How to rotate a rigidbody with torque around only the XY axes?

I'm trying to rotate a rigidbody, based on mouse movement, using torque. If the mouse moves horizontally, the rigidbody should rotate horizontally (around the Y axis). If the mouse moves vertically, the rigidbody should rotate vertically (around the X axis). The Z axis of the rigidbody should never change (always 0).

I've tried doing this in a few different ways, but I can't figure out the math to get the proper axes to rotate around. The below code works when rotating around the X axis. However, rotating around the Y axis results in the Z rotation changing if the X rotation is not 0. If the X rotation is 0, then the Y rotation works as expected.

Using the local Y (transform.up) also will give the incorrect results as the object will rotate in all directions if the X rotation is not 0.

What would be the proper axis to rotate around for the Y rotation?

Code (CSharp):

1. public class TestRotater : MonoBehaviour {
2.  
3.      private Rigidbody rb;
4.      private float rotX;
5.      private float rotY;
6.      public float speed = 10;
7.  
8.      void Awake() {
9.          rb = GetComponent<Rigidbody>();
10.      }
11.  
12.      void FixedUpdate() {
13.          rotY = Input.GetAxis("Mouse X") * speed;
14.          rotX = Input.GetAxis("Mouse Y") * speed;
15.          rb.AddTorque(transform.right * rotX + Vector3.up * rotY);
16.      }
17. }

 

Your problem is coming from the rb maintaining angular momentum from previous torque applications. The quick and dirty fix for this is to change your code to this:

Code (CSharp):

1. public class TestRotater : MonoBehaviour
2. {
3.  
4.     private Rigidbody rb;
5.     private float rotX;
6.     private float rotY;
7.     public float speed = 1f;
8.  
9.     void Awake()
10.     {
11.         rb = GetComponent<Rigidbody>();
12.     }
13.  
14.     void FixedUpdate()
15.     {
16.         rb.angularVelocity = Vector3.zero; // <-------------------
17.         rotY = Input.GetAxis("Mouse X") * speed;
18.         rotX = Input.GetAxis("Mouse Y") * speed;
19.         rb.AddTorque(transform.right * rotX + Vector3.up * rotY);
20.     }
21. }

Obviously, this means your angular velocity will not be reliably influenced by external physics forces (it will get reset and over-ridden each FixedUpdate). Equally, you should be able to see that if any velocity exists from a previous rotation you are going to have a very tricky time regaining control of the new angular velocity.