How to render spheres in SceneKit at user's tap position?

Question

I am working on an iOS app with Swift and SceneKit. When user taps on screen, I want to render a dot in SceneKit at the location that user just tapped. How can I do that?

I am using unprojectPoint() method but my object is not rendered at expected location.

I am having a SceneView, with the camera like this:

cameraNode.position = SCNVector3(x: 0, y: 0, z: 15)

And then I render the dot at tap position like this:

func handleTap(_ gestureRecognize: UIGestureRecognizer) {
    // retrieve the SCNView
    let scnView = self.view as! SCNView
    
    // create dot
    let geo = SCNSphere(radius: CGFloat(0.1))
    geo.firstMaterial?.diffuse.contents = UIColor.red
    let dotNode = SCNNode(geometry: geo)
    
    let p = gestureRecognize.location(in: scnView)
    let uP = scnView.unprojectPoint(SCNVector3(p.x, p.y, 5))
    // HERE - how to set position for the dot ?
    dotNode.position = SCNVector3(uP.x, uP.y, 5)
    
    scnView.scene?.rootNode.addChildNode(dotNode)
}

Here is my Github repo: https://github.com/rudyhuynh/SceneKitExample

Answer 1

Unprojecting a Point

To render dot nodes at expected position, use the following approach. Don't forget that you are looking at the scene in perspective projection. Unprojecting a point whose Z-coordinate is 0.0 returns a point on the near clipping plane. Unprojecting a point whose Z-coordinate is 1.0 returns a point on the far clipping plane.

import SceneKit

class GameViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()            
        let sceneView = self.view as! SCNView
        sceneView.scene = SCNScene()
        sceneView.backgroundColor = UIColor.black
                
        let cameraNode = SCNNode()
        cameraNode.camera = SCNCamera()
        cameraNode.simdPosition = simd_float3(0, 0, 1)
        sceneView.scene?.rootNode.addChildNode(cameraNode)
        
        let tapGesture = UITapGestureRecognizer(target: self,
                                                action: #selector(tap))
        sceneView.addGestureRecognizer(tapGesture)
    }

    @objc func tap(_ gestureRecognizer: UITapGestureRecognizer) {
        let scnView = self.view as! SCNView
        
        let geo = SCNSphere(radius: 0.01)
        geo.firstMaterial?.diffuse.contents = UIColor.white
        let dotNode = SCNNode(geometry: geo)

        let point = gestureRecognizer.location(in: scnView)
        let unprojected = scnView.unprojectPoint(.init(point.x, point.y, 0))
 
        dotNode.position = SCNVector3(unprojected.x, unprojected.y, -0.01)
        scnView.scene?.rootNode.addChildNode(dotNode)
    }
}

To run a camera in orthographic projection, use:

cameraNode.camera?.usesOrthographicProjection = true