ARKit – 如何显示来自放置在 SCNPlane 上的虚拟 SCNCamera 的提要?

经过一番研究和睡眠，我得出了以下可行的解决方案(包括一些无法解释的障碍):

After some research and sleep, I came to the following, working solution (including some inexplainable obstacles):

目前，额外的 SCNCamera 提要未链接到 SCNPlane 上的 SCNMaterial，因为这是最初的想法，但我将使用额外的 SCNView(暂时)

Currently, the additional SCNCamera feed is not linked to a SCNMaterial on a SCNPlane, as it was the initial idea, but I will use an additional SCNView (for the moment)

在定义中，我添加了另一个视图，如下所示:

In the definitions I add an other view like so:

let overlayView   = SCNView() // (also tested with ARSCNView(), no difference)
let camera        = SCNCamera()
let cameraNode    = SCNNode()

然后，在 viewDidLoad 中，我设置了这样的东西...

then, in viewDidLoad, I setup the stuff like so...

camera.automaticallyAdjustsZRange = true
camera.usesOrthographicProjection = false
cameraNode.camera                 = camera
cameraNode.camera?.focalLength    = 50
sceneView.scene.rootNode.addChildNode(cameraNode) // add the node to the default scene

overlayView.scene                    = scene // the same scene as sceneView
overlayView.allowsCameraControl      = false
overlayView.isUserInteractionEnabled = false
overlayView.pointOfView              = cameraNode // this links the new SCNView to the created SCNCamera
self.view.addSubview(overlayView)    // don't forget to add as subview

// Size and place the view on the bottom
overlayView.frame  = CGRect(x: 0, y: 0, width: self.view.bounds.width * 0.8, height: self.view.bounds.height * 0.25)
overlayView.center = CGPoint(x: self.view.bounds.width * 0.5, y: self.view.bounds.height - 175)

然后，在其他一些函数中，我将包含 SCNCamera 的节点放置到我想要的位置和角度.

then, in some other function, I place the node containing the SCNCamera to my desired position and angle.

// (exemplary)
cameraNode.position = initialStartPosition + SCNVector3(x: -0.5, y: 0.5, z: -(Float(shiftCurrentDistance * 2.0 - 2.0)))          
cameraNode.eulerAngles = SCNVector3(-15.0.degreesToRadians, -15.0.degreesToRadians, 0.0)

结果，是屏幕底部的一种窗口(新的 SCNView)，显示与主场景视图中相同的 SceneKit 内容，通过 SCNCamera 的视角及其节点位置进行查看，这非常好.

The result, is a kind of window (the new SCNView) at the bottom of the screen, displaying the same SceneKit content as in the main sceneView, viewed trough the perspective of the SCNCamera plus its node position, and that very nicely.

在一个常见的 iOS/Swift/ARKit 项目中，这种构造会产生一些副作用，人们可能会遇到这些副作用.

In a common iOS/Swift/ARKit project, this construct generates some side effects, that one may struggle into.

1) 主要是，新的 SCNView 从所需的角度显示 SceneKit 内容，但背景始终是实际的物理相机源.我无法弄清楚如何通过仍然显示所有 SceneKit 内容来使背景成为静态颜色.更改新场景的背景属性也会影响整个主场景，实际上是不想要的.

1) Mainly, the new SCNView shows SceneKit content from the desired perspective, but the background is always the actual physical camera feed. I could not figure out, how to make the background a static color, by still displaying all the SceneKit content. Changing the new scene's background property affects also the whole main scene, what is actually NOT desired.

2) 听起来可能令人困惑，但是一旦包含以下代码(这对于使其工作至关重要):

2) It might sound confusing, but as soon as the following code get's included (which is essential to make it work):

overlayView.scene = scene

整个场景的动画速度(两者)双倍！(为什么?)

the animation speed of the entire scenes (both) DOUBLES! (Why?)

我通过添加/更改以下属性纠正了这个问题，这几乎像正常(默认)一样恢复了动画速度行为:

I got this corrected by adding/changing the following property, which restores the animation speed behavour almost like normal (default):

// add or change this in the scene setup
scene.physicsWorld.speed = 0.5

3)如果项目中有SCNAction.playAudio之类的动作，所有的效果都将不再播放——只要我不这样做:

3) If there are actions like SCNAction.playAudio in the project, all the effects will no longer play - as long as I don't do this:

overlayView.scene = nil

当然，额外的 SCNView 停止工作，但其他一切都会恢复正常.

Of course, the additional SCNView stops working but everything else gets gets back to its normal.