Need to rotate object in specific ways - Stuck with Gimbal Lock?

Question

I am working on a small mini-game that requires the rotation of a cube 90 degrees in the appropriate direction based on the direction you swipe. So you could swipe up and it would rotate up 90 degrees, and them immediately after, swipe left, and it would swipe 90 degrees to the left from your current rotation (so it would stay rotated up 90 degrees as well). I feel like this should be really simple, but it's giving me a ton of trouble.

I would like to use Lerp/Slerp so that the rotation looks nice, though it isn't entirely necessary. The way I currently have it implemented, each time I call my "SlerpRotateLeft()" function for example, it only rotates to the exact same exact rotation relative to the world each time (instead of the current rotation + 90 degrees in the correct direction).

I have been reading up on Quaternions and Euler angles all day, but I'm still not entirely sure what my problem is.

I am currently using states to determine when the object is currently rotating and in what direction, though I feel like I may be overcomplicating it. Any possible solution to this problem (where you can swipe in a particular direction, in any order, in succession, to rotate a cube 90 degrees in that particular direction). Previously, I attempted to use coroutines, but those didn't have the desired effect either (and I was unable to reset them).

Here is my class. It works and you can test it by dropping the script into any cube object in-editor, but it doesn't work as intended. You will see what my problem is by testing it (I recommend placing an image on the cube's front face to track which one it is). I'm not sure if I explained my problem properly, so please let me know if any more information is needed.

****UPDATE: I have accepted @Draco18s's answer as correct, because their solution worked. However, I did not completely understand the solution, or how to store the value. I found an answer to a similar question that also used Transform.Rotate, and stored the value, which helped clear the solution up. The key seemed to be storing it in a GameObject instead of in a Quaternion like I originally thought. I thought I should provide this code in case anyone stumbles upon this and is equally confused, though you may not need the swipe detection:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Rotater : MonoBehaviour
{

 private GameObject endRotation;


//SWIPE VARIABLES
public Vector2 touchStart = new Vector2(0, 0);

public Vector2 touchEnd = new Vector2(0, 0);

public Vector2 currentSwipe = new Vector2(0, 0);

public Vector2 currentSwipeNormal = new Vector2(0, 0);

// Use this for initialization
void Start()
{
    endRotation = new GameObject();

}

// Update is called once per frame
void Update()
{


    if (Input.GetMouseButtonDown(0))
    {
        touchStart = new Vector2(Input.mousePosition.x, Input.mousePosition.y);
        //Debug.Log("Touched at: " + touchStart);

    }

    if (Input.GetMouseButtonUp(0))
    {
        touchEnd = new Vector2(Input.mousePosition.x, Input.mousePosition.y);

        //Get Swipe Vector information
        currentSwipe = new Vector2(touchEnd.x - touchStart.x, touchEnd.y - touchStart.y);

        //Normalize Swipe Vector
        currentSwipeNormal = currentSwipe;
        currentSwipeNormal.Normalize();

        //Swipe up
        if (currentSwipeNormal.y > 0 && currentSwipeNormal.x > -0.5 && currentSwipeNormal.x < 0.5)
        {
            endRotation.transform.Rotate(-Vector3.left, 90, Space.World);

        }
        //Swipe down
        if (currentSwipeNormal.y < 0 && currentSwipeNormal.x > -0.5 && currentSwipeNormal.x < 0.5)
        {
            endRotation.transform.Rotate(Vector3.left, 90, Space.World);


        }
        //Swipe left
        if (currentSwipeNormal.x < 0 && currentSwipeNormal.y > -0.5 && currentSwipeNormal.y < 0.5)
        {
            endRotation.transform.Rotate(Vector3.up, 90, Space.World);



        }
        //Swipe right
        if (currentSwipeNormal.x > 0 && currentSwipeNormal.y > -0.5 && currentSwipeNormal.y < 0.5)
        {
            endRotation.transform.Rotate(-Vector3.up, 90, Space.World);

        }

    }

    LerpRotate();
}

void LerpRotate()
{
    transform.rotation = Quaternion.Lerp(transform.rotation, endRotation.transform.rotation, Time.deltaTime * 10);

}
}

Answer 1

Use Transform.RotateAround

You're encountering an issue where you take the current Euler angles and try and add/subtract 90, which does not necessarily correlate to the desired position, due to the rotated nature of the rotated reference frame.

But using RotateAround, you can pass in the global Up, Left, and Forward vectors, which is what you're trying to do.