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

[Olivier G. <gal...@po...>]

On Fri, Feb 27, 2009 at 11:02:58AM -0500, Matt J wrote:
> 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?

The max.

That formula has more fabs than sense.  A quick rewrite would be:
x = abs(v.x); y = abs(v.y); z = abs(v.z)
return (x < max(y,z)) << (z >= y)

which is obviously correct.

  OG.

> 
> 
> 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));
> > > }
> >

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