Need to modify image before displaying on AVCaptureVideoPreviewLayer

Question

I'm using AVCaptureSession to capture video from my iPhone 3G and I need to capture the image and change it before displaying on my AVCaptureVideoPreviewLayer. In my current implementation, I am simply implementing the captureOutput: method and displaying the UIImage in a UIImageView, but that does not work for some reason.

Any ideas? Subclass AVCaptureSession somehow?

Thanks!