[Insight-developers] itk::Vector Normalize method

[Arnaud Gelas]

I would agree with you about throwing an exception in the case of NULL  
vector.
However, when computing mesh normals on a mesh with boundaries it  
sometimes
happens on some point of the boundary. In such a case, I am not sure  
an exception
should be thrown.

But how should we define normalization for NULL vectors?
(I have just checked in VTK, the normalization of a NULL vector is a  
NULL vector and it returns the norm).

Can I proceed and test these changes? any suggestions / thoughts?

Arnaud

On Jan 14, 2010, at 12:43 PM, Luis Ibanez wrote:

> Hi Arnaud,
>
> All your suggestions sound very reasonable to me.
>
> I would just add that when checking for NULL in the
> Normalize() method we probably should use the
> Assert and Throw Exception macro (or a variant of it).
>
> In your current suggested modification, calling Normalize
> in a Null vector will still return a null vector.... well... I guess
> it comes down to picking a mathematical definition of what
> should be the normalization of a null vector...
>
>
> I would vote for throwing an Exception in that case....
>
>
>    Any thoughts ?
>
>
>           Luis
>
>
> --------------------------------
> On Thu, Jan 14, 2010 at 10:47 AM, Gelas, Arnaud Joel Florent
> <Arnaud_Gelas at hms.harvard.edu> wrote:
>> Hi guys,
>>
>> I was going through the implementation of the normalize method of  
>> itk::Vector, and I was wondering if there is any reason not to test  
>> if the norm is null (see the implementation below)?
>> I guess it can create some problems when trying to normalize null  
>> vector...
>>
>> /**
>> * Divide vector's components by vector's norm
>> */
>> template<class T, unsigned int TVectorDimension>
>> void
>> Vector<T, TVectorDimension>
>> ::Normalize( void )
>> {
>>  const RealValueType norm = this->GetNorm();
>>  for( unsigned int i=0; i<TVectorDimension; i++)
>>    {
>>    (*this)[i] = static_cast<T> (static_cast<RealValueType>((*this) 
>> [i]) / norm);
>>    }
>> }
>>
>> What about the following modification?
>>
>> /**
>> * Divide vector's components by vector's norm
>> */
>> template<class T, unsigned int TVectorDimension>
>> void
>> Vector<T, TVectorDimension>
>> ::Normalize( void )
>> {
>>  const RealValueType norm = this->GetNorm();
>>  if( norm > 0 )
>>    {
>>    for( unsigned int i=0; i<TVectorDimension; i++)
>>      {
>>      (*this)[i] = static_cast<T> (static_cast<RealValueType>((*this) 
>> [i]) / norm);
>>      }
>>    }
>> }
>>
>> I also think that the method GetNorm could be slightly optimized in  
>> the case where the norm is null.
>>
>> /**
>> * Returns vector's Euclidean Norm
>> */
>> template<class T, unsigned int TVectorDimension>
>> typename Vector<T, TVectorDimension>::RealValueType
>> Vector<T, TVectorDimension>
>> ::GetNorm( void ) const
>> {
>> return RealValueType( vcl_sqrt(double(this->GetSquaredNorm()) ));
>> }
>>
>> modification
>> /**
>> * Returns vector's Euclidean Norm
>> */
>> template<class T, unsigned int TVectorDimension>
>> typename Vector<T, TVectorDimension>::RealValueType
>> Vector<T, TVectorDimension>
>> ::GetNorm( void ) const
>> {
>>  double sq_norm = this->GetSquaredNorm();
>>  if( sq_norm > 0. )
>>    {
>>    return RealValueType( vcl_sqrt( sq_norm ) );
>>    }
>>  else
>>   {
>>   return static_cast<RealValueType >( sq_norm );
>>   }
>> }
>>
>> or
>>
>> double sq_norm = this->GetSquaredNorm();
>> return ( sq_norm > 0. ) : static_cast<RealValueType  
>> >( vcl_sqrt( sq_norm ) ), sq_norm;
>>
>> Any objection?
>>
>> Arnaud
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