我正在iOS上使用SceneKit开发一些代码，在我的代码中，我想确定全局z平面上的x和y坐标，其中z为0.0，而x和y由轻击手势确定。我的设置如下：

    override func viewDidLoad() {
    super.viewDidLoad()

    // create a new scene
    let scene = SCNScene()

    // create and add a camera to the scene
    let cameraNode = SCNNode()
    let camera = SCNCamera()
    cameraNode.camera = camera
    scene.rootNode.addChildNode(cameraNode)
    // place the camera
    cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)

    // create and add an ambient light to the scene
    let ambientLightNode = SCNNode()
    ambientLightNode.light = SCNLight()
    ambientLightNode.light.type = SCNLightTypeAmbient
    ambientLightNode.light.color = UIColor.darkGrayColor()
    scene.rootNode.addChildNode(ambientLightNode)

    let triangleNode = SCNNode()
    triangleNode.geometry = defineTriangle();
    scene.rootNode.addChildNode(triangleNode)

    // retrieve the SCNView
    let scnView = self.view as SCNView

    // set the scene to the view
    scnView.scene = scene

    // configure the view
    scnView.backgroundColor = UIColor.blackColor()
    // add a tap gesture recognizer
    let tapGesture = UITapGestureRecognizer(target: self, action: "handleTap:")
    let gestureRecognizers = NSMutableArray()
    gestureRecognizers.addObject(tapGesture)
    scnView.gestureRecognizers = gestureRecognizers
}

func handleTap(gestureRecognize: UIGestureRecognizer) {
    // retrieve the SCNView
    let scnView = self.view as SCNView
    // check what nodes are tapped
    let p = gestureRecognize.locationInView(scnView)
    // get the camera
    var camera = scnView.pointOfView.camera

    // screenZ is percentage between z near and far
    var screenZ = Float((15.0 - camera.zNear) / (camera.zFar - camera.zNear))
    var scenePoint = scnView.unprojectPoint(SCNVector3Make(Float(p.x), Float(p.y), screenZ))
    println("tapPoint: (\(p.x), \(p.y)) scenePoint: (\(scenePoint.x), \(scenePoint.y), \(scenePoint.z))")
}

func defineTriangle() -> SCNGeometry {

    // Vertices
    var vertices:[SCNVector3] = [
        SCNVector3Make(-2.0, -2.0, 0.0),
        SCNVector3Make(2.0, -2.0, 0.0),
        SCNVector3Make(0.0, 2.0, 0.0)
    ]

    let vertexData = NSData(bytes: vertices, length: vertices.count * sizeof(SCNVector3))
    var vertexSource = SCNGeometrySource(data: vertexData,
        semantic: SCNGeometrySourceSemanticVertex,
        vectorCount: vertices.count,
        floatComponents: true,
        componentsPerVector: 3,
        bytesPerComponent: sizeof(Float),
        dataOffset: 0,
        dataStride: sizeof(SCNVector3))

    // Normals
    var normals:[SCNVector3] = [
        SCNVector3Make(0.0, 0.0, 1.0),
        SCNVector3Make(0.0, 0.0, 1.0),
        SCNVector3Make(0.0, 0.0, 1.0)
    ]

    let normalData = NSData(bytes: normals, length: normals.count * sizeof(SCNVector3))
    var normalSource = SCNGeometrySource(data: normalData,
        semantic: SCNGeometrySourceSemanticNormal,
        vectorCount: normals.count,
        floatComponents: true,
        componentsPerVector: 3,
        bytesPerComponent: sizeof(Float),
        dataOffset: 0,
        dataStride: sizeof(SCNVector3))

    // Indexes
    var indices:[CInt] = [0, 1, 2]
    var indexData  = NSData(bytes: indices, length: sizeof(CInt) * indices.count)
    var indexElement = SCNGeometryElement(
        data: indexData,
        primitiveType: .Triangles,
        primitiveCount: 1,
        bytesPerIndex: sizeof(CInt)
    )

    var geo = SCNGeometry(sources: [vertexSource, normalSource], elements: [indexElement])

    // material
    var material = SCNMaterial()
    material.diffuse.contents  = UIColor.redColor()
    material.doubleSided = true
    material.shininess = 1.0;
    geo.materials = [material];

    return geo
}

如你看到的。我有一个三角形，该三角形的高度为4个单位，宽为4个单位，并设置在以x，y（0.0，0.0）为中心的z平面（z =
0）上。相机是默认的SCNCamera，它沿负z方向看，我将其放置在（0，0，15）处。zNear和zFar的默认值分别为1.0和100.0。在我的handleTap方法中，我获取水龙头的x和y屏幕坐标，并尝试在z
= 0.0的情况下查找x和y全局场景坐标。我正在使用unprojectPoint的电话。

unprojectPoint的文档指示

取消投影其z坐标为0.0的点将返回近裁剪平面上的一个点；取消投影z坐标为1.0的点将返回远裁剪平面上的点。

尽管并没有特别说明，对于两者之间的点，近平面和远平面之间存在线性关系，但我做出了这一假设，并将screenZ的值计算为z
=的近平面和远平面之间的距离百分比。 0平面位于。要查看答案，我可以单击三角形的角附近，因为我知道它们在全局坐标中的位置。

我的问题是，当我开始更改相机上的zNear和zFar裁剪平面时，我没有获得正确的值，也没有获得一致的值。所以我的问题是，我该怎么做？最后，我将创建一个新的几何并将其放置在与用户单击位置相对应的z平面上。

在此先感谢您的帮助。