Just providing context... After re-reading my statements: The problem actually isn't that we do "color * constant", the problem is that the constant should be "ambience + specular". What we have right now can behave like ambient shading or specular shading, and something "in the middle" - but that's not quite the same thing as the sum.

[Edit] Btw, all of this comes directly from here, e.g. from an attempt to make a good general approximation to your shader. Not trying to shift "blame" or anything, just wanted to highlight the history here. Strictly speaking you even had the sum in there, which I promptly optimised out in the "approximated pow curve" bit :)

> Could the "magic" not be seperated out then

That was actually pretty much the idea. From what I can see, the only special case that applies to other shaders is that it needs to shade two pixels? We could remove that by mixing the pixels up-front, but that would give us worse anti-aliasing properties on the borders. I am fairly sure I tested that, and it used to be pretty noticeable.