Mike Ash Friday Q&A: Obtaining and Interpreting Image Data.
"Cocoa provides some great abstractions for working with images. NSImage lets you treat an image as an opaque blob that you can just draw where you want it. Core Image wraps a lot of image processing in an easy-to-use API that frees you from worrying about how individual pixels are represented. However, sometimes you really just want to get at the raw pixel data in code. Scott Luther suggested today's topic: fetching and manipulating that raw pixel data."