ios - 场景中的 ARKit 3D 头部跟踪

Question

我正在使用 ARKit 创建增强相机应用。当 ARSession 初始化时，一个 3d 字符显示在 ARSCNView 中。我正在尝试获取角色的跟踪 ARCamera 的视角，以便在用户移动拍照时它们始终注视着相机。

我使用了 Apple 的变色龙演示，它添加了一个使用 SCNLookAtConstraint 跟踪相机视角的焦点节点，但我越来越奇怪的行为。当 ARCamera 平移时，头部会掉到一边并旋转。如果我添加一个 SCNTransformConstraint 来限制头部向上/向下/左右移动，它保持垂直但随后看向别处并且不跟踪。

我尝试将变色龙演示拆开，看看为什么我的演示不起作用，但几天后我就卡住了。

我使用的代码是:

class Daisy: SCNScene, ARCharacter, CAAnimationDelegate {

    // Rig for animation
    private var contentRootNode: SCNNode! = SCNNode()
    private var geometryRoot: SCNNode!
    private var head: SCNNode!
    private var leftEye: SCNNode!
    private var rightEye: SCNNode!

    // Head tracking properties
    private var focusOfTheHead = SCNNode()
    private let focusNodeBasePosition = simd_float3(0, 0.1, 0.25)

    // State properties
    private var modelLoaded: Bool = false
    private var headIsMoving: Bool = false
    private var shouldTrackCamera: Bool = false



    /*
    * MARK: - Init methods
    */

    override init() {

        super.init()

        loadModel()
        setupSpecialNodes()
        setupConstraints()

    }


    /*
    * MARK: - Setup methods
    */

    func loadModel() {

        guard let virtualObjectScene = SCNScene(named: "daisy_3.dae", inDirectory: "art.scnassets") else {
            print("virtualObjectScene not intialised")
            return
        }

        let wrapper = SCNNode()

        for child in virtualObjectScene.rootNode.childNodes {
            wrapper.addChildNode(child)
        }

        self.rootNode.addChildNode(contentRootNode)

        contentRootNode.addChildNode(wrapper)

        hide()

        modelLoaded = true

    }


    private func setupSpecialNodes() {

        // Assign characters rig elements to nodes
        geometryRoot = self.rootNode.childNode(withName: "D_Rig", recursively: true)
        head = self.rootNode.childNode(withName: "D_RigFBXASC032Head", recursively: true)
        leftEye = self.rootNode.childNode(withName: "D_Eye_L", recursively: true)
        rightEye = self.rootNode.childNode(withName: "D_Eye_R", recursively: true)

        // Set up looking position nodes
        focusOfTheHead.simdPosition = focusNodeBasePosition

        geometryRoot.addChildNode(focusOfTheHead)

    }


    /* 
    * MARK: - Head animations
    */ 

    func updateForScene(_ scene: ARSCNView) {

        guard shouldTrackCamera, let pointOfView = scene.pointOfView else {
            print("Not going to updateForScene")
            return
        }

        followUserWithHead(to: pointOfView)

    }

    private func followUserWithHead(to pov: SCNNode) {

        guard !headIsMoving else { return }

        // Update the focus node to the point of views position
        let target = focusOfTheHead.simdConvertPosition(pov.simdWorldPosition, to: nil)

        // Slightly delay the head movement and the animate it to the new focus position
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.2, execute: {
            let moveToTarget = SCNAction.move(to: SCNVector3(target.x, target.y, target.z), duration: 1.5)
            self.headIsMoving = true
            self.focusOfTheHead.runAction(moveToTarget, completionHandler: {
                self.headIsMoving = false
            })
        })

    }



    private func setupConstraints() {

        let headConstraint = SCNLookAtConstraint(target: focusOfTheHead)
        headConstraint.isGimbalLockEnabled = true

        let headRotationConstraint = SCNTransformConstraint(inWorldSpace: false) { (node, transform) -> SCNMatrix4 in

            // Only track the up/down and side to side movement
            var eulerX = node.presentation.eulerAngles.x
            var eulerZ = node.presentation.eulerAngles.z

            // Restrict the head movement so it doesn't rotate too far
            if eulerX < self.rad(-90) { eulerX = self.rad(-90) }
            if eulerX > self.rad(90) { eulerX = self.rad(90) }

            if eulerZ < self.rad(-30) { eulerZ = self.rad(-30) }
            if eulerZ > self.rad(30) { eulerZ = self.rad(30) }

            let tempNode = SCNNode()
            tempNode.transform = node.presentation.transform
            tempNode.eulerAngles = SCNVector3(eulerX, 0, eulerZ)
            return tempNode.transform

        }

        head?.constraints = [headConstraint, headRotationConstraint]

    }

    // Helper to convert degrees to radians
    private func rad(_ deg: Float) -> Float {
        return deg * Float.pi / 180
    }

}

场景编辑器中的模型是:

Answer 1

我已经解决了我遇到的问题。有 2 个问题:

1. followUserWithHead 中的目标应该已经为它的父级转换了 simdWorldPosition 并且从(而不是)转换为

   focusOfTheHead.parent!.simdConvertPosition(pov.simdWorldPosition, from: nil)

2. 头节点的本地坐标不正确。 z 轴应该是 x 轴，所以当我获得焦点时头部运动跟踪，耳朵始终跟随相机。

我没有意识到 Xcode 中的Debug View Hierarchy 会显示 SCNScene 的细节。这帮助我调试场景并找到节点跟踪的位置。您可以将场景导出为 dae，然后加载到 SceneKit 编辑器中

编辑:我将 localFront 用作下面评论中建议的 mnuages，这使跟踪工作朝着正确的方向进行。不过，头部确实偶尔会动一下。我已将此归因于在模型上运行的动画，该动画试图应用一个转换，然后在下一个更新周期中更改该转换。我决定从头部移除跟踪并使用相同的方法仅跟踪眼睛。