How to Rotate an Object Around Only One Axis in Realitykit

How do I rotate an object around only one axis in RealityKit?

In RealityKit there are, at least, three ways to rotate an object around single axis.

In each example we rotate an object counterclockwise (CCW).

First approach

let boxScene = try! Experience.loadBox()

boxScene.steelBox?.orientation = simd_quatf(angle: .pi/4,    /* 45 Degrees */
                                             axis: [0,0,1])  /* About Z axis */

Second approach

boxScene.steelBox?.transform = Transform(pitch: 0, 
                                           yaw: 0, 
                                          roll: .pi/4)      /* Around Z axis */

pitch, yaw and roll are rotations about X, Y and Z axis expressed in radians.

Third approach

let a: Float = cos(.pi/4)
let b: Float = sin(.pi/4)

let matrix = float4x4([ a, b, 0, 0 ],        /* column 0 */
                      [-b, a, 0, 0 ],        /* column 1 */
                      [ 0, 0, 1, 0 ],        /* column 2 */
                      [ 0, 0, 0, 1 ])        /* column 3 */

boxAnchor.steelBox?.setTransformMatrix(matrix, relativeTo: nil)

Visual representation of rotation matrix looks like this:

let a: Float = cos(.pi/4)
let b: Float = sin(.pi/4)

//  0  1  2  3
 ┌              ┐
 |  a -b  0  0  |
 |  b  a  0  0  |
 |  0  0  1  0  |
 |  0  0  0  1  |
 └              ┘

If you wanna know more about Rotation Matrices, read this post.

RealityKit – Can't rotate object properly

Use *= compound multiply operator instead of += compound addition.

Here is the explanation.

Change a rotation of AnchorEntity in RealityKit

Try move(...) instance method:

import UIKit
import RealityKit
import SceneKit

class ViewController: UIViewController {
    
    @IBOutlet var arView: ARView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let box = try! Experience.loadBox()
        let entity = box.steelBox!
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 3.0) {

            let currentMatrix = entity.transform.matrix

            // Nothing prevents you from using even SceneKit's matrix methods
            let rotation = simd_float4x4(SCNMatrix4MakeRotation(.pi/2, 0,1,0))

            let transform = simd_mul(currentMatrix, rotation)               
            entity.move(to: transform, relativeTo: nil, duration: 3.0)
        }
        arView.scene.anchors.append(box)
    }
}