Fading up scene geometry as a post process

Im trying to fade up the scene geometry when I have established tracking. (as a nice user experience effect)
I create a post process shader but it seems the input texture is the entire composited scene (objects + image from device camera). That is to be expected I guess. Probably what I need to do is lerp between the device camera image and the composited camera+geometry image.

So my question is how do I get the raw device camera image from ARKit. ? Is there a render texture available somewhere that I can pass to my post-process shader?

- Anupam