如何获取SCNCamera正对的网格(SCNNode)点的localCoordinates与localNormal

问题 #

我创建了一个SCNScene，其中包含一个网格节点（SCNNode）和一个挂载SCNCamera的节点，代码如下：

let sceneView = SCNView()
let scene = SCNScene()
let meshNode: SCNNode()
lazy var cameraNode: SCNNode = {
    let node = SCNNode()
    let camera = SCNCamera()
    node.camera = camera
    return node
}()

用户可旋转相机查看场景，我需要随时获取相机当前正对的网格点的坐标，以及该点的法向量。

简单来说，就是模拟相机向前发射一条射线，获取射线与网格交点的局部坐标（localCoordinates）和局部法向量（localNormal），类似SCNHitTestResult提供的信息：

let hitResults = sceneView.hitTest(touchPoint, options: [:])
if let result: SCNHitTestResult = hitResults.first {
    let localCoordinates = result.localCoordinates
    let localNormal = result.localNormal
}

请问如何获取相机正对的网格点的localCoordinates和localNormal？

解决方法 #

通过节点的线段检测方法可实现该需求，具体代码如下：

func sceneSpacePosition(inFrontOf node: SCNNode, atDistance distance: Float) -> SCNVector3 {
    let localPosition = SCNVector3(x: 0, y: 0, z: Float(CGFloat(-distance)))
    let scenePosition = node.convertPosition(localPosition, to: nil)
    return scenePosition
}

let to: SCNVector3 = sceneSpacePosition(inFrontOf: cameraNode, atDistance: Float(radius))

if let hitTest = scene.rootNode.hitTestWithSegment(from: cameraNode.position, to: to, options: .none).first {
    let localCoordinates = hitTest.localCoordinates
    let localNormal = hitTest.localNormal
}

内容的提问来源于Stack Exchange，提问作者Ihor Kram