Fade scene up in ArKit

How does one go about fading the scene up in AR kit like the Apple Park example.
I tried using a replacement shader but since I use multiple different shaders in my scenes that does not properly account for all objects in the scene. I could implement a replacement shader for each shader type but that would be tedious.

At the end of the day seems to me the best, most performant way to do this is to utilize a post process shader.
I did an experiment but seems the input color in post processing includes the ipad camera pixels. Is there a way to just do the post processing fade on scene objects, then comp the camera pixels afterword?

Please share your approach.
- Anupam

 

anyone?