Chapter 21: World Tracking & Persistent AR Experiences
前言
這個新app的主要功能就是可以把ar體驗記錄下來，下次開啟app時可以繼續上次的ar體驗。
這app可以在實體世界的任何地方進行塗鴉，然後紀錄。下次重啟app只要經過同樣的地方就可以看到之前留下的塗鴉。
大綱
Getting started
這個app只能在iOS12上才能使用。 Sketch Button: 按下去後。可以隨意地移動裝置進行塗鴉。 Reset Button: 就是清空當前所有塗鴉。 ------
Placing anchors
要達到Persistent，要依賴ARKit幾個重要class - ARWorldMap, ARSession, ARAnchor。
ARWorldMap: 可以想像是用來記錄環境特徵，例如當app移到相同的場景，可以把之前的ar塗鴉讀取出來放在場景上，以達到Persistent的功能。
ARSession: 主要是用來協助進行存檔，讀檔。
ARAnchor: 這就是要被存起來或讀取的資料。
Adding ARAnchor objects to your ARSession is a requirement because the objects are the means to restore the state of the app when it relaunches in the same physical environment
// MARK: - Place AR content
- func addLineObject(sourcePoint: SCNVector3, destinationPoint: SCNVector3) {
- let lineNode = SCNLineNode(from: sourcePoint, to: destinationPoint, radius: 0.02, color: lineColor)
- guard let hitTestResult = sceneView
- .hitTest(self.viewCenter!, types: [.existingPlaneUsingGeometry, .estimatedHorizontalPlane])
- .first
+ func addLineAnchorForObject(sourcePoint: SCNVector3?, destinationPoint: SCNVector3?) {
+ guard let hitTestResult = sceneView.hitTest(self.viewCenter!, types: [.existingPlaneUsingGeometry, .estimatedHorizontalPlane]).first
else { return }
- lineNode.transform = SCNMatrix4(hitTestResult.worldTransform)
- sceneView.scene.rootNode.addChildNode(lineNode)
+
+ currentLineAnchorName = "virtualObject\(count)"
+ count = count + 1
+ // 每次都根據currentLineAnchorName產生唯一的line anchor
+ let lineAnchor = ARLineAnchor(name: currentLineAnchorName!, transform: hitTestResult.worldTransform, sourcePoint: sourcePoint, destinationPoint: destinationPoint)
+ // (這步就是讓session可以協助將anchor記錄起來)將這唯一的line anchor加入session中
+ sceneView.session.add(anchor: lineAnchor)
+ lineObjectAnchors.append(lineAnchor)
}
} func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
let lineARAnchor = anchor as? ARLineAnchor
if let lineAnchor = lineARAnchor,
let source = lineARAnchor?.sourcePoint,
let destination = lineARAnchor?.destinationPoint {
lineObjectAnchors.append(lineAnchor)
let lineNode = SCNLineNode(from: source, to: destination, radius: 0.02, color: lineColor)
node.addChildNode(lineNode)
}
}Saving your world map
要取得穩定可靠(取得很多辨識度良好的環境特徵)的world map是要看時機的。
可以透過
ARFrame.WorldMappingStatus觀察是否是好時機。
func session(_ session: ARSession, didUpdate frame: ARFrame) {
switch frame.worldMappingStatus {
case .extending, .mapped:
if let lastLineAnchor = lineObjectAnchors.last,
lineObjectAnchors.count > 0 && frame.anchors.contains(lastLineAnchor) {
// 最適合進行存檔的時機
saveExperienceButton.isEnabled = true
saveExperienceButton.isHidden = false
}
default:
saveExperienceButton.isHidden = true
saveExperienceButton.isEnabled = false
}
mappingStatusLabel.text = """
Mapping: \(frame.worldMappingStatus.description)
Tracking: \(frame.camera.trackingState.description)
"""
}
// 存檔
@IBAction func saveExperience(_ sender: UIButton) {
// 從session中取得當前的WorldMap
sceneView.session.getCurrentWorldMap { (worldMap, error) in
guard let map = worldMap else {
// 當無法獲取map, map = nil
self.showAlert(title: "Can't get current world map", message: error!.localizedDescription)
return
}
// get the SnapshotAnchor from sceneView.
// SnapshotAnchor is a custom ARAnchor for saving a snapshot in a world map
guard let snapshotAnchor = SnapshotAnchor(capturing: self.sceneView) else {
fatalError("Can't take snapshot")
}
map.anchors.append(snapshotAnchor)
do {
// 將map轉成data
let data = try NSKeyedArchiver.archivedData(withRootObject: map, requiringSecureCoding: true)
// 將data記錄在適當的位置上
try data.write(to: self.mapSaveURL, options: [.atomic])
DispatchQueue.main.async {
self.loadExperienceButton.isHidden = false
self.loadExperienceButton.isEnabled = true
}
} catch {
fatalError("Can't save map: \(error.localizedDescription)")
}
}
}Loading and restoring content
可以把AR記錄起來，當來就要可以讀取出來。
func getWorldMap() -> ARWorldMap {
// 確認有存檔的data
guard let data = mapDataFromFile else {
fatalError("""
Map data should already be verified to exist
before Load button is enabled.
""")
}
do {
// 從data還原成world map
guard let worldMap = try NSKeyedUnarchiver.unarchivedObject(ofClass: ARWorldMap.self, from: data) else {
fatalError("No ARWorldMap in archive.")
}
return worldMap
} catch {
fatalError("Can't unarchive ARWorldMap from file data: \(error)")
}
}
////////////////////////////////////////////////
@IBAction func loadExperience(_ sender: Any) {
// 取得先前紀錄的world map
let worldMap: ARWorldMap = getWorldMap()
// 這個snapshot是用來幫助使用者知道當初紀錄的worldmap是長怎樣
// 從world map 讀取data到snapshotThumbnailImageView
if let snapshotData = worldMap.snapshotAnchor?.imageData, let snapshot = UIImage(data: snapshotData) {
self.snapshotThumbnailImageView.image = snapshot
} else {
print("No snapshot image in world map")
}
worldMap.anchors.removeAll(where: { $0 is SnapshotAnchor })
let configuration = self.defaultConfiguration
// 建立新config的session, 其中初始化的worldmap是當初紀錄的worldmap
configuration.initialWorldMap = worldMap
sceneView.session.run(configuration, options: [.resetTracking, .removeExistingAnchors])
// 重置相關參數
isRelocalizingMap = true
lineObjectAnchors.removeAll()
}Resetting content
@IBAction func resetTracking(_ sender: UIButton?) {
sceneView.session.run(defaultConfiguration,
options: [.resetTracking, .removeExistingAnchors])
isRelocalizingMap = false
lineObjectAnchors.removeAll()
}Last updated
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