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matt271

Wed Mar 28, 2012 5:16 pm


A Brief History of InvSqrt

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Just finished my Honours Thesis about the fast InvSqrt function found in Quake III,

float Q_rsqrt(float number) {
    long i;
    float x2, y;
    const float threehalfs = 1.5F;

    x2 = number * 0.5F;
    y = number;
    i = *(long *) &y;
    i = 0x5f3759df - (i >> 1);
    y = *(float *) &i;
    y = y * (threehalfs - (x2 * y * y));
//  y = y * (threehalfs - (x2 * y * y));

    return y;
}


http://eggroll.unbsj.ca/rsqrt/

Questions, comments, suggestions?


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Dreadnought

Wed Mar 28, 2012 6:22 pm


Re: A Brief History of InvSqrt

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Wow, that's so amazing! It's probably the coolest (and weirdest) function I've ever seen.

Nice job, on the thesis too, it was fun to read.

Btw, if you plan on printing from that pdf, some of the numbers in the appendix run off the page.


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matt271

Wed Mar 28, 2012 6:41 pm


Re: A Brief History of InvSqrt

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Thanks for the tip! I will fix it before I give print outs to my readers tomorrow!


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matt271

Wed Sep 11, 2013 9:38 pm


RE:A Brief History of InvSqrt

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Complete version, for anybody who's interested...

http://shelfflag.com/rsqrt.pdf