ARKit – Collision with Real world objects

Question

I need to detect when the virtual objects gets in contact with the real world object using ARKit.

Is there any way to find it out?

Answer 1

At first you need to create a collision category struct that conforms to OptionSet protocol and has properties with bitset types:

import ARKit

struct Category: OptionSet {

    let rawValue: Int
    
    static let sphereCategory = Category(rawValue: 1 << 0)
    static let targetCategory = Category(rawValue: 1 << 1)
}

Then set a physics delegate to SCNPhysicsContactDelegate protocol inside lifecycle method:

class ViewController: UIViewController, SCNPhysicsContactDelegate {

    @IBOutlet var sceneView: ARSCNView!

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        sceneView.scene = SCNScene()
        sceneView.scene.physicsWorld.contactDelegate = self

        let config = ARWorldTrackingConfiguration()
        config.planeDetection = [.horizontal]
        sceneView.session.run(config)
    }
}

SCNPhysicsContactDelegate contains 3 optional physicsWorld() methods (we'll use 1st later):

public protocol SCNPhysicsContactDelegate: NSObjectProtocol {
        
    optional func physicsWorld(_ world: SCNPhysicsWorld, 
                      didBegin contact: SCNPhysicsContact)

    optional func physicsWorld(_ world: SCNPhysicsWorld, 
                     didUpdate contact: SCNPhysicsContact)

    optional func physicsWorld(_ world: SCNPhysicsWorld, 
                        didEnd contact: SCNPhysicsContact)
}

After this define categoryBitMask and collisionBitMask for sphere collider:

fileprivate func createSphere() -> SCNNode {

    var sphere = SCNNode()
    sphere.geometry = SCNSphere(radius: 0.1)

    sphere.physicsBody =  .init(type: .kinematic, 
                               shape: .init(geometry: sphere.geometry!, 
                                             options: nil))

    sphere.physicsBody?.isAffectedByGravity = true

    sphere.physicsBody?.categoryBitMask = Category.sphereCategory.rawValue
    sphere.physicsBody?.collisionBitMask = Category.targetCategory.rawValue
    sphere.physicsBody?.contactTestBitMask = Category.targetCategory.rawValue

    return sphere
}

...and define bit-masks in reversed order for real world detected plane:

fileprivate func visualizeDetectedPlane() -> SCNNode {

    var plane = SCNNode()
    plane.geometry = SCNPlane(width: 0.7, height: 0.7)

    plane.physicsBody =  .init(type: .kinematic, 
                              shape: .init(geometry: plane.geometry!, 
                                            options: nil))

    plane.physicsBody?.isAffectedByGravity = false

    plane.physicsBody?.categoryBitMask = Category.targetCategory.rawValue
    plane.physicsBody?.collisionBitMask = Category.sphereCategory.rawValue
    plane.physicsBody?.contactTestBitMask = Category.sphereCategory.rawValue

    return plane
}

And only when you've added your SCNPlanes to real-world detected planes and append an SCNSphere to your SCNScene, you could use physicsWorld(_:didBegin:) instance method to detect collisions:

func physicsWorld(_ world: SCNPhysicsWorld, 
         didBegin contact: SCNPhysicsContact) {

    if contact.nodeA.physicsBody?.categoryBitMask == 
                                          Category.targetCategory.rawValue |
       contact.nodeB.physicsBody?.categoryBitMask == 
                                          Category.targetCategory.rawValue {

        print("BOOM!")
    }
}