How to get SceneKit node placed over detected image to stay put as camera moves? - scenekit

Question

I'm very new to ARKit, and have built a small app based on Apple's "Detecting Images in an AR Experience" sample app. The sample app places a plane over the detected image; my app places an SCNBox at the center of it instead.
If the detected image is displayed on my monitor, I can move the phone all around and the box will stay fixed at the center of the image, which is what I want. If, however, the image is displayed on another iPhone, the box will move around as I move the phone running my app. Possibly due to the smaller screen size?
Here is my implementation of the didAdd delegate method:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
guard let imageAnchor = anchor as? ARImageAnchor else { return }
updateQueue.async {
let cube = self.createCubeNode() // returns an SCNNode with SCNBox geometry
let position = SCNVector3(x: imageAnchor.transform.columns.3.x,
y: imageAnchor.transform.columns.3.y,
z: imageAnchor.transform.columns.3.z)
cube.worldPosition = position
self.sceneView.scene.rootNode.addChildNode(cube)
}
}

Answer 1

I found the answer. Instead of positioning and adding my cube node in didAdd, returning the node from nodeFor and letting the system place it completely fixes the problem. So I have removed the above code and replaced it with:
func renderer(_ renderer: SCNSceneRenderer, nodeFor anchor: ARAnchor) -> SCNNode? {
// make sure this is an image anchor, otherwise bail out
guard let _ = anchor as? ARImageAnchor else { return nil }
return self.createCubeNode() // returns an SCNNode with SCNBox geometry
}
I got this idea from a Hacking With Swift article: https://www.hackingwithswift.com/example-code/arkit/how-to-detect-images-using-arimagetrackingconfiguration