SCNCamera moving camera's pivot

Question

I need to focus to certain node, for example a pyramid. Then apply distance to the camera, then move the camera based on user click. My approach is like this :

import SceneKit

class GameViewController: UIViewController {
let scene = SCNScene()
override func viewDidLoad() {
    super.viewDidLoad()
    let camera = SCNCamera()
    camera.usesOrthographicProjection = true
    camera.orthographicScale = 4
    camera.zNear = 1
    camera.zFar = 100
    let cameraNode = SCNNode()
    cameraNode.position = SCNVector3(x: 0, y: 0, z: 6)
    cameraNode.camera = camera
    let cameraOrbit = SCNNode()
    cameraOrbit.name = "orbit"
    cameraOrbit.addChildNode(cameraNode)
    scene.rootNode.addChildNode(cameraOrbit)

    let Py = SCNPyramid(width: 2, height: 3, length: 2)
    Py.firstMaterial?.diffuse.contents = UIColor.purple()
    let P = SCNNode(geometry: Py)
    P.position = SCNVector3(x:0,y:0,z:2) //see the note
    scene.rootNode.addChildNode(P)

    /* N O T E :
     the position of the pyramid must not be changed
     as my intention is to rotate the camera
     not the pyramid node
     I repeat, I don't want to rotate the pyramid
     */

    let scnView = self.view as! SCNView
    scnView.scene = scene
    scnView.allowsCameraControl = false
    scnView.backgroundColor = UIColor.black()

    // user rotates the camera by tapping
    let tapGesture = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
    scnView.addGestureRecognizer(tapGesture)
}


//the function which does camera rotation :
func handleTap(_ gestureRecognize: UIGestureRecognizer) {
    //I guess the solution is around here
    //like modify the cameraOrbit.position ?
    //or cameraNode.position ?
    //tried both but doesn't work
    //or I used them wrong
    let cameraOrbit = scene.rootNode.childNode(withName: "orbit", recursively: true)!
    SCNTransaction.begin()
    SCNTransaction.animationDuration = 2
    cameraOrbit.eulerAngles.z += Float(M_PI_2) //see below
    SCNTransaction.commit()
    /*
     I realise that by performing rotation on the camera
     (the camera position is unchanged) works for z rotation,
     but this is not what I want. What I want is a solution,
     which also works for eulerAngles.x and eulerAngles.y.

     I used eulerAngles.z as example since it's easier to observe.

     I guess the true solution involves moving the camera
     with specific trajectory, not only rotation of "anchored" camera.
     */
     }

//...
}

The result is :

the triangle rotates around a point below the apex

What I want to achieve is to make the rotation relative to its centre :

like this

My question is, how to adjust the pivot so I can achieve the rotation relative to the pyramid's centre?

Note : I don't want to rotate the pyramid.

Chess Engineer Chess Engineer · Accepted Answer · 2016-08-09T15:55:12

I found the solution, but I can't explain why. Please comment if you can explain.

The camera orbit position must be set to match the object's location (in this case, the pyramid). So

cameraOrbit.position = P.position

Then here comes the mystery solution, add cameraOrbit.position.y by half of pi :

cameraOrbit.position.y += Float(M_PI_2)
//therefore we have the final cameraOrbit.position as (0, pi/2, 2)

Tested and works perfectly for all cameraOrbit.eulerAngles. But I don't have a clue why it works. If pi/2 is coming from some projection thingy, then why it's tucked on y only? I mean, when I do any of the cameraOrbit.eulerAngles, I don't need to assign this pi/2 to either x or z.

Here it is the complete code

import SceneKit

class GameViewController: UIViewController {
let scene = SCNScene()
override func viewDidLoad() {
    super.viewDidLoad()
    let camera = SCNCamera()
    camera.usesOrthographicProjection = true
    camera.orthographicScale = 4
    camera.zNear = 1
    camera.zFar = 100
    let cameraNode = SCNNode()
    cameraNode.position = SCNVector3(x: 0, y: 0, z: 6)
    cameraNode.camera = camera
    let cameraOrbit = SCNNode()
    cameraOrbit.name = "orbit"
    cameraOrbit.addChildNode(cameraNode)
    scene.rootNode.addChildNode(cameraOrbit)

    let Py = SCNPyramid(width: 2, height: 3, length: 2)
    Py.firstMaterial?.diffuse.contents = UIColor.purple()
    let P = SCNNode(geometry: Py)
    P.position = SCNVector3(x:0,y:0,z:2)
    scene.rootNode.addChildNode(P)

    // S O L U T I O N :
    cameraOrbit.position = P.position
    cameraOrbit.position.y += Float(M_PI_2)
    //therefore we have the final cameraOrbit.position as (0, pi/2, 2)

    let scnView = self.view as! SCNView
    scnView.scene = scene
    scnView.allowsCameraControl = false
    scnView.backgroundColor = UIColor.black()

    // user rotates the camera by tapping
    let tapGesture = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
    scnView.addGestureRecognizer(tapGesture)
}

//the function which does camera rotation :
func handleTap(_ gestureRecognize: UIGestureRecognizer) {
    //I was wrong, the solution is not here
    let cameraOrbit = scene.rootNode.childNode(withName: "orbit", recursively: true)!
    SCNTransaction.begin()
    SCNTransaction.animationDuration = 2
    cameraOrbit.eulerAngles.z += Float(M_PI_2) //works also for x and y
    SCNTransaction.commit()

     }

...
}

SCNCamera moving camera's pivot

1 Answers