ARkit - 将物体放置在相机视野内

Question

我想在当前相机视图的边界中放置 2 条线作为流：

来源 - SceneKit docs

从 ARKit 文档中，我了解到我需要 projectionMatrix，但是如何计算从“zNear”到“zFar”和 x\y 的差异？

我从这段代码开始：

let cameraProjectionMatrix = session.currentFrame?.camera.projectionMatrix 
let cameraPosition = SCNVector3.positionFromTransform(cameraProjectionMatrix)
let rightBoxNode = SCNNode(geometry: SCNBox(...))
rightBoxNode.position = SCNVector3(???)
sceneView.scene.rootNode.addChildNode(rightBoxNode)

对于左边的我可能需要

var leftPos = rightboxNode.position
leftPos.x = rightboxNode.position.x * -1
leftBoxNode.position = leftPos

但我在尝试计算 rightboxNode.position 时失败了：

rightBoxNode.position = SCNVector3(x: x1 ,y: y1 z: zNear)

Answer 1

您可以使用SCNSceneRenderer 在相机空间中取消投影屏幕边界：

func renderer(_ sender: SCNSceneRenderer,  updateAtTime time: TimeInterval) {
  let position = sender.unprojectPoint(SCNVector3(0, 0, 0))
  // x, y in screen coordinate space, z in [0, 1] corresponds to [zNear, zFar] in a way that I don't think is proportional
  let node = SCNNode()
  node.position = position
  ...
}