Capturing iphone camera input w/out overlay

Question

I'm trying to capture the camera input, analyse this image and then adding something on top of it. This is a continuous process. Problem is when I do my image capture it includes the item I am overlaying on top of the camera input. The code I'm using is:

CGImageRef screen = UIGetScreenImage();

Does anyone have an idea how to capture just the camera input?

Cheers!