[ITK-users] VectorImage vs image of vectors

[Williams, Norman K]

2 observations:

1. I think the redundant SetSize you mentioned is almost certainly
unnecessary.  I¹ve seen similar unnecessary vector/array sizing other
places in ITK; I think it arose out of an abundance of caution, and just
never showed up on anyone¹s optimization radar yet.

2. There¹s a technique you can use to get around the
itk::Image<itk::Vector<TScalar,TVecLength>, TDim> problem, if there is a
small, finite number of possible vector lengths.  Put the code using the
vector image inside a function templated over TVecLength.  Then at
runtime, determine the vector length and invoke the template function with
that vector length.

E.G.

template<typename TScalar,unsigned NVecLength, unsigned NDim>
void DoSomething()
{
  typedef itk::Image< itk::Vector<TScalar, NVecLength>, NDim > ImageType;
        ...
}

int main(int,char *[])
{
  ...
  switch(vecLength)
  {
  case 3:
     DoSomething<double,3,3>();
     break;
  case 11:
     DoSomething<double,11,3>();
  Š
  }
}

This is of course in most cases bad C++ programming practice, but it does
allow you to accommodate different data types in a program.

On 11/3/14, 8:39 AM, "Olivier Commowick" <Olivier.Commowick at inria.fr>
wrote:

>Hello all,
>
>I encountered some weird behavior in ITK. For some reason, I have in my
>code some vector images that I need to smooth using a Gaussian filter. It
>appears that whether the image is stored as a VectorImage or an Image of
>itk Vectors is actually extremely important in terms of performance for
>this task. Namely, if using VectorImages, the processing time can be much
>worse (30s vs 1s with an image of vectors). My problem is that I cannot
>really use the templated itk::Image <itk::Vectors <Type, Size> , Ndim >
>since I do not know Size at compilation time.
>
>Looking deeper into the code, I have two questions for whoever would know
>about these images:
>- the first one is on the Variable length vector: it seems to me that the
>operator= method always starts by destroying current data, even if the
>vector size is the same. It doesn¹t seem really efficient so I was
>wondering if there was any reason for that
>- the second one is on the VectorImage itself: is there something in it
>that would justify the computation times to be much worse ?
>
>If someone has any info on which image type I should be using or why
>VectorImage would be slower, I would be grateful.
>
>Thanks in advance
>
>---
>
>Olivier Commowick, Ph.D.
>Research Scientist
>INRIA Rennes - Bretagne Atlantique, VISAGES Team
>Campus de Beaulieu
>35042 Rennes
>FRANCE
>
>Phone: +33 2 99 84 25 92
>Email: Olivier.Commowick at inria.fr
>Web: http://olivier.commowick.org/
>
>
>
>
>
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