Redirect Unity3D input into a letterboxed fixed area. Good for fixed resolution games with aspect ratio preserved scaling.

RedirectInput.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class RedirectInput : StandaloneInputModule {

	// letterboxed fixed area
	[SerializeField] Vector2 fixedResolution;
	private Vector2 m_cursorPos;
	public override void UpdateModule()
	{
		//scale input to our fixed resolution
		Rect zone = GetInputZone();
		float scale = GetScale();
		Vector2 pos = new Vector2(
			(Input.mousePosition.x - zone.xMin) / scale,
			(Input.mousePosition.y - zone.yMin) / scale
		);

		//m_cursorPos = Input.mousePosition;
		m_cursorPos = pos;
	}

	private float GetScale() {
		float screenAspect = (float)Screen.width / (float)Screen.height;
		float rectAspect = fixedResolution.x / fixedResolution.y;

		float scaleFactor;
		if (screenAspect > rectAspect)
			scaleFactor = Screen.height / fixedResolution.y;
		else
			scaleFactor = Screen.width / fixedResolution.x;

		return scaleFactor;
	}

	private Rect GetInputZone(){
		var area = fixedResolution * GetScale();
		return new Rect(
			Screen.width / 2 - area.x / 2,
			Screen.height / 2 - area.y / 2,
			area.x, area.y
		);
	}

	private readonly MouseState m_MouseState = new MouseState();
	protected override MouseState GetMousePointerEventData(int id = 0)
	{
		// Populate the left button...
		PointerEventData leftData;
		var created = GetPointerData(kMouseLeftId, out leftData, true);

		leftData.Reset();

		if (created)
            leftData.position = m_cursorPos;
        
		Vector2 pos = m_cursorPos;
		leftData.delta = pos - leftData.position;
		leftData.position = pos;
		leftData.scrollDelta = Input.mouseScrollDelta;
		leftData.button = PointerEventData.InputButton.Left;
		eventSystem.RaycastAll(leftData, m_RaycastResultCache);
		var raycast = FindFirstRaycast(m_RaycastResultCache);
		leftData.pointerCurrentRaycast = raycast;
		m_RaycastResultCache.Clear();

		// copy the apropriate data into right and middle slots
		PointerEventData rightData;
		GetPointerData(kMouseRightId, out rightData, true);
		CopyFromTo(leftData, rightData);
		rightData.button = PointerEventData.InputButton.Right;

		PointerEventData middleData;
		GetPointerData(kMouseMiddleId, out middleData, true);
		CopyFromTo(leftData, middleData);
		middleData.button = PointerEventData.InputButton.Middle;

		m_MouseState.SetButtonState(PointerEventData.InputButton.Left, StateForMouseButton(0), leftData);
		m_MouseState.SetButtonState(PointerEventData.InputButton.Right, StateForMouseButton(1), rightData);
		m_MouseState.SetButtonState(PointerEventData.InputButton.Middle, StateForMouseButton(2), middleData);

		return m_MouseState;
	}
}