AR multiple cameras dont line up - Solution

I embarked on a multi-hour debugging session to find out why having multiple cameras with augmented reality does not work as expected (they don't line up).

However i found the culprit and a solution (hope this helps some of you):
- The ARCameraBackground changes the projection matrix of the main camera (presumably to get the fov to match)
- When you have child cameras they have the same position/rotation as the main camera, however the projection matrix is still different

In order to solve this you can put this script on any child camera in AR:

Code (CSharp):

1. [RequireComponent(typeof(Camera))]
2.   public class UICamera : MonoBehaviour
3.   {
4.     private Camera _uiCamera;
5.     private Camera _sourceCamera;
6.  
7.     private void Awake()
8.     {
9.       _sourceCamera = transform.parent.GetComponent<Camera>();
10.       _uiCamera = GetComponent<Camera>();
11.     }
12.  
13.     private void Update()
14.     {
15.       _uiCamera.projectionMatrix = _sourceCamera.projectionMatrix;
16.     }
17.   }

@unityteam: Maybe this could be improved -> maybe include a script for child cameras?
Use case: If you need a separate UI camera because i.e. you don't want post-processing on your ui elements (world space)