A Brief History of InvSqrt

[matt271]

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

c:

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?

[Dreadnought]

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.

[matt271]

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

[matt271]

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

http://shelfflag.com/rsqrt.pdf