
Origin of Quake3's Fast InvSqrt() - rms
http://beyond3d.com/content/articles/8
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teuobk
The thrilling follow-up, in which the true author of the algorithm is
revealed:

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

~~~
jlangenauer
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.

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jrockway
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...

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hyperbovine
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>

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Patient0
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)".

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benw
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

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dlsspy
I so enjoy the subtleties of xkcd...

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

