some q3 code

[abcdefgqwerty]

I know this has been looked at before a lot, but I was wondering what a line in this q3 code does. This code is very confusing partly because of that crazy number they invent.


float Q_rsqrt( float number )
{
long i;
float x2, y;

y = number;
i = * ( long * ) &y; //what does this do?
i = 0x5f3759df - ( i >> 1 );
y = * ( float * ) &i;
y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration

return y;
}

What exactly does casting the address of a float do?

[creatorul]

Well .. i = * ( long * ) &y
&y - gives the address of y variable
( long * ) &y - is casting that float address to an address of type long
* ( long * ) &y - gets the value using our new pointer of type long

[TheCPUWizard]

It is a bad way of doing the following


union FloatBits
{
float Float;
int Bits;
};

{
float f = 123.456;
FloatBits fb;
fb.Float = f;
int i = fb.Bits;
}

[wien]

A bit OT, but here's a link to an article on the origin of the function in question: Origin of Quake3's Fast InvSqrt() (http://www.beyond3d.com/articles/fastinvsqrt/). Doesn't go too much into detail, but you might find it interesting. It also links to an article on the math behind the function if you're curious. :)

[Zaccheus]

Interesting article.

It is a bad way of doing the following

Isn't it also bad to write to one member, and then read from another member, of the same union?

[TheCPUWizard]

Isn't it also bad to write to one member, and then read from another member, of the same union?


Not if you understand the implications :D :D

There are many analogies I am thinking of, but each one is sure to embarass/offend someone in this international community. :blush:

[abcdefgqwerty]

ah ok thanks. I guess it just confused me