Using SceneKit for hitTesting not returning a hit with SCNNode - swift

Question

The documentation in XCode clearly states that hitTesting a geometry in SceneKit can be done with SCNRender, SCNView or the SCNNode themselves when one plans to test a 3D line segment. I have a use for SCNScene with its nodes without a renderer or a view, therefore I am planning to use SCNNode hitTesting. I create a SCNScene, put a SCNNode in it and test a simple ray that goes through, but I always get an empty hitList and I don't understand why:
import Swift
import SceneKit
let boxGeometry = SCNBox(width: 1.0, height: 1.0, length: 1.0, chamferRadius: 0)
let boxNode = SCNNode(geometry: boxGeometry)
var scene = SCNScene()
scene.rootNode.addChildNode(boxNode)
let from = SCNVector3(x: 0, y: -2, z: 0)
let to = SCNVector3(x: 0, y: 2 , z: 0)
var hits = scene.rootNode.hitTestWithSegmentFromPoint(from, toPoint: to, options:nil) // this is always empty
if hits != nil {
if hits!.count > 0 {
var hit = (hits!.first as! SCNHitTestResult).node as SCNNode
}
}
I have tried passing various forms of options but nothing changes.
SCNHitTestFirstFoundOnlyKey: yes or no does not change anything
SCNHitTestSortResultsKey: yes or no does not change anything
SCNHitTestClipToZRangeKey: invalid for SCNNode
SCNHitTestBackFaceCullingKey: yes or no does not change anything
SCNHitTestBoundingBoxOnlyKey: yes or no does not change anything
SCNHitTestRootNodeKey: rootNOde of scene or boxNode does not change
anything
SCNHitTestIgnoreHiddenNodesKey: yes or no does not change anything
What am I doing wrong?

Answer 1

I have found the answer, which is either a bug or a feature: using SCNScene and its nodes SCNNode for 3D hitTesting, in particular the method: "hitTestWithSegmentFromPoint(toPoint:options:)" does not return a hit unless the scene is included in an SCNView. It appears it cannot be used offscreen. My guess is yours for why this is the case, although I can imagine it has something to do with performing some of these quite expensive calculations on the graphics card.
I have tested this using the GameView SCNScene starter project. The critical line is self.gameView!.scene = scene
override func awakeFromNib(){
let scene = SCNScene()
let boxGeometry = SCNBox(width: 1.0, height: 1.0, length: 1.0, chamferRadius: 0.0)
let boxNode = SCNNode(geometry: boxGeometry)
boxNode.position=SCNVector3(x: 0, y: 0, z: 0)
scene.rootNode.addChildNode(boxNode)
// create and add a camera to the scene
let cameraNode = SCNNode()
cameraNode.camera = SCNCamera()
scene.rootNode.addChildNode(cameraNode)
// place the camera
cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)
// create and add a light to the scene
let lightNode = SCNNode()
lightNode.light = SCNLight()
lightNode.light!.type = SCNLightTypeOmni
lightNode.position = SCNVector3(x: 0, y: 10, z: 10)
scene.rootNode.addChildNode(lightNode)
// create and add an ambient light to the scene
let ambientLightNode = SCNNode()
ambientLightNode.light = SCNLight()
ambientLightNode.light!.type = SCNLightTypeAmbient
ambientLightNode.light!.color = NSColor.darkGrayColor()
scene.rootNode.addChildNode(ambientLightNode)
// set the scene to the view
// uncomment this to fail
self.gameView!.scene = scene
// allows the user to manipulate the camera
self.gameView!.allowsCameraControl = true
// show statistics such as fps and timing information
self.gameView!.showsStatistics = true
// configure the view
self.gameView!.backgroundColor = NSColor.blackColor()
let hitList = scene.rootNode.hitTestWithSegmentFromPoint(SCNVector3(x:-10,y:0,z:0), toPoint: SCNVector3(x:10,y:0,z:0), options:[SCNHitTestBackFaceCullingKey:false, SCNHitTestSortResultsKey:true, SCNHitTestIgnoreHiddenNodesKey:false])
if hitList?.count > 0 {
println("Hit found: \n\n\( hitList![0] )") // assign self.gameView!.scene = scene to reach this point.
} else {
println("No hit") // uncomment self.gameView!.scene = scene to reach this point.
}
}

Answer 2

I've also had trouble with hitTestWithSegmentFromPoint.
I was calling it in viewDidLoad() and it returned a 0 elements array, though I was sure there was a hit.
Calling it in viewDidAppear() (or later) solved my problem.