Quaternions; how do I rotate an object around only 2 axis based on another rotation?

Unity Engine
1

So I have a camera that rotates around an object, always facing it. What I would like to do now is to have that object rotate with the camera, but only along the xz plane (in other words; around the y axis).

Imagine a skyscraper with a camera rotating around it in all directions, always facing it. I want the skyscraper to always face the camera, but not leave the ground.

I don’t have much code so far, but this is what I have tried:

Quaternion difference = Quaternion.Inverse(cameraRigging.rotation) * masterCamera.transform.rotation;

            // rotate camera reletive to main camera
            cameraRigging.rotation = masterCamera.transform.rotation;
           
            // rotate the object such that it is still facing the
            // same direction reletivly to the camera along z and
            // x axes (NOT WORKING)
            target.transform.rotation *= difference;

This isn’t really working; the rotation doesn’t stay on Y (as expected), but it also is not rotating the way I would expect. Target almost faces the same direction as camera, but not quite.

2

I just tried SetLookRotation, but that also does not work.

3

I’m not entirely positive what you’re asking here, but I’m assuming you want the skyscraper to face the camera while having it’s Up vector perpendicular to the X and Z vectors. There’s a couple ways to do this, but a fairly simple one is to take the direction from the position of your skyscraper to the camera and project it onto the ZX plane.

Vector3 direction = masterCamera.transform.position - target.transform.position;
direction = Vector3.ProjectOnPlane(direction, Vector3.up);
target.transform.rotation = Quaternion.LookRotation(direction);

This will set the target’s rotation to point in the direction of it’s local Z, the direction being a Vector pointing at the mainCamera but laying flat on the ZX plane. I think this is what you’re after.

4

This is exactly why I wanted. Thank you!!! Rotations drive me nuts; just can’t seem to wrap my head around them