Rotate an object to a position only through one axis

Question

I'm about to rip my hair out trying to do something that should be relatively simple.

I have an Vehicle A, which when upside-down I want to rotate slowly so it aligns back to the ground, but only through the X-asis and Z-Axis (Player is able to steer the object left and right, and I don't want to lock them out of controlling the Y axis while it rotates). The problem, of course, is that angles in Unity is bananas. Instead of going from 0-360, angles goes from 0 to 90 and back down to 0, then jumps to 360, down to 270, and back up to 360 again. And Quaternions requires a ph.d.

Is there a simple solution to this I'm just not seeing?

edit: This replicated pretty closely what I want:

 Quaternion rt = Quaternion.RotateTowards(transform.rotation, new Quaternion(0, 0, 0, 1), rotateSpeed * Time.fixedDeltaTime);
 transform.rotation = new Quaternion(rt.x, transform.rotation.y, rt.z, rt.w);

And if you're looking at this in horror going "what in the he..?"; you and me both. The problem with this is that it still grips the Y axis a little bit, making steering slower, and when the rotation is done it starts rotating around the Y-axis ever so slightly. Due to my complete lack of knowledge about Quaternions, I have no idea why.

Answer 1

I might be able to give you a hand with your PhD if you need!

 public float timeTaken = 5f;
 
 private bool standingUp = false;
 
 void Update()
 {
     // Usual turning
     transform.rotation *= Quaternion.AngleAxis(Input.GetAxis("Horizontal"), transform.InverseTransformDirection(Vector3.up));
 
     // Flip up
     if (Input.GetKeyDown(KeyCode.Q) && !standingUp && isFlipped())
     {
         standingUp = true;
         StartCoroutine(StandUp(timeTaken));
     }
 
     // Flip over
     if (Input.GetKeyDown(KeyCode.W))
     {
         transform.rotation = Quaternion.AngleAxis(180, Vector3.forward) * Quaternion.AngleAxis(Random.Range(-180, 180), Vector3.up);
     }
 }
 
 private bool isFlipped()
 {
     return transform.up.y < 0.1f;
 }
 
 private IEnumerator StandUp(float timeToTake)
 {
     float t = 0;
     while(t < timeToTake)
     {
         Quaternion target = findTargetRotation(transform);
 
         // Try to keep the 'degreesToChange' as constant as possible whilst realising that it may go up or down
         // if other rotations are applied whilst this happens
         float timeLeft = timeToTake - t;
         float leftOverDegrees = Quaternion.Angle(transform.rotation, target);
         float degreesToChange = Time.deltaTime * (leftOverDegrees / timeLeft);
             
         transform.rotation = Quaternion.RotateTowards(transform.rotation, target, degreesToChange);
         t += Time.deltaTime;
         yield return null;
     }
 
     transform.rotation = findTargetRotation(transform);
     standingUp = false;
 }
 
 public Quaternion findTargetRotation(Transform t)
 {
     // Project the current forward axis onto the horizontal plane (hence the normal axis is Vector3.up)
     Vector3 proj = Vector3.ProjectOnPlane(t.forward, Vector3.up);
 
     // Use lovely built in methods to get a rotation such that we point the forward axis along the
     // closest axis on the horizontal plane. We also want our up vector to be the world up vector
     // once we've finished rotating.
     return Quaternion.LookRotation(proj, Vector3.up);
 }

Check the in-code comments for some pointers but let me know if you need any parts explained in more detail!

Scribe

Rotate an object to a position only through one axis

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