Ah, thanks, that makes sense, and thank you also for the extra info. it helps to visualize it better. Oh, by the way, Schlick's approx. does get included into CookTorrance, right? I was following this gem here: http://content.gpwiki.org/index.php/D3DBook:%28Lighting%29_Cook-Torrance Never knew Cook had Schlick's in it.