Texture2D.ReadPixels not working

I am trying to grab a texture as it is displayed onscreen so I can use it elsewhere. In my scene, I have a plane with a material shader that takes in a WebCamTexture and a Mask texture. the shader masks the WebCamTexture input to only show whats within the mask image, and also pixelates it.

I want to take a snapshot of what I see onscreen and apply it to a Sprite to use elsewhere.

I have tried using Graphics.Blit() and Texture2D.ReadPixels() to output (at this point any/all of the screen - though ideally I will have to select only a specific portion) the texture2D, but I keep getting a black image.

I got this code from another thread. It's currently what I'm working with:

     // Initialize and render
     RenderTexture rt = new RenderTexture(Screen.width, Screen.height, 24);
     cam.targetTexture = rt;
     cam.Render();
     RenderTexture.active = rt;

     // Create Texture
     Texture2D tex = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);

     // Read pixels
     tex.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0, false);

     // Clean up
     //cam.targetTexture = null;
     //RenderTexture.active = null; // added to avoid errors 
     //DestroyImmediate(rt);
     tex.Apply();
     return tex;

 

Can anyone tell me why this isn't working?