Question Black Hole Rendering on Limited Depth camera - How to apply renderTexture on Sprite ?

I am trying to simulate a Black Hole in the Galaxy, using a dedicated shader. The light coming behind the hole is modified going to the following effect.

A shader is applied on main camera, using the following script. The shader in included in the material myMat.

Code (CSharp):

1. private void OnRenderImage(RenderTexture source, RenderTexture destination)
2.     {
3.         if (IsBlackHoleVisible)
4.             Graphics.Blit(source, destination, myMat);
5.     }

But some objects, that are in front of the black hole (blue ring) shall not be modified.

So I have created a second camera exactly at the same position and FOV than the Main Camera, but with depth starting at BlackHole position up to infinity.
The second camera is rendered in a target render texture. Then I want to copy part of this texture in a Sprite that is set at the BlockHole position.

I try the following script on Sprite object. But Sprite is blank or nothing...

Code (CSharp):

1.    
2.     public RenderTexture renderTexture;
3.     public Texture2D texture2D;
4.     public Vector2 blackholePos;
5.     public Vector2 blackholeSize;
6.     public SpriteRenderer mySprite;
7.     // Start is called before the first frame update
8.     void Start()
9.     {
10.         //Define texture2D
11.         texture2D = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);
12.         //set SpriteRenderer
13.         mySprite = GetComponent<SpriteRenderer>();
14.     }
15.     // Update is called once per frame
16.     void Update()
17.     {
18.         //first method tested - copy renderTexture in texture2D
19.         Graphics.CopyTexture(renderTexture, 0, 0, (int)blackholePos.x, (int)blackholePos.y, (int)blackholeSize.x, (int)blackholeSize.y, texture2D, 0, 0, 0, 0);
20.  
21.         //second method tested - copy renderTexture in texture2D
22.         RenderTexture.active = renderTexture;
23.         texture2D.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
24.         texture2D.Apply();
25.         RenderTexture.active = null;
26.  
27.         //then copy the texture2D as the Sprite
28.         mySprite.sprite = Sprite.Create(texture2D, new Rect((int)blackholePos.x, (int)blackholePos.y, (int)blackholeSize.x, (int)blackholeSize.y), new Vector2(0, 0));
29.     }
30.  

How to copy only part of a renderTexture in a Texture2D and apply it on a Sprite ?
Thanks for your help...

 

I finaly manage the problem by using a Cube Mesh with a very small Z size, and not a Sprite.
I apply on this mesh a material with my Shader. And in the shader I extrat the screen part corresponding to the area where the cube is. Simple thanks to Unity.
You can see the result hereafter.