What is the real benefit of using Raycast in ARKit and RealityKit?

问题

What is a ray-casting in ARKit and RealityKit for?

And when I need to use a makeRaycastQuery instance method:

func makeRaycastQuery(from point: CGPoint, 
                 allowing target: ARRaycastQuery.Target, 
                       alignment: ARRaycastQuery.TargetAlignment) -> ARRaycastQuery?

Any help appreciated.

回答1:

Ray-Casting, the same way as Hit-Testing, helps find a 3D position on a real-world surface by projecting an imaginary ray from a screen point. I've found a following definition of ray-casting in Apple documentation:

Ray-casting is the preferred method for finding positions on surfaces in the real-world environment, but the hit-testing functions remain present for compatibility. With tracked ray-casting, ARKit continues to refine the results to increase the position accuracy of virtual content you place with a ray-cast.

When the user wants to place a virtual content onto some surface, it's a good idea to have a tip for this. Many AR apps draw a focus circle or square that give the user visual confirmation of the shape and alignment of the surfaces that ARKit is aware of. So, to find out where to put a focus circle or a square in the real world, you may use an ARRaycastQuery to ask ARKit where any surfaces exist in the real world.

Here's some abstract example where you could see ray-casting method makeRaycastQuery():

import RealityKit

class ViewController: UIViewController {

    @IBOutlet var arView: ARView!
    let model = try! Entity.load(named: "car")

    func rayCastingMethod() {

        // target iOS 13.0+, Xcode 11.0+

        guard let query = arView.makeRaycastQuery(from: arView.center,
                                              allowing: .estimatedPlane,
                                             alignment: .vertical) 
        else {
            return
        }

        guard let result = arView.session.raycast(query).first 

        else {
            return
        }

        let transform = Transform(matrix: result.worldTransform)
        model.transform = transform

        let raycastAnchor = AnchorEntity(raycastResult: result)
        raycastAnchor.addChild(model)
        arView.scene.anchors.append(raycastAnchor)
    }
}

If you want to know how to use a Hit-Testing method in RealityKit, read THIS POST, please.

来源：https://stackoverflow.com/questions/56466773/what-is-the-real-benefit-of-using-raycast-in-arkit-and-realitykit