How to place an AR object on plane WITHOUT ARHitTest? - swift

Question

I'm working on an AR project and attempting to add an object to a horizontal plane. My intention is for the user to search around for the object in their environment and once a horizontal plane is detected for the object to appear on the center point of the plane. Here's the code:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
if anchor is ARPlaneAnchor {
guard let planeAnchor = anchor as? ARPlaneAnchor else { return }
let plane = SCNPlane(width: CGFloat(planeAnchor.extent.x), height: CGFloat(planeAnchor.extent.z))
let planeNode = SCNNode()
planeNode.position = SCNVector3(x: planeAnchor.center.x, y: 0, z: planeAnchor.center.z)
planeNode.geometry = plane
print("Plane Node: \(planeNode.position)")
planeNode.transform = SCNMatrix4MakeRotation(-Float.pi / 2, 1, 0, 0)
node.addChildNode(planeNode)
let chestScene = SCNScene(named: "art.scnassets/empty_treasure_chest copy.scn")
guard let chestNode = chestScene?.rootNode.childNode(withName: "chest", recursively: true) else { return }
sceneView.scene.rootNode.addChildNode(chestNode)
chestNode.position = planeNode.position
print("Chest Node: \(chestNode.position)")
sceneView.scene.rootNode.enumerateChildNodes { (node, _) in
if node.name == "chest_lid_top_wood" {
print("found top")
top = node
top.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(node: top, options: nil))
} else if node.name == "chest_bottom_wood" {
print("found bottom")
bottom = node
bottom.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(node: bottom, options: nil))
}
}
} else {
return
}
}
The app will detect the plane properly however the object won't be placed directly on the plane, most often it is elevated off of the plane at the world origin position. Even more confusing, I have print statements showing the position of the planeNode and chestNode, these values end up being different despite me assigning the position of the chestNode to the planeNode position. I understand most people use ARHitTest to place AR objects but doing that would hinder the UX I'm trying to achieve.
Any suggestions? Thanks in advance!
Cheers!

Answer 1

From my experience ARKit calls the delegate methods "to early", before the worldPosition of the node has been calculated. It is still [0, 0, 0], which is why your objects appear at the world origin.
There are two workarounds:
You could add your chestNode as a child of node instead of the scnene's rootNode.
However, this would result in the node moving every time ARKit updates the plane, because that would update the position of the node. This is usually what you want.
The alternative would be to wait for at least 1 frame before updating the position of your chestNode by wrapping the actual placement of your node inside something like
DispatchQueue.main.asyncAfter(deadline: .now() + 0.05) { [weak sceneView] in
sceneView?.scene.rootNode.addChildNode(chestNode)
chestNode.position = planeNode.position
}