How do I read live camera data on the iPhone (like Microsoft Tag Reader)?

Question

While the user is still in UIImagePickerController, I'd like to be able to get a UIImage representation of the image in the camera, make some changes to the image, and then display the altered image instead of the image coming directly from the camera.

How can I do this? I tried setting a timer and calling takePicture, but calling it seemed to have no effect. The documentation says:

Calling this method while an image is being captured has no effect. You must wait until the associated delegate object receives an imagePickerController:didFinishPickingMediaWithInfo: message before you can capture another picture.

So how can I do this? I know that it's possible, since apps like Tag Reader are able to detect when something specific rolls in front of the camera.

Thanks!