Multi tool use

ARKit Place a SCNNode facing the camera

I'm using ARKit to display 3D objects. I managed to place the nodes in the real world in front of the user (aka the camera). But I don't manage to make them to face the camera when I drop them.

ARKit

let tap_point=CGPoint(x: x, y: y)
let results=arscn_view.hitTest(tap_point, types: .estimatedHorizontalPlane)
guard results.count>0 else{
return
}
guard let r=results.first else{
return
}

let hit_tf=SCNMatrix4(r.worldTransform)
let new_pos=SCNVector3Make(hit_tf.m41, hit_tf.m42+Float(0.2), hit_tf.m43)

guard let scene=SCNScene(named: file_name) else{
return
}
guard let node=scene.rootNode.childNode(withName: "Mesh", recursively: true) else{
return
}
node.position=new_pos
arscn_view.scene.rootNode.addChildNode(node)


The nodes are well positioned on the plane, in front of the camera. But they are all looking in the same direction. I guess I should rotate the SCNNode but I didn't manage to do this.

SCNNode

4 Answers
4

First, get the rotation matrix of the camera:

let rotate = simd_float4x4(SCNMatrix4MakeRotation(sceneView.session.currentFrame!.camera.eulerAngles.y, 0, 1, 0))


Then, combine the matrices:

let rotateTransform = simd_mul(r.worldTransform, rotate)


Lastly, apply a transform to your node, casting as SCNMatrix4:

node.transform = SCNMatrix4(rotateTransform)


Hope that helps

EDIT

here how you can create SCNMatrix4 from simd_float4x4

let rotateTransform = simd_mul(r.worldTransform, rotate)

node.transform = SCNMatrix4(m11: rotateTransform.columns.0.x, m12: rotateTransform.columns.0.y, m13: rotateTransform.columns.0.z, m14: rotateTransform.columns.0.w, m21: rotateTransform.columns.1.x, m22: rotateTransform.columns.1.y, m23: rotateTransform.columns.1.z, m24: rotateTransform.columns.1.w, m31: rotateTransform.columns.2.x, m32: rotateTransform.columns.2.y, m33: rotateTransform.columns.2.z, m34: rotateTransform.columns.2.w, m41: rotateTransform.columns.3.x, m42: rotateTransform.columns.3.y, m43: rotateTransform.columns.3.z, m44: rotateTransform.columns.3.w)


SCNVector3

touchesBegan

textNode

rotateTransform

Do you want the nodes to always face the camera, even as the camera moves? That's what SceneKit constraints are for. Either SCNLookAtConstraint or SCNBillboardConstraint can keep a node always pointing at the camera.

SCNLookAtConstraint

SCNBillboardConstraint

Do you want the node to face the camera when placed, but then hold still (so you can move the camera around and see the back of it)? There are a few ways to do that. Some involve fun math, but a simpler way to handle it might just be to design your 3D assets so that "front" is always in the positive Z-axis direction. Set a placed object's transform based on the camera transform, and its initial orientation will match the camera's.

Here's how I did it:

func faceCamera() {
guard constraints?.isEmpty ?? true else {
return
}
SCNTransaction.begin()
SCNTransaction.animationDuration = 5
SCNTransaction.completionBlock = { [weak self] in
self?.constraints =
}
constraints = [billboardConstraint]
SCNTransaction.commit()
}

private lazy var billboardConstraint: SCNBillboardConstraint = {
let constraint = SCNBillboardConstraint()
constraint.freeAxes = [.Y]
return constraint
}()


As stated earlier a SCNBillboardConstraint will make the node always look at the camera. I am animating it so the node doesn't just immediately snap into place, this is optional. In the SCNTransaction.completionBlock I remove the constraint, also optional.

SCNBillboardConstraint

SCNTransaction.completionBlock

Also I set the SCNBillboardConstraint's freeAxes, which customizes on what axis the node follows the camera, again optional.

SCNBillboardConstraint

freeAxes

here's my code for the SCNNode facing the camera..hope help for someone

let location = touches.first!.location(in: sceneView)

var hitTestOptions = [SCNHitTestOption: Any]()
hitTestOptions[SCNHitTestOption.boundingBoxOnly] = true

let hitResultsFeaturePoints: [ARHitTestResult] = sceneView.hitTest(location, types: .featurePoint)

let hitTestResults = sceneView.hitTest(location)
guard let node = hitTestResults.first?.node else {
if let hit = hitResultsFeaturePoints.first {
let rotate = simd_float4x4(SCNMatrix4MakeRotation(sceneView.session.currentFrame!.camera.eulerAngles.y, 0, 1, 0))
let finalTransform = simd_mul(hit.worldTransform, rotate)
sceneView.session.add(anchor: ARAnchor(transform: finalTransform))
}
return
}


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