Origin of Quake3's Fast InvSqrt()

The thrilling follow-up, in which the true author of the algorithm is revealed:

http://beyond3d.com/content/articles/15

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...

If, like me, you were left hoping for some explanation of the actual magic behind this code, try this: http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf

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://stereopsis.com/computermath101/

-Ben

I so enjoy the subtleties of xkcd...

This post seems to come up a lot in hn: http://www.google.co.uk/search?q=site%3Anews.ycombinator.com...