[Insight-developers] CovariantVector::GetSquaredNorm()

[Matt McCormick]

Hey David,

Sounds like trouble to me.

Thanks,
Matt

On Fri, Aug 31, 2012 at 8:23 PM, David Doria <daviddoria at gmail.com> wrote:
> The current implementation of CovariantVector::GetSquaredNorm() seems to
> assume that the component type may have a problem being multiplied and added
> (overflow). However, when the component type IS capable of these operations,
> a penalty is still paid in the conversion of everything to double
> (itk::NumericTraits<every POD>::RealValueType = double).
>
> In my program, I have images of CovariantVector<int, 3> pixels and I get
> about a 20% speed up if I change the current implementation:
>
> template< class T, unsigned int NVectorDimension >
> typename CovariantVector< T, NVectorDimension >::RealValueType
> CovariantVector< T, NVectorDimension >
> ::GetSquaredNorm(void) const
> {
>   RealValueType sum = NumericTraits< RealValueType >::Zero;
>
>   for ( unsigned int i = 0; i < NVectorDimension; i++ )
>     {
>     const RealValueType value = ( *this )[i];
>     sum += value * value;
>     }
>   return sum;
> }
>
> to
>
>  template< class T, unsigned int NVectorDimension >
>  typename CovariantVector< T, NVectorDimension >::ValueType
>  CovariantVector< T, NVectorDimension >
>  ::GetSquaredNorm(void) const
>  {
>    ValueType sum = NumericTraits< ValueType >::Zero;
>
>    for ( unsigned int i = 0; i < NVectorDimension; i++ )
>      {
>      const ValueType value = ( *this )[i];
>      sum += value * value;
>      }
>    return sum;
>  }
>
> (note that the return type as well as the internal type has changed).
>
> I can't think of a reasonable way to determine automatically if there will
> be a problem (it is not just a signed/unsigned type of problem, but rather
> it actually depends how large the values are relative to the size of the
> type). Would it make sense to add a second function called something like
> GetSquaredNormNoCast() or something like that that can be used in cases
> where the developer knows that there will be no chance of overflow and would
> like to get the speed savings?
>
> Thanks,
>
> David
>
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