ARKit - ARRreferenceImage tracking - ios11

Question

I'm playing around with ARReferenceImages in ARKit and I'm trying to add an SCNNode when a reference image is recognised and then leave that node in place regardless of whether the same reference image is then recognised elsewhere.
I can add my SCNode correctly, but if I move my marker it picks it up again and moves my placed node to the position of the marker.
My code to add is as follows:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
guard let imageAnchor = anchor as? ARImageAnchor else { return }
let referenceImage = imageAnchor.referenceImage
print("MAPNODE IS NIL = \(self.mapNode == nil)")
updateQueue.async {
if self.mapNode == nil {
// Create a plane to visualize the initial position of the detected image.
let plane = SCNPlane(width: 1.2912,
height: 1.2912)
let planeNode = SCNNode(geometry: plane)
planeNode.opacity = 1
/*
`SCNPlane` is vertically oriented in its local coordinate space, but
`ARImageAnchor` assumes the image is horizontal in its local space, so
rotate the plane to match.
*/
planeNode.eulerAngles.x = -.pi / 2
self.mapNode = planeNode
/*
Image anchors are not tracked after initial detection, so create an
animation that limits the duration for which the plane visualization appears.
*/
// Add the plane visualization to the scene.
node.addChildNode(planeNode)
}
}
}
reading the docs here https://developer.apple.com/documentation/arkit/recognizing_images_in_an_ar_experience#2958517 it states that
Apply Best Practices
This example app simply visualizes where ARKit detects each reference
image in the user’s environment, but your app can do much more. Follow
the tips below to design AR experiences that use image detection well.
Use detected images to set a frame of reference for the AR scene.
Instead of requiring the user to choose a place for virtual content,
or arbitrarily placing content in the user’s environment, use detected
images to anchor the virtual scene. You can even use multiple detected
images. For example, an app for a retail store could make a virtual
character appear to emerge from a store’s front door by recognizing
posters placed on either side of the door and then calculating a
position for the character directly between the posters.
Note
Use the ARSession setWorldOrigin(relativeTransform:) method to
redefine the world coordinate system so that you can place all anchors
and other content relative to the reference point you choose.
Design your AR experience to use detected images as a starting point
for virtual content. ARKit doesn’t track changes to the position or
orientation of each detected image. If you try to place virtual
content that stays attached to a detected image, that content may not
appear to stay in place correctly. Instead, use detected images as a
frame of reference for starting a dynamic scene. For example, your app
might recognize theater posters for a sci-fi film and then have
virtual spaceships appear to emerge from the posters and fly around
the environment.
So I tried setting my world transform to be equal to the transform of my image anchor
self.session.setWorldOrigin(relativeTransform: imageAnchor.transform)
However my mapNode follows the imageAnchor where ever it moves. I haven't implemented the renderer update method so I'm not sure why this keeps moving.
I'm assuming that the setWorldOrigin method is constantly updating to the imageAnchor.transform and not just that moment in time, which is weird as that code is only called once. Any ideas?

Answer 1

If you want to add the mapNode at the position of the ARImageAnchor you could set the position of your mapNode at the transform of the ARImageAnchor and add it to the scene but not linked to the reference image if that makes sense.
This could be done like so:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
//1. If Out Target Image Has Been Detected Than Get The Corresponding Anchor
guard let currentImageAnchor = anchor as? ARImageAnchor else { return }
//2. An ImageAnchor Is Only Added Once For Each Identified Target
print("Anchor ID = \(currentImageAnchor.identifier)")
//3. Add An SCNNode At The Position Of The Identified ImageTarget
let nodeHolder = SCNNode()
let nodeGeometry = SCNBox(width: 0.02, height: 0.02, length: 0.02, chamferRadius: 0)
nodeGeometry.firstMaterial?.diffuse.contents = UIColor.cyan
nodeHolder.geometry = nodeGeometry
nodeHolder.position = SCNVector3(currentImageAnchor.transform.columns.3.x,
currentImageAnchor.transform.columns.3.y,
currentImageAnchor.transform.columns.3.z)
augmentedRealityView?.scene.rootNode.addChildNode(nodeHolder)
}
In another part of your question you seem to imply that you want to detect multiple occurrences of the same image. I could be wrong but I think the only way to do this is to remove the corresponding ARImageAnchor for the reference image, which can be done like so (by adding it at the end of the last code snippet):
augmentedRealitySession.remove(anchor: currentImageAnchor)
The issue here is that once the ARImageAnchor is removed, any time it is detected again, you would have to handle whether content should be added, which is tricky since the ARImageAnchor.identifier is always the same for the referenceImage regardless of whether it is removed and then re-added thus making it difficult to store in a dictionary etc. As such depending on your needs you would then need to find a way to determine if content existed at that location and whether to re add it etc.
The last part of your question about setWorldOrigin seems a bit odd like you said, but maybe you could add a Bool to prevent it from potentially changing e.g:
var hasSetWorldOrigin = false
Then based on this you could ensure that it is only set once e.g:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
//1. If Out Target Image Has Been Detected Than Get The Corresponding Anchor
guard let currentImageAnchor = anchor as? ARImageAnchor else { return }
//2. If We Havent Set The World Origin Set It Based On The ImageAnchorTranform
if !hasSetWorldOrigin{
self.augmentedRealitySession.setWorldOrigin(relativeTransform: currentImageAnchor.transform)
hasSetWorldOrigin = true
//3. Create Two Nodes To Add To The Scene And Distribute Them
let nodeHolderA = SCNNode()
let nodeGeometryA = SCNBox(width: 0.04, height: 0.04, length: 0.04, chamferRadius: 0)
nodeGeometryA.firstMaterial?.diffuse.contents = UIColor.green
nodeHolderA.geometry = nodeGeometryA
let nodeHolderB = SCNNode()
let nodeGeometryB = SCNBox(width: 0.04, height: 0.04, length: 0.04, chamferRadius: 0)
nodeGeometryB.firstMaterial?.diffuse.contents = UIColor.red
nodeHolderB.geometry = nodeGeometryB
if let cameraTransform = augmentedRealitySession.currentFrame?.camera.transform{
nodeHolderA.simdPosition = float3(cameraTransform.columns.3.x,
cameraTransform.columns.3.y,
cameraTransform.columns.3.z)
nodeHolderB.simdPosition = float3(cameraTransform.columns.3.x + 0.2,
cameraTransform.columns.3.y,
cameraTransform.columns.3.z)
}
augmentedRealityView?.scene.rootNode.addChildNode(nodeHolderA)
augmentedRealityView?.scene.rootNode.addChildNode(nodeHolderB)
}
}
Hopefully my answer will provide a useful starting point to you assuming of course I have interpreted your question correctly,