GetDepthBufferValue.swift

let viewBoundingBox = viewSpaceBoundingBox(fromNormalizedImageBoundingBox: boundingBox,
                                           usingDisplayTransform: displayTransform)
        
let midPoint = CGPoint(x: viewBoundingBox.midX,
                       y: viewBoundingBox.midY)

// Get the feature point that is closed to our detected rectangle's center
let results = sceneView.hitTest(midPoint, types: .featurePoint)

let depthBufferZ = calculateFragmentDepth(usingCamera: camera,
                                          distanceToTarget: Double(result.distance),
                                          usesReverseZ: sceneView.usesReverseZ)

private func calculateFragmentDepth(usingCamera camera: SCNCamera, 
                                    distanceToTarget: Double, 
                                    usesReverseZ: Bool) -> Float 
{
    // SceneKit uses a reverse z buffer since iOS 13. In case it uses a reverse buffer.
    // We'll need to swap the near and far planes.
    let zFar = usesReverseZ ? camera.zNear : camera.zFar
    let zNear = usesReverseZ ? camera.zFar : camera.zNear
    let range = 2.0 * zNear * zFar
    // The depth value in in normalized device coordinates [-1, 1].
    let fragmentDepth = (zFar + zNear - range / distanceToTarget) / (zFar - zNear)
    // Convert to normalized coordinates [0, 1].
    return Float((fragmentDepth + 1.0) / 2.0)
}