Coordinates of ARImageAnchor transform matrix are way too different from the ARPlaneAnchor ones

Question

I am doing this simple thing:

Vertical plane detection
Image recognition on a vertical plane

The image is hanged on the detected plane (on my wall). In both case I implement the renderer:didAddNode:forAnchor: function from ARSCNViewDelegate. I stand at the place for the vertical plane detection and the image recognition.

func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
    guard let shipScene = SCNScene(named: "ship.scn"), let shipNode = shipScene.rootNode.childNode(withName: "ship", recursively: false) else { return }
    shipNode.position = SCNVector3(anchor.transform.columns.3.x, anchor.transform.columns.3.y, anchor.transform.columns.3.z)
    sceneView.scene.rootNode.addChildNode(shipNode)
    print(anchor.transform)
}

In the case of a vertical plane detection the anchor will be an ARPlaneAnchor. In the case of an image recognition the anchor will be an ARImageAnchor.

Why are the transform matrices of those two anchors so different? I'm printing the anchor.transform and I get those results:

1.

simd_float4x4([
    [0.941312,  0.0,        -0.337538,  0.0)],
    [0.336284,  -0.0861278, 0.937814,   0.0)],
    [-0.0290714,-0.996284,  -0.0810731, 0.0)],
    [0.191099,  0.172432,   -1.14543,   1.0)]
])

2.

simd_float4x4([
    [0.361231,  0.10894,    0.926093,   0.0)],
    [-0.919883, -0.121052,  0.373049,   0.0)],
    [0.152743,  -0.986651,  0.0564843,  0.0)],
    [75.4418,   10.9618,    -14.3788,   1.0)]
])

So if I want to place a 3D object on the detected vertical plane I can simply use [x = 0.191099, y = 0.172432, z = -1.14543] as coordinates to set the position of my node (myNode), and then add this node to the scene with sceneView.scene.rootNode.addChildNode(myNode) but if I want to place a 3D object at the detected image's anchor, I cannot use [x = 75.4418, y = 10.9618, z = -14.3788].

What should I do to place a 3D object on the detected image's anchor? I really don't understand the transform matrix of the ARImageAnchor.

BlackMirrorz BlackMirrorz · Accepted Answer · 2018-03-13T23:36:23

Here is an example for you in which I use the func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) method:

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 }

    let x = currentImageAnchor.transform
    print(x.columns.3.x, x.columns.3.y , x.columns.3.z)

    //2. Get The Targets Name
    let name = currentImageAnchor.referenceImage.name!

    //3. Get The Targets Width & Height In Meters
    let width = currentImageAnchor.referenceImage.physicalSize.width
    let height = currentImageAnchor.referenceImage.physicalSize.height

    print("""
    Image Name = \(name)
    Image Width = \(width)
    Image Height = \(height)
    """)

    //4. Create A Plane Geometry To Cover The ARImageAnchor
    let planeNode = SCNNode()
    let planeGeometry = SCNPlane(width: width, height: height)
    planeGeometry.firstMaterial?.diffuse.contents = UIColor.white
    planeNode.opacity = 0.25
    planeNode.geometry = planeGeometry

    //5. Rotate The PlaneNode To Horizontal
    planeNode.eulerAngles.x = -.pi/2

    //The Node Is Centered In The Anchor (0,0,0)
    node.addChildNode(planeNode)


    //6. Create AN SCNBox
    let boxNode = SCNNode()
    let boxGeometry = SCNBox(width: 0.1, height: 0.1, length: 0.1, chamferRadius: 0)

    //7. Create A Different Colour For Each Face
    let faceColours = [UIColor.red, UIColor.green, UIColor.blue, UIColor.cyan, UIColor.yellow, UIColor.gray]
    var faceMaterials = [SCNMaterial]()

    //8. Apply It To Each Face
    for face in 0 ..< 5{
        let material = SCNMaterial()
        material.diffuse.contents = faceColours[face]
        faceMaterials.append(material)
    }
    boxGeometry.materials = faceMaterials
    boxNode.geometry = boxGeometry

    //9. Set The Boxes Position To Be Placed On The Plane (node.x + box.height)
    boxNode.position = SCNVector3(0 , 0.05, 0)

    //10. Add The Box To The Node
    node.addChildNode(boxNode)


}

From my understanding (I could of course by wrong) you would know that your placement area is the width and height of the referenceImage.physicalSize which is expressed in Metres:

let width = currentImageAnchor.referenceImage.physicalSize.width
let height = currentImageAnchor.referenceImage.physicalSize.height

As such you would need to scale your content (if needed to fit) within these boundaries assuming you wanted it to appear to overlay the image.

Coordinates of ARImageAnchor transform matrix are way too different from the ARPlaneAnchor ones

1 Answers