ARKit – Is there a way to know where the object is located in space related to ARCamera?

Question

I have an object in ARKit's screen and I want to know the horizontal angle between it and the camera direction, or at least is the object on the left, center or right of the screen?

Answer 1

Definitely (well kind of)

First, we need to understand ARKit.

1. ARKit objects are not in actual space and are based off of what the camera sees.

2. The camera is not perfect; You will notice "stationary" objects moving around either due to lighting, surroundings, etc.

3. No one's hand is steady; not only will the object be moving, but the phone will be doing small adjustments as well.

Now, provided this information, we can use the variables we know to calculate the angle between 3 known positions (in 3D Space of course!).

Using this answer, we can see that through lots of math we can get to an angle from 3 3D points

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Now, getting into the code of it.

Let's get our variables

1. You can get your phone position using a function provided by SCNSceneRendererDelegate

func renderer(_ renderer: SCNSceneRenderer, willRenderScene scene: SCNScene, atTime time: TimeInterval) {
    guard let pointOfView = sceneView.pointOfView else { return }
    let transform = pointOfView.transform
    let orientation = SCNVector3(-transform.m31, -transform.m32, transform.m33)
    let location = SCNVector3(transform.m41, transform.m42, transform.m43)
    let currentPositionOfCamera = orientation + location
    print(currentPositionOfCamera)
}

func +(lhv:SCNVector3, rhv:SCNVector3) -> SCNVector3 {
     return SCNVector3(lhv.x + rhv.x, lhv.y + rhv.y, lhv.z + rhv.z)
}

2. You can get your object by doing

let anObject = SCNNode()
let pos = anObject.position

3. You can get your userViewDirection in a slightly creative way -- the best way I can come up with is to create a new SCNNode and place it some distance in front of you. We can do this by using this link

func updatePositionAndOrientationOf(_ node: SCNNode, withPosition position: SCNVector3, relativeTo referenceNode: SCNNode) {
    let referenceNodeTransform = matrix_float4x4(referenceNode.transform)

    // Setup a translation matrix with the desired position
    var translationMatrix = matrix_identity_float4x4
    translationMatrix.columns.3.x = position.x
    translationMatrix.columns.3.y = position.y
    translationMatrix.columns.3.z = position.z

    // Combine the configured translation matrix with the referenceNode's transform to get the desired position AND orientation
    let updatedTransform = matrix_multiply(referenceNodeTransform, translationMatrix)
    node.transform = SCNMatrix4(updatedTransform)
}

//Create a global node 
let lookingNode:SCNNode = SCNNode()

//Now update node say `2` away in the Z (looking direction)
let position = SCNVector3(x: 0, y: 0, z: -2)
updatePositionAndOrientationOf(lookingNode, withPosition: position, relativeTo: cameraNode)

Now we have our 3 variables.

We can san simply do math on them :)

//Vertex is pos1
func calculateAngleBetween3Positions(pos1:SCNVector3, pos2:SCNVector3, pos3:SCNVector3) -> Float {
    let v1 = SCNVector3(x: pos2.x-pos1.x, y: pos2.y-pos1.y, z: pos2.z-pos1.z)
    let v2 = SCNVector3(x: pos3.x-pos1.x, y: pos3.y-pos1.y, z: pos3.z-pos1.z)

    let v1Magnitude = sqrt(v1.x * v1.x + v1.y * v1.y + v1.z * v1.z)
    let v1Normal = SCNVector3(x: v1.x/v1Magnitude, y: v1.y/v1Magnitude, v1.z/v1Magnitude)
  
    let v2Magnitude = sqrt(v2.x * v2.x + v2.y * v2.y + v2.z * v2.z)
    let v2Normal = SCNVector3(x: v2.x/v2Magnitude, y: v2.y/v2Magnitude, v2.z/v2Magnitude)

    let result = v1Normal.x * v2Normal.x + v1Normal.y * v2Normal.y + v1Normal.z * v2Normal.z
    let angle = acos(result)

    return angle
}