BackgroundRoller.cs

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

public class BackgroundRoller : MonoBehaviour
{
    //rolls with constant speed now, could be adjusted to the movement speed of the platforms or the offset of the player



    private Camera camera;
    public List<Sprite> bgSprites;
    private int currentSprite = 0;
    public GameObject currentBackgroundInstance;
    public GameObject upcomingBackgroundInstance;
    private BoxCollider2D bgCollider;

    public GameObject backgroundInstancePrefab;


    void Start()
    {
        camera = Camera.main;
        bgCollider = currentBackgroundInstance.GetComponent<BoxCollider2D>();
        settUpBackgroundInstance(currentBackgroundInstance);
    }

    // Update is called once per frame
    void Update()
    {
        Debug.Log("current spire" + currentSprite);

        if (currentSprite >= bgSprites.Count - 1)
        {
            currentSprite = 0; //never do this at home kids
        }

        //if there are sprites left & we are over half the height, we instantiate another sprite at the top
        if (currentSprite < bgSprites.Count - 1 &&
            camera.transform.position.y > currentBackgroundInstance.transform.position.y + bgCollider.size.y / 2 - camera.orthographicSize &&
            upcomingBackgroundInstance == null)
        {
            upcomingBackgroundInstance = Instantiate(backgroundInstancePrefab);
            settUpBackgroundInstance(upcomingBackgroundInstance);
            upcomingBackgroundInstance.transform.position = new Vector3(0, currentBackgroundInstance.transform.position.y + bgCollider.size.y * currentBackgroundInstance.transform.localScale.y, 0.1f);

        }


    }

    public void SetCurrentBgInstance(GameObject bg)
    {
        currentBackgroundInstance = bg.gameObject;
        upcomingBackgroundInstance = null;
        bgCollider = bg.GetComponent<BoxCollider2D>();

    }

    public void settUpBackgroundInstance(GameObject upcomingBackgroundInstance)
    {
        //https://answers.unity.com/questions/890148/scale-and-place-gameobject-as-a-background-of-the.html
        //source
        upcomingBackgroundInstance.transform.localScale = new Vector2((Camera.main.orthographicSize * 2 * Screen.width * 0.28f) / Screen.height, Camera.main.orthographicSize * 2 * 0.28f);
        upcomingBackgroundInstance.transform.SetParent(transform);
        upcomingBackgroundInstance.GetComponent<SpriteRenderer>().sprite = bgSprites[currentSprite];
        upcomingBackgroundInstance.GetComponent<BackgroundInstance>().roller = this;
        currentSprite++;
    }
}