Also interesting that the real author of the fast InvSqrt ( Greg Walsh) was working at Ardent when he invented it, which is the same company that Steve Blank has been blogging about recently.
Walsh worked on the Xerox PARC Bravo word processor with Charles Simonyi and Ben Wegbreit. Followed Wegbreit to Convergent, Ardent and E.piphany.
Brilliant architect and coder.
Personally, I think the origin of Quake3's fast InvSqrt is from a bubble in the space-time continuum resulting from posting articles about Quake3's fast InvSqrt routine to social news sites every week. Yeah, it's kind of self-referential, but that's exactly how these bubbles are formed...
What I found confusing was the name. "InvSqrt" reads to me as "Inverse square root"... Well, the inverse function of the square root function is just to square the number!
Apparently, what they meant was "1/sqrt", so ideally the function should have been called "ReciprocalSqrt" or something like that.
In fact, this is so confusing that I would argue that's actually just an incorrect name for what the function does.
It's true that mathematicians write "sin^-1" to mean the "inverse sin" function, for example, but they never mean 1/sin(x) when they do this, and they would never say "inverse sin" to mean "1/sin(x)".
I developed this trick independently while working at MetaCreations in the late 1990's. I wrote it up in an article in 2001. I doubt this is where the Quake guys got it, because the magic number is slightly different, but it's the same basic idea.
http://beyond3d.com/content/articles/15