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

Question

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.

Answer 1

Simple 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 onto detected plane. In Apple documentation (2019) there's a following definition of ray-casting:

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 and RealityKit 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 detected 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 an example where you could see how to implement ray-casting methods makeRaycastQuery() and raycast():

import UIKit
import RealityKit

class ViewController: UIViewController {
    
    @IBOutlet var arView: ARView!
    let model = try! Entity.loadModel(named: "usdzModel")
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        self.raycasting()
    }

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

        guard let result = arView.session.raycast(query).first
        else { return }

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

If you wanna know how to use a Convex-Ray-Casting in RealityKit, read THIS POST.

If you wanna know how to use Hit-Testing in RealityKit, read THIS POST.