right.z) * 0.5)

Finally, I calculate the distance:

let distance = distance(start: cameraPos!, end: center)

distance is defined as:

func distance(start: SCNVector3, end: SCNVector3) -> Float {
  let dx = start.x - end.x
  let dy = start.y - end.y
  let dz = start.z - end.z

  let distance = sqrt(dx * dx + dy * dy + dz * dz)

  return round(distance * 100 * 10) / 10.0
}

Runtime result is incorrect.

• Actual distance: ~20 cm
• In-app distance: ~3 cm

Can someone tell me where the problem lies, even another solution? Thanks.

Answer 1

Assuming center is the midpoint between the eyes, then shouldn't the formula be:

Midpoint: (x1, y1, z1) and (x2, y2, z2) is (x1+x2 )/2,(y1+y2 )/2,(z1+z2 )/2.

Edit: Taking a guess here, but...

Example: So that a projectile will actually launch from a turret with a long barrel cannon exactly where the barrel is rotated to at the time of firing, you have to calculate that position at the end of the tube as it relates to the position of the node that the barrel is attached to, otherwise the shot will not look like it came from the right spot.

Requires a little imagination, but this is your face moving around = turret is moving around. I "think" that's what's happening to your math. I don't think you are getting the right LF/RF positions because you didn't mention converting the point. The link you sent [The face mesh consists of hundreds of vertices that make up the face, and is defined relative to the center pose.] Relative to the center pose - I'm pretty sure that means you have to convert LF with relation to the center to get the real position.

// Convert position something like this:

    let REAL_LF = gNodes.gameNodes.convertPosition(LF.presentation.position, from: POSE_POSITION)

convertPosition(_:to:) Converts a position from the node’s local coordinate space to that of another node