Another lighting model similar to Cook-Torrance in that it attempts to modify the lambertian model to compensate for a rough surface. The key difference to Cook-Torrance is that Oren-Nayar's doesn't include a specular component. Most implementations attach a standard Blinn-Phong specular term if it's needed.
So, the equation I'm using (which has been the hardest to comprehend so far [headshake]) is as follows:
With regards to the generic base that I'm trying to derive, the following is noteworthy:
And, as you should be expecting, a visio diagram to encapsulate the above steps: