How do I rotate a SCNNode around a point?

Question

I have a SCNNode - sceneNode - which is a child of rootNode and contains all of my child nodes. Upon the user tapping a button, I want to rotate the scene around a certain point on the y-axis. For example, the camera's point of view is known, and I want to rotate everything by 90º around the camera. The camera is no longer at 0, 0, 0.

I've tried playing around with the SCNMatrix4MakeTranslation function, and then changing the y value on the euler angles, but I've not been able to get it to work expectedly.

Answer 1

Your question is rather ambiguous, if you were to rotate the entire scene 90degrees “around the camera” the scene would end up next to the camera and you wouldn’t see it.

To rotate a SCNNode around a point other than its own pivot, as the question in the title, create a translation matrix that moves it to that point, multiply with a rotation matrix to the SCNNode, and then multiply with the inverse of the translation matrix.

However, what you probably want to do instead is do basically the same process with the camera node instead of the sceneNode. That will make the camera move around the sceneNode, giving the appearance the scene is rotating in place.

For example:

//get the current camera's transform and store it in cammat
GLKMatrix4 cammat = SCNMatrix4ToGLKMatrix4(self.myView.pointOfView.transform);

//create a rotation matrix of 90 degrees over axis Y
GLKQuaternion quaty = GLKQuaternionMakeWithAngleAndAxis(M_PI/2, 0, 1, 0);
GLKMatrix4 rotMat = GLKMatrix4MakeWithQuaternion(quaty);

//set the camera transform to rotMat * cammat, which basically rotates the camera first, and then moves it back to the same offset it was. 
self.myView.pointOfView.transform = SCNMatrix4FromGLKMatrix4(GLKMatrix4Multiply(rotMat, cammat));