c# - Unity smooth local rotation around one Axis, oriented toward a target

I want to acheive the same rotation you see in the inspector. In short: a smooth local Y rotation, toward a target

I have a Vector3 orientation: Vector3 dirOrientation = targetToLook.position - movingCamera.position;

So, every Update, what should I do ? I try many thing, but I don't realy understand Quaternion...

I just want to rotate localy in Y, toward a precise moving target, NOT in X or Z. and smoothly with some Lerp or Slerp.

Thanks !

Answer

Using the trick discussed in a similar question here and also here, we can define a helper method to point our local Y axis exactly in a particular direction, while turning our local Z axis to face (as close as possible) toward another direction.

Quaternion TurretLookRotation(Vector3 approximateForward, Vector3 exactUp)
{
    Quaternion rotateZToUp = Quaternion.LookRotation(exactUp, -approximateForward);
    Quaternion rotateYToZ = Quaternion.Euler(90f, 0f, 0f);

    return rotateZToUp * rotateYToZ;
}

Now we can use that in Update to face toward our target:

void Update() {
    // Form the direction we want to look towards
    Vector3 offsetToTarget = target.position - transform.position;

    // Preserve our current up direction
    // (or you could calculate this as the direction away from the planet's center)
    Vector3 up = transform.up;

    // Form a rotation facing the desired direction while keeping our
    // local up vector exactly matching the current up direction.

    Quaternion desiredOrientation = TurretLookRotation(offsetToTarget, up);

    // Move toward that rotation at a controlled, even speed regardless of framerate.
    transform.rotation = Quaternion.RotateTowards(
                            transform.rotation,
                            desiredOrientation,
                            maxDegreesPerSecond * Time.deltaTime
                         );

}