ARKit - getting distance from camera to anchor - ios

Question

I'm creating an anchor and adding it to my ARSKView at a certain distance in front of the camera like this:
func displayToken(distance: Float) {
print("token dropped at: \(distance)")
guard let sceneView = self.view as? ARSKView else {
return
}
// Create anchor using the camera's current position
if let currentFrame = sceneView.session.currentFrame {
// Create a transform with a translation of x meters in front of the camera
var translation = matrix_identity_float4x4
translation.columns.3.z = -distance
let transform = simd_mul(currentFrame.camera.transform, translation)
// Add a new anchor to the session
let anchor = ARAnchor(transform: transform)
sceneView.session.add(anchor: anchor)
}
}
then the node gets created for the anchor like this:
func view(_ view: ARSKView, nodeFor anchor: ARAnchor) -> SKNode? {
// Create and configure a node for the anchor added to the view's session.
if let image = tokenImage {
let texture = SKTexture(image: image)
let tokenImageNode = SKSpriteNode(texture: texture)
tokenImageNode.name = "token"
return tokenImageNode
} else {
return nil
}
}
This works fine and I see the image get added at the appropriate distance. However, what I'm trying to do is then calculate how far the anchor/node is in front of the camera as you move. The problem is the calculation seems to be off immediately using fabs(cameraZ - anchor.transform.columns.3.z). Please see my code below that is in the update() method to calculate distance between camera and object:
override func update(_ currentTime: TimeInterval) {
// Called before each frame is rendered
guard let sceneView = self.view as? ARSKView else {
return
}
if let currentFrame = sceneView.session.currentFrame {
let cameraZ = currentFrame.camera.transform.columns.3.z
for anchor in currentFrame.anchors {
if let spriteNode = sceneView.node(for: anchor), spriteNode.name == "token", intersects(spriteNode) {
// token is within the camera view
//print("token is within camera view from update method")
print("DISTANCE BETWEEN CAMERA AND TOKEN: \(fabs(cameraZ - anchor.transform.columns.3.z))")
print(cameraZ)
print(anchor.transform.columns.3.z)
}
}
}
}
Any help is appreciated in order to accurately get distance between camera and the anchor.

Answer 1

The last column of a 4x4 transform matrix is the translation vector (or position relative to a parent coordinate space), so you can get the distance in three dimensions between two transforms by simply subtracting those vectors.
let anchorPosition = anchor.transforms.columns.3
let cameraPosition = camera.transform.columns.3
// here’s a line connecting the two points, which might be useful for other things
let cameraToAnchor = cameraPosition - anchorPosition
// and here’s just the scalar distance
let distance = length(cameraToAnchor)
What you’re doing isn’t working right because you’re subtracting the z-coordinates of each vector. If the two points are different in x, y, and z, just subtracting z doesn’t get you distance.

Answer 2

This one is for scenekit, I'll leave it here though.
let end = node.presentation.worldPosition
let start = sceneView.pointOfView?.worldPosition
let dx = (end?.x)! - (start?.x)!
let dy = (end?.y)! - (start?.y)!
let dz = (end?.z)! - (start?.z)!
let distance = sqrt(pow(dx,2)+pow(dy,2)+pow(dz,2))

Answer 3

With RealityKit there is a slightly different way to do this. If you're using the world tracking configuration, your AnchorEntity object conforms to HasAnchoring which gives you a target. Target is an enum of AnchoringComponent.Target. It has a case .world(let transform). You can compare your world transform to the camera's world transform like this:
if case let AnchoringComponent.Target.world(transform) = yourAnchorEntity.anchoring.target {
let theDistance = distance(transform.columns.3, frame.camera.transform.columns.3)
}
This took me a bit to figure out but I figure others that might be using RealityKit might benefit from this.

Answer 4

As mentioned above by #codeman, this is the right solution:
let distance = simd_distance(YOUR_NODE.simdTransform.columns.3, (sceneView.session.currentFrame?.camera.transform.columns.3)!);

Answer 5

3D distance - You can check these utils,
class ARSceneUtils {
/// return the distance between anchor and camera.
class func distanceBetween(anchor : ARAnchor,AndCamera camera: ARCamera) -> CGFloat {
let anchorPostion = SCNVector3Make(
anchor.transform.columns.3.x,
anchor.transform.columns.3.y,
anchor.transform.columns.3.z
)
let cametaPosition = SCNVector3Make(
camera.transform.columns.3.x,
camera.transform.columns.3.y,
camera.transform.columns.3.z
)
return CGFloat(self.calculateDistance(from: cametaPosition , to: anchorPostion))
}
/// return the distance between 2 vectors.
class func calculateDistance(from: SCNVector3, to: SCNVector3) -> Float {
let x = from.x - to.x
let y = from.y - to.y
let z = from.z - to.z
return sqrtf( (x * x) + (y * y) + (z * z))
}
}
And now you can call:
guard let camera = session.currentFrame?.camera else { return }
let anchor = // you anchor
let distanceAchorAndCamera = ARSceneUtils.distanceBetween(anchor: anchor, AndCamera: camera)