Custom Pan gesture handler for a model rotation

Question

I would like to allow users to rotate the model with a pan gesture. The rotation should follow the finger movement. The possible movement could look like https://www.youtube.com/watch?v=50EjlzVsQM8 but with rotation along the z-axis as well.

I implemented a pan gesture handler:

@objc private func handlePan(recognizer: UIPanGestureRecognizer) {
    let currentTouchPoint = recognizer.location(in: self.arView)
    guard let ray = self.arView.ray(through: currentTouchPoint) else {
        return
    }
    anchorEntity.transform.rotation = .init(angle: .pi, axis: ray.direction)
}

but now, there are several problems with this code:

1. Since the angle is set to PI, the gesture flips the model completely and I can rotate it from there which is not what I want. But if I set it to .zero, I can’t rotate it at all.
2. It requires a lot of effort to rotate in a sense that I need to do very long finger movements and it is impossible to do a full rotation / move it back to the start point.

I believe I need to somehow adapt the angle / direction, but not sure where to start, I am not really familiar with the 3D transformations. Any help would be appreciated.

Answer 1

Since the 2D pan gesture has just x/y coordinates, with pan, you're able to control the rotation of your model around definite pair of axes, such as x/y or y/z. Here's the code:

import RealityKit
import ARKit

class ViewController : UIViewController {
    @IBOutlet var arView: ARView!
    let boxPrim = ModelEntity(mesh: .generateBox(size: 0.75))
    var previousPoint = CGPoint()
    let anchor = AnchorEntity()
    var deg: Float = .zero
    
    @objc func panning(recognizer: UIPanGestureRecognizer) {
        deg += 5.0
        var currentPoint = recognizer.location(in: arView)
        let rad = deg * .pi / 180
        
        if currentPoint.x > previousPoint.x {
            boxPrim.transform = Transform(yaw: rad * 1.5)  // Y
            currentPoint.y = 0
        }
        if currentPoint.x < previousPoint.x {
            boxPrim.transform = Transform(yaw: -rad * 1.5) // Y
            currentPoint.y = 0
        }
        if currentPoint.y > previousPoint.y {
            boxPrim.transform = Transform(pitch: rad)      // X
        }
        if currentPoint.y < previousPoint.y {
            boxPrim.transform = Transform(pitch: -rad)     // X
        }
        previousPoint = currentPoint
    }
    func gestureRecognizer() {
        let recognizer = UIPanGestureRecognizer(target: self,
                                                action: #selector(panning))
        arView.addGestureRecognizer(recognizer)
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        anchor.addChild(boxPrim)
        arView.scene.addAnchor(anchor)
        self.gestureRecognizer()
    }
}