How to use Swift Scenekit Contact Test

I am developing a game where users can use their finger to move objects, and when the user releases their finger the game checks for overlap between objects and move the moved object back to it's original position if there is overlap. I am using SCNScene.PhysicsWorld.contactTest(with: ) to check for overlap between my nodes. However, the method only works correctly when nodes have physicsbodys using .convexHull, when I change it to .concavePolyHedron everything stops working and no contact is reported. I have set the physicbodys to be static so I am at a loss of what to do.

Here is my code configuring the physicsbody for each node

parentNode.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(node: parentNode, options: [.type: SCNPhysicsShape.ShapeType.concavePolyhedron, .collisionMargin: 0.0, .scale: scaleVector]))

Here is my code calling contact test :

if let test = currentNode?.physicsBody {
     let list = view.scene!.physicsWorld.contactTest(with: test) {
          ...
     }
}

Edit: Here is a GameViewController of a basic game demonstrating my problem. The collision detection does not work with concavePolyHedron but changing the option to convexHull the code works correctly.

import UIKit
import QuartzCore
import SceneKit

class GameViewController: UIViewController {
  
  var dragGesture: UIPanGestureRecognizer?
  var currentNode: SCNNode?
  var beginningPos: SCNVector3?
  var previousLoc: CGPoint?
  var scnView: SCNView?
  
  override func viewDidLoad() {
    super.viewDidLoad()
    
    // create a new scene
    let scene = SCNScene()
    
    // create and add a camera to the scene
    let cameraNode = SCNNode()
    cameraNode.camera = SCNCamera()
    scene.rootNode.addChildNode(cameraNode)
    
    // place the camera
    cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)
    
    
    // create and add a light to the scene
    let lightNode = SCNNode()
    lightNode.light = SCNLight()
    lightNode.light!.type = .omni
    lightNode.position = SCNVector3(x: 0, y: 10, z: 10)
    scene.rootNode.addChildNode(lightNode)
    
    // create and add an ambient light to the scene
    let ambientLightNode = SCNNode()
    ambientLightNode.light = SCNLight()
    ambientLightNode.light!.type = .ambient
    ambientLightNode.light!.color = UIColor.darkGray
    scene.rootNode.addChildNode(ambientLightNode)
    
    
    // retrieve the SCNView
    scnView = self.view as! SCNView
    
    // set the scene to the view
    scnView?.scene = scene
    
    // allows the user to manipulate the camera
    scnView?.allowsCameraControl = true
    
    // show statistics such as fps and timing information
    scnView?.showsStatistics = true
    
    // configure the view
    scnView?.backgroundColor = UIColor.black
    
    let node1 = SCNNode(geometry: SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0))
    node1.position = SCNVector3(x: 0, y: 0, z: 0)
    node1.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(node: node1, options:
                                                                              [.type: SCNPhysicsShape.ShapeType.concavePolyhedron, .collisionMargin: 0.0,]))
    scene.rootNode.addChildNode(node1)
    
    
    let node2 = SCNNode(geometry: SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0))
    node2.position = SCNVector3(x: 2, y: 2, z: 0)
    node2.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(node: node2, options:
                                                                              [.type: SCNPhysicsShape.ShapeType.concavePolyhedron, .collisionMargin: 0.0,]))
    scene.rootNode.addChildNode(node2)
    
    
    let tapGesture = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
    scnView?.addGestureRecognizer(tapGesture)
    
    
  }
  
  func setupGesture() {
    dragGesture = UIPanGestureRecognizer(target: self, action: #selector(handlePan(_:)))
    view.addGestureRecognizer(dragGesture!)
  }
  
  func removeGesture() {
    if let dragGesture = dragGesture {
      view.removeGestureRecognizer(dragGesture)
      self.dragGesture = nil // Set the dragGesture to nil after removing it
    }
  }
  
  @objc
  func handlePan(_ sender: UIPanGestureRecognizer) {
    var delta = sender.translation(in: self.view)
    let loc = sender.location(in: self.view)
    
    
    if sender.state == .began {
      if let location = currentNode?.position {
        beginningPos = location
        previousLoc = loc
      }
    }
    
    if sender.state == .changed {
      if let currentNode {
        delta = CGPoint.init(x: 2 * (loc.x - previousLoc!.x), y: 2 * (loc.y - previousLoc!.y))
        currentNode.position = SCNVector3.init(currentNode.position.x  + Float(delta.x * 0.0075), currentNode.position.y - Float(delta.y * (0.0075)), currentNode.position.z)
      }
      previousLoc = loc
    }
    
    
    if sender.state == .ended {
      if let test = currentNode?.physicsBody {
        let list = scnView?.scene?.physicsWorld.contactTest(with: test)
        
        if list!.count > 0 {
          if let beginningPos {
            currentNode?.position = beginningPos
            
          }
        }
      }
    }
    
  }
  
  @objc func handleTap(_ gestureRecognize: UIGestureRecognizer) {
    
    // check what nodes are tapped
    let p = gestureRecognize.location(in: scnView)
    let hitResults = scnView?.hitTest(p, options: [:])
    
    if let node = currentNode {
      
      removeGesture()
      scnView?.allowsCameraControl = true
      
      
      if let material = node.geometry?.firstMaterial {
        SCNTransaction.begin()
        SCNTransaction.animationDuration = 0.5
        
        material.multiply.contents = UIColor.white
        
        SCNTransaction.commit()
      }
      currentNode = nil
    }
    
    // check that we clicked on at least one object
    if hitResults!.count > 0 {
      // retrieved the first clicked object
      
      currentNode = hitResults!.first?.node
      
      
      
      scnView?.allowsCameraControl = false
      
      
      
      setupGesture()
      
      
      let pos = currentNode!.position
      
      
      if let material = currentNode?.geometry?.firstMaterial {
        
        
        // highlight it
        SCNTransaction.begin()
        SCNTransaction.animationDuration = 0.5
        
        material.multiply.contents = UIColor.red
        SCNTransaction.commit()
      }
    }
  }
  
  
  
  override var prefersStatusBarHidden: Bool {
    return true
  }
  
  override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
    if UIDevice.current.userInterfaceIdiom == .phone {
      return .allButUpsideDown
    } else {
      return .all
    }
  }
  
}

Solution

Indeed, the behaviour of that convexHull and concavePolyhedron are a little bit strange here.

I suppose, your goal is to place one cube like on top of the existing one (or where ever.) With your config the margin at the surface overlapped the real cube size by a tiny distance, so the cube flips to its original position, even when it did not touch the existing cube. I played around a little bit and this is my best result, I achieved:

  let node1 = SCNNode(geometry: SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0))
  node1.position = SCNVector3(x: 0, y: 0, z: 0)
  node1.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(geometry: node1.geometry!))
  scene.rootNode.addChildNode(node1)
  
  
  let node2 = SCNNode(geometry: SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0))
  node2.position = SCNVector3(x: 2, y: 2, z: 0)
  node2.physicsBody = SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(geometry: node2.geometry!))
  scene.rootNode.addChildNode(node2)

As you can see, I do not force some special configuration to the physicsBody. And this is the most close, I can move the cubes together now.

Using: SCNPhysicsBody(type: .static, shape: SCNPhysicsShape(geometry: node2.geometry!))

I hope it will help.