检查 ARReferenceImage 是否在相机视图中不再可见

我想检查一下是否AR参考图像 https://developer.apple.com/documentation/arkit/arreferenceimage在相机视图中不再可见。目前我可以检查图像的节点是否在相机的视图中，但是当AR参考图像 https://developer.apple.com/documentation/arkit/arreferenceimage被其他图像覆盖或图像被删除时。

func renderer(_ renderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
    guard let node = self.currentImageNode else { return }

    if let pointOfView = sceneView.pointOfView {
        let isVisible = sceneView.isNode(node, insideFrustumOf: pointOfView)
        print("Is node visible: \(isVisible)")
    }
}

因此，我需要检查图像是否不再可见，而不是图像的节点可见性。但我不知道这是否可能。第一个屏幕截图显示了找到下面的图像时添加的三个框。当找到的图像被覆盖时（参见屏幕截图 2），我想删除这些框。

我设法解决了这个问题！使用了一点 Maybe1 的代码和他的概念来解决问题，但是以不同的方式。下面这行代码仍然用于重新激活图像识别。

// Delete anchor from the session to reactivate the image recognition
sceneView.session.remove(anchor: anchor) 

让我解释。首先我们需要添加一些变量。

// The scnNodeBarn variable will be the node to be added when the barn image is found. Add another scnNode when you have another image.    
var scnNodeBarn: SCNNode = SCNNode()
// This variable holds the currently added scnNode (in this case scnNodeBarn when the barn image is found)     
var currentNode: SCNNode? = nil
// This variable holds the UUID of the found Image Anchor that is used to add a scnNode    
var currentARImageAnchorIdentifier: UUID?
// This variable is used to call a function when there is no new anchor added for 0.6 seconds    
var timer: Timer!

完整代码，下面有注释。

/// - Tag: ARImageAnchor-Visualizing
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
    guard let imageAnchor = anchor as? ARImageAnchor else { return }

    let referenceImage = imageAnchor.referenceImage

    // The following timer fires after 0.6 seconds, but everytime when there found an anchor the timer is stopped.
    // So when there is no ARImageAnchor found the timer will be completed and the current scene node will be deleted and the variable will set to nil
    DispatchQueue.main.async {
        if(self.timer != nil){
            self.timer.invalidate()
        }
        self.timer = Timer.scheduledTimer(timeInterval: 0.6 , target: self, selector: #selector(self.imageLost(_:)), userInfo: nil, repeats: false)
    }

    // Check if there is found a new image on the basis of the ARImageAnchorIdentifier, when found delete the current scene node and set the variable to nil
    if(self.currentARImageAnchorIdentifier != imageAnchor.identifier &&
        self.currentARImageAnchorIdentifier != nil
        && self.currentNode != nil){
            //found new image
            self.currentNode!.removeFromParentNode()
            self.currentNode = nil
    }

    updateQueue.async {

        //If currentNode is nil, there is currently no scene node
        if(self.currentNode == nil){

            switch referenceImage.name {
                case "barn":
                    self.scnNodeBarn.transform = node.transform
                    self.sceneView.scene.rootNode.addChildNode(self.scnNodeBarn)
                    self.currentNode = self.scnNodeBarn
                default: break
            }

        }

        self.currentARImageAnchorIdentifier = imageAnchor.identifier

        // Delete anchor from the session to reactivate the image recognition
        self.sceneView.session.remove(anchor: anchor)
    }

}

当计时器结束表明没有找到新的 ARImageAnchor 时删除该节点。

@objc
    func imageLost(_ sender:Timer){
        self.currentNode!.removeFromParentNode()
        self.currentNode = nil
    }

这样当图像被覆盖或者发现新的图像时，当前添加的scnNode就会被删除。

遗憾的是，该解决方案并没有解决图像的定位问题，原因如下：

ARKit 不会跟踪每个检测到的图像的位置或方向的变化。