So no one has found where the 1597463007 constant came from? Is John Carmack from an advanced alien intelligence? I Googled the constant in hexadecimal and in decimal and no results were returned; what are you guys talking about?

the value is simple. it is build by two parts: the part that fits onto the mantissa, and the part that fits onto the exponent of a float.


1/sqrt(x) = x^-.5

1/sqrt(m*2^e) = m^-.5 * 2^(e*-.5)

you're bitshifting the whole float one to the right, to get the *.5 in the exponent part. you subtract it from another constant to negate that exponent 0 - e>>1 simply (dunno, i think 0 for the exponent is actually not binary 00000.. check the value, &0xFF and you see what "zero" is for exponents)


now, the m = m^-.5 part.. well.. as the mantissa is represented without the 1 in front, but the sign bit is in front (value is always positive, you can't sqrt from a negative one anyways), you get a clear defined other value if you shift to the right by one bit.

now the idea is simple. the actual mantissa part is (1+m) (as the one is not stored in the float), and you can expand (1+m)^-.5 in a taylor serie, and the first part is actually something in the form constant - m/2 + ......

and that constant - m/2 is exactly what is done with this bitshifting and subtracting from a constant. more or less. rest is just finding a good constant value..

and that quite accurate starting value then gets feed into an iterative solving equation. converging to the exact solution..


its all just math, no alien technology.. (while i was damn impressed the first time i've read it, myself:D)

"take a look around" - limp bizkit
www.google.com

[edited by - davepermen on February 26, 2003 3:20:39 AM]

quote:Original post by 63616C68h
Is John Carmack from an advanced alien intelligence?

Probably yes

Could someone explain this yet again? --Without the word "mantissa." So whay you''re saying is: you let the sub-language architecture of your processor deal with exponential expressions? If I''m correct, then could someone plz explain the pointer aritmetic going on in that illegible code? If we could understand how the code works, then we most likely could implement that ingenious concept more comfortably.

no one cared. necro-ap''s should get disabled, just as every other ap.



If that''s not the help you''re after then you''re going to have to explain the problem better than what you have. - joanusdmentia

davepermen.net