Unity - change only z axis rotation to 0

Question

I have a child where I want the rotation to work like (parentRot, parentRot, 0).

So first I set its localRotation to 0, meaning it will get the parents rotation: child.localRotation = Quaternion.identity;

But how can I make sure the world rotation (not local) value of the z axis stays at 0?

Answer 1

Here's another way to think of it:

First, we figure out which direction in the parent space points toward world "up":

Vector3 parentSpaceUp = transform.parent.InverseTransformDirection(Vector3.up);

Then we orient ourselves to point forward in the parent's local coordinate, while aligning our own up vector as close as possible to this vertical:

transform.localRotation = Quaternion.LookRotation(Vector3.forward, parentSpaceUp);

Wherever possible, I like to use direction vectors and quaternions over Euler angles, both to avoid unnecessary trig functions, and also to make sure the code doesn't exhibit bugs at edge cases where the Euler angles wrap around.

Answer 2

I have no idea what you're trying to do, but this does precisely what you described.

public class AxisLock : MonoBehaviour
{
    void Update()
    {

        Vector3 target;
        target.x = transform.parent.eulerAngles.x;
        target.y = transform.parent.eulerAngles.y;
        target.z = 0;
        transform.rotation = Quaternion.Euler(target);
    }
}