BackgroundParallax.cs

using UnityEngine;
using UnityEngine.Assertions;

public class BackgroundParallax : BaseBehaviour
{
	public Transform[] backgrounds;			// Array of all the backgrounds to be parallaxed.
	public float parallaxScale;				// The proportion of the camera's movement to move the backgrounds by.
	public float parallaxReductionFactor;	// How much less each successive layer should parallax.
	public float smoothing;					// How smooth the parallax effect should be.

	private Transform cam;					// Shorter reference to the main camera's transform.
	private Vector3 previousCamPos;			// The postion of the camera in the previous frame.

	void Start()
	{
		cam = Camera.main.transform;
		Assert.IsNotNull(cam);

		previousCamPos = cam.position;
	}

	void Update()
	{
		// The parallax is the opposite of the camera movement since the previous frame, multiplied by the scale.
		float parallax = (previousCamPos.x - cam.position.x) * parallaxScale;

		// For each successive background...
		for (int i = 0; i < backgrounds.Length; i++)
		{
			// ... set a target x position which is their current position plus the parallax multiplied by the reduction.
			float backgroundTargetPosX = backgrounds[i].position.x + parallax * (i * parallaxReductionFactor + 1);

			// Create a target position which is the background's current position but with it's target x position.
			var backgroundTargetPos = new Vector3(backgroundTargetPosX, backgrounds[i].position.y, backgrounds[i].position.z);

			// Lerp the background's position between itself and it's target position.
			backgrounds[i].position = Vector3.Lerp(backgrounds[i].position, backgroundTargetPos, smoothing * Time.deltaTime);
		}

		// Set the previousCamPos to the camera's position at the end of this frame.
		previousCamPos = cam.position;
	}
}