Re: [Algorithms] How to get 3dvector largest coordinate index?

[Matt J <mjo...@gm...>]

Yeah, okay, i get it, i got it backwards. I just said that =)

Anyway, here another way that only has two conditionals, no shifts, and no
max's and easier to follow. Not sure if it is faster though. The FP uses a
stack either way, right? I think this will work but never tested

const float f[3] = { fabsf(n.x), fabsf(n.y), fabsf(n.z) };

unsigned int i = ( f[1] > f[0] ) ? 1 : 0;

return (f[2] > f[i]) ? 2 : i;

..........Finally, an all integer way:

const unsigned int u[3] = {
 0x7FFFFFFF & *(unsigned int *)&n.x,
 0x7FFFFFFF & *(unsigned int *)&n.y,
 0x7FFFFFFF & *(unsigned int *)&n.z };

unsigned int i = ( u[1] > u[0] ) ? 1 : 0;

return (u[2] > u[i]) ? 2 : i;

 If (y > x) == 0 then that means x <= y.
> nope:
> If (y > x) == 0 then that means x >= y.
>
> 2009/2/27 Matt J <mjo...@gm...>
>
>>
>> Hmm, I fail to see how this works:
>>
>> If (y > x) == 0 then that means x <= y.
>>
>> So,
>>
>> If x <= y and z < y then 0 << 0 = 0
>> If x <= y and z >= y then 0 << 1 = 0
>>
>> In the first case, y is the greatest component In the second case, z is
>> the greatest component.
>>
>> Both are encoded as 0. Something I'm missing?
>>
>>
>> Hi,
>>>
>>> I need to compute the index (not the actual value) of the largest
>>> coordinate of a normal, for some space hashing.
>>>
>>> I'm not sure how fast you guys usually find this index, but I've just
>>> created the following trick which I think is quite fast:
>>>
>>> > inline uint LargestCoordinate(const Vector3d &v)
>>> > {
>>> >       const float x = fabs(v.x);
>>> >       const float z = fabs(v.z);
>>> >       const float y = Maths::max( fabs(v.y), z );
>>> >       return uint(fabs(y)>fabs(x)) << uint(fabs(z)>=fabs(y));
>>> > }
>>>
>>