Allows the mouse to rotate two game object transforms.

MouseLook.cs

using System;
using UnityEngine;

/* ********************************************************************************************
 * Program:       MouseLook.cs
 * Purpose:       Allows the mouse to rotate two game object transforms.
 * Date:          3/04/2017
 * Author:        Maxstupo
 * ****************************************************************************************** */
public class MouseLook : MonoBehaviour {

    public Transform yAxisTransform;

    [Header("Input")]
    public String xAxisInput = "Mouse X";
    public float xSensitivity = 2;

    public String yAxisInput = "Mouse Y";
    public float ySensitivity = 2;

    [Header("Clamping")]
    public bool clampVerticalRotation = true;

    [Range(-120, 120)]
    public float minimumX = -90;

    [Range(-120, 120)]
    public float maximumX = 90;

    [Header("Smoothing")]
    public bool interpolate = true;
    public float interpolateAmount = 20f;

    [Header("Misc")]
    public bool lockCursor = true;

    private Quaternion characterTargetRot;
    private Quaternion cameraTargetRot;
    
    void Start() {
        if (yAxisTransform == null)
            Debug.LogWarning("MouseLook yAxisTransform is null! Disabling y-axis rotation!");

        characterTargetRot = transform.localRotation;
        if (yAxisTransform != null)
            cameraTargetRot = yAxisTransform.localRotation;

        if (lockCursor)
            Cursor.lockState = CursorLockMode.Locked;
    }

    private void Update() {

        if (lockCursor && Cursor.lockState != CursorLockMode.Locked) {
            if (Input.GetMouseButtonDown(0)) {
                Cursor.lockState = CursorLockMode.Locked;
            } else {
                return;
            }
        }


        // Avoids the mouse looking if the game is effectively paused
        if (Mathf.Abs(Time.timeScale) < float.Epsilon)
            return;

        float yRot = Input.GetAxis(xAxisInput) * xSensitivity;
        float xRot = Input.GetAxis(yAxisInput) * ySensitivity;

        if (yAxisTransform != null)
            cameraTargetRot *= Quaternion.Euler(-xRot, 0, 0f);
        
        characterTargetRot *= Quaternion.Euler(0, yRot, 0);

        if (clampVerticalRotation)
            cameraTargetRot = ClampRotationAroundXAxis(cameraTargetRot);

        if (interpolate) {
            transform.localRotation = Quaternion.Slerp(transform.localRotation, characterTargetRot, interpolateAmount * Time.deltaTime);
            if (yAxisTransform != null)
                yAxisTransform.localRotation = Quaternion.Slerp(yAxisTransform.localRotation, cameraTargetRot, interpolateAmount * Time.deltaTime);
        } else {
            transform.localRotation = characterTargetRot;
            if (yAxisTransform != null)
                yAxisTransform.localRotation = cameraTargetRot;
        }
    }

    private Quaternion ClampRotationAroundXAxis(Quaternion q) {
        q.x /= q.w;
        q.y /= q.w;
        q.z /= q.w;
        q.w = 1.0f;

        float angleX = 2.0f * Mathf.Rad2Deg * Mathf.Atan(q.x);

        angleX = Mathf.Clamp(angleX, minimumX, maximumX);

        q.x = Mathf.Tan(0.5f * Mathf.Deg2Rad * angleX);
        return q;
    }
}