[vtk-developers] Speeding up raycasting on x86

Fri Jul 12 12:33:04 EDT 2002

Whoops. Thanks, I'll look into this further...

> -----Original Message-----
> From: David Gobbi [mailto:dgobbi at imaging.robarts.ca]
> Sent: Friday, July 12, 2002 12:27 PM
> To: Volpe, Christopher R (Research)
> Subject: RE: [vtk-developers] Speeding up raycasting on x86
> 
> 
> Hi Chris,
> 
> Your float->int conversion seems to have a major flaw:
> 
> The FPU does 'round-to-nearest,even' meaning that
>   1.5 rounds to 2.0
>   3.5 rounds to 4.0
> but
>   2.5 rounds to 2.0
>   4.5 rounds to 4.0
> 
> This is IEEE standard, but isn't quite what we want.
> Using your QuickTrunc() I get
> 
> floor of 1  ->  0
> floor of 2  ->  2
> floor of 3  ->  3
> floor of 4  ->  3
> 
> The truth is that for most places where floor() is used (calculating
> integer + fraction for the purpose of interpolation) this result
> is fine, the fractional portion is just returned as 1.0.  But it
> seems just a bit too subtle to be safe.
> 
>  - David
> 
> 
> On Fri, 12 Jul 2002, David Gobbi wrote:
> 
> > Hi Lisa,
> >
> > I've converted the vtkFloorFuncMacro() to Chris' version.
> > His version is indeed safe, there is no 1/65536 error in it,
> > and it is more efficient than my version.
> >
> > There are still plenty of (int) casts left in the ray casting
> > code i.e. all the conversions of float scalar values to int,
> > and if these were converted to use vtkRoundFuncMacro() there
> > is probably a few more % of efficiency to be gained.
> >
> > I don't really like the fact that VTK always converts float scalars
> > to int scalars using 'cast' operations.  It seems to me 
> that it really
> > should be rounding instead.
> >
> >  - David
> >
> >
> > On Thu, 11 Jul 2002, Volpe, Christopher R (Research) wrote:
> >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Lisa Avila [mailto:lisa.avila at kitware.com]
> > > > Sent: Thursday, July 11, 2002 5:06 PM
> > > > To: Volpe, Christopher R (Research); 'David Gobbi'; Lisa Avila
> > > > Cc: vtk-developers at public.kitware.com
> > > > Subject: RE: [vtk-developers] Speeding up raycasting on x86
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > >2) It's not completely correct. Tests I ran not too long ago
> > > > show that the
> > > > >process causes the value
> > > > >to be *rounded*, but this is not readily apparent because
> > > > the rounding is,
> > > > >in this case, not to the
> > > > >nearest int but to the nearest fraction of 1/2**16. So, for
> > > > example, a
> > > > >value greater than (3.0 -
> > > > >1/2**16), but less than 3.0,  will be rounded up to 3. 
> This problem
> > > > >becomes very readily apparent if
> > > > >you tweak the values so that you use the full 32 bits for
> > > > the integer
> > > > >portion and don't reserve any
> > > > >bits for fixed point fractional values, thus causing 
> rounding to the
> > > > >nearest integer.
> > > >
> > > > Being "not completely correct" may cause big problems in the
> > > > ray caster.
> > >
> > > Yes, butit's easily fixable. My version, for example, 
> doesn't have the problem.
> > >
> > > -Chris
> > > _______________________________________________
> > > vtk-developers mailing list
> > > vtk-developers at public.kitware.com
> > > http://public.kitware.com/mailman/listinfo/vtk-developers
> > >
> >
> >
> 
> 

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