[Insight-developers] Numeric Constants

[Luis Ibanez]

Tom, Brad,

I agree, we should centralize these constants.

It is worth trying to do this for the upcoming release.


Please note that we anticipate to freeze by Saturday...


      Luis


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On Thu, Aug 6, 2009 at 11:21 AM, Tom Vercauteren
<tom.vercauteren at gmail.com>wrote:

> Hi Brad,
>
> Moving these mathematical constants to a more visible place sounds
> good to me. However, handling floating point constants in c++ is a
> pain. For example, the following code is not standard and won't be
> usable on several systems (mac gcc 4.0 at least):
>
> class No_Good {
>  static double const d = 1.0;
> };
>
> I guess that's why boost chose to implement these constants as
> template functions:
>
> http://www.boost.org/doc/libs/1_39_0/libs/math/doc/sf_and_dist/html/math_toolkit/toolkit/internals1/constants.html
>
> The other option is to decouple the definition and declaration as done
> in vnl  (with some ugly macro):
>
> class Now_Better
> {
>    static double const d;
> };
>
> double const Now_Better::d = 1.0;
>
> More information can be found here:
>
> http://stackoverflow.com/questions/370283/why-cant-i-have-a-non-integral-static-const-member-in-a-class
>
> No matter the implementation, I would definitely vote for replacing
> the current hard-coded values by a common thing such as vnl_math::pi,
> itk::Math::pi, or  itk::Math::pi<double>().
>
> My two cents,
> Tom
>
> On Thu, Aug 6, 2009 at 16:40, Bradley Lowekamp<blowekamp at mail.nih.gov>
> wrote:
> > Hello,
> >
> >        I was looking at:
> >
> > BasicFilters/itkBilateralImageFilter.txx:  rangeGaussianDenom =
> > m_RangeSigma*vcl_sqrt(2.0*3.1415927);
> >
> > It took me a little bit to figure out that we most likely should be using
> > vnl_math::pi, along with their "e and all that". I do wonder if these
> > constants were in itk::Math and in doxygen if it would help with
> increased
> > usage.
> >
> > So I began greping in the "Code" directory and came up with the
> following:
> >
> > grep -r 3.1415 *
> > BasicFilters/itkBilateralImageFilter.txx:  rangeGaussianDenom =
> > m_RangeSigma*vcl_sqrt(2.0*3.1415927);
> > BasicFilters/itkVectorGradientMagnitudeImageFilter.txx:  const double dpi
> =
> > 3.14159265358979323846;
> > Common/itkGaussianDerivativeSpatialFunction.txx:    prefixDenom *=
> > 2*vcl_pow( 2 * 3.1415927, VImageDimension / 2.0);
> > Common/itkMersenneTwisterRandomVariateGenerator.h:  double phi = 2.0 *
> > 3.14159265358979323846264338328
> > Numerics/itkCumulativeGaussianOptimizer.cxx:  m_ComputedAmplitude =  sum
> /
> > (m_ComputedStandardDeviation * vcl_sqrt(2*3.14159265));
> >
> > grep -r 0.707 *
> > BasicFilters/itkBSplineResampleImageFilterBase.txx:      m_G[0]  =
> >  0.707107;
> >
> > grep -r 1.414 *
> > Numerics/Statistics/itkGaussianDistribution.cxx:    dq  = 0.5e+0 *
> vnl_erfc(
> > dx / 1.414213562373095e+0 ) - dp;
> > Review/Statistics/itkGaussianDistribution.cxx:    dq  = 0.5e+0 *
> vnl_erfc(
> > dx / 1.414213562373095e+0 ) - dp;
> >
> > grep -r vnl_math:: * | wc
> >      69     571    7652
> >
> > Well I guess that is almost 90% usage of the numeric constants, so its
> not
> > bad at all.
> >
> > I'll work on a patch tonight, and likely commit something tomorrow if no
> one
> > sees a problem with this.
> >
> > Brad
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