SCNNode releasing children with altered transform and pivot properties - ios

Question

I'm dealing with children nodes with pivot and position that have been altered. I found a lot of SCNNode transformation topics, but it seems none of them represent my situation.
I have six balls : (can't post more than 2 links, image is at i.stack.imgur.com/v3Lc4.png )
And I select the top four of them, adjust the pivot, adjust the position (to counter the pivot translation effect), and rotate. This is the code I use :
//core code
let fourBalls = SCNNode()
for i in 1...4
{
let ball = scene.rootNode.childNode(withName: "b" + String(i), recursively: false)!
ball.removeFromParentNode()
fourBalls.addChildNode(ball)
}
scene.rootNode.addChildNode(fourBalls)
//adjust the pivot of the fourBalls node
fourBalls.pivot = SCNMatrix4MakeTranslation(-1.5, 0.5, -5)
//fix the position
fourBalls.position = SCNVector3Make(-1.5, 0.5, -5)
//rotate
let action = SCNAction.rotateBy(x: 0, y: 0, z: CGFloat(M_PI_2), duration: 2)
fourBalls.run(action)
It did the job well :
Now, I need to release back the fourBalls child nodes into the rootNode, I use this code which I put as completion block :
//core problem
//how to release the node with the transform?
for node in fourBalls.childNodes
{ node.transform = node.worldTransform
node.removeFromParentNode()
self.scene.rootNode.addChildNode(node)
}
And here comes the problem, I released them wrongly :
So my question is, how to release the children nodes to the rootNode with correct pivot, position, and transform properties?
Here is my full GameViewController.swift for you who want to try :
import SceneKit
class GameViewController: UIViewController {
let scene = SCNScene()
override func viewDidLoad() {
super.viewDidLoad()
let ball1 = SCNSphere(radius: 0.4)
let ball2 = SCNSphere(radius: 0.4)
let ball3 = SCNSphere(radius: 0.4)
let ball4 = SCNSphere(radius: 0.4)
let ball5 = SCNSphere(radius: 0.4)
let ball6 = SCNSphere(radius: 0.4)
ball1.firstMaterial?.diffuse.contents = UIColor.purple()
ball2.firstMaterial?.diffuse.contents = UIColor.white()
ball3.firstMaterial?.diffuse.contents = UIColor.cyan()
ball4.firstMaterial?.diffuse.contents = UIColor.green()
ball5.firstMaterial?.diffuse.contents = UIColor.black()
ball6.firstMaterial?.diffuse.contents = UIColor.blue()
let B1 = SCNNode(geometry: ball1)
B1.position = SCNVector3(x:-2,y:1,z:-5)
scene.rootNode.addChildNode(B1)
B1.name = "b1"
let B2 = SCNNode(geometry: ball2)
B2.position = SCNVector3(x:-1,y:1,z:-5)
scene.rootNode.addChildNode(B2)
B2.name = "b2"
let B3 = SCNNode(geometry: ball3)
B3.position = SCNVector3(x:-2,y:0,z:-5)
scene.rootNode.addChildNode(B3)
B3.name = "b3"
let B4 = SCNNode(geometry: ball4)
B4.position = SCNVector3(x:-1,y:0,z:-5)
scene.rootNode.addChildNode(B4)
B4.name = "b4"
let B5 = SCNNode(geometry: ball5)
B5.position = SCNVector3(x:-2,y:-1,z:-5)
scene.rootNode.addChildNode(B5)
B5.name = "b5"
let B6 = SCNNode(geometry: ball6)
B6.position = SCNVector3(x:-1,y:-1,z:-5)
scene.rootNode.addChildNode(B6)
B6.name = "b6"
let cameraNode = SCNNode()
cameraNode.camera = SCNCamera()
cameraNode.position = SCNVector3Make(-1.5,0,2)
scene.rootNode.addChildNode(cameraNode)
// create and add an ambient light to the scene
let ambientLightNode = SCNNode()
ambientLightNode.light = SCNLight()
ambientLightNode.light!.type = SCNLightTypeAmbient
ambientLightNode.light!.color = UIColor.yellow()
scene.rootNode.addChildNode(ambientLightNode)
let scnView = self.view as! SCNView
scnView.scene = scene
scnView.allowsCameraControl = false
scnView.backgroundColor = UIColor.orange()
//core code
let fourBalls = SCNNode()
for i in 1...4
{
let ball = scene.rootNode.childNode(withName: "b" + String(i), recursively: false)!
ball.removeFromParentNode()
fourBalls.addChildNode(ball)
}
scene.rootNode.addChildNode(fourBalls)
//adjust the pivot of the fourBalls node
fourBalls.pivot = SCNMatrix4MakeTranslation(-1.5, 0.5, -5)
//fix the position
fourBalls.position = SCNVector3Make(-1.5, 0.5, -5)
//rotate
let action = SCNAction.rotateBy(x: 0, y: 0, z: CGFloat(M_PI_2), duration: 2)
fourBalls.run(action, completionHandler:
{
//core problem
for node in fourBalls.childNodes
{
node.transform = node.worldTransform
node.removeFromParentNode()
self.scene.rootNode.addChildNode(node)
}
})
}
override func shouldAutorotate() -> Bool {
return true
}
override func prefersStatusBarHidden() -> Bool {
return true
}
override func supportedInterfaceOrientations() -> UIInterfaceOrientationMask {
if UIDevice.current().userInterfaceIdiom == .phone {
return .allButUpsideDown
} else {
return .all
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Release any cached data, images, etc that aren't in use.
}
}

Answer 1

SCNActions update the presentation tree directly, not the model tree, this is probably best explained in the 2014 WWDC video (skip to 16:38). What this means is that throughout the animation the current transform for each of the animating nodes is only available from the presentationNode. Your code works if we get the transform from here instead.
Apologies for the Swift 2 backport...
//rotate
let action = SCNAction.rotateByX(0, y: 0, z: CGFloat(M_PI_2), duration: 2)
fourBalls.runAction(action, completionHandler: {
//core problem
for node in fourBalls.childNodes
{
node.transform = node.presentationNode.worldTransform
node.removeFromParentNode()
self.scene.rootNode.addChildNode(node)
}
})
TBH I was expecting node.worldTransform == node.presentationNode.worldTransform when it got to the completion handler.