[Insight-developers] itk::ImageBase::TransformIndexToPhysicalPoint/TransformPointToContinuousIndex ignores spacing?

[Luis Ibanez]

Hi Kent,

May it be that you are mixing up expectations between the methods:


    TransformIndexToPhysicalPoint             and
    TransformContinuousIndexToPhysicalPoint ?


as well as


    TransfromPhysicalPointToIndex             and
    TransformPhysicalPointToContinuousIndex ?


Can you post to the list the actual computation that led to you
think that spacing is not being taken into account ?


     Thanks,


         Luis


--------------------------
kent williams wrote:
> It looks like I'm going to have to roll my own functions to go between
> indices and points for the WarpImageFilter because this doesn't work:
> 
> while( !outputIt.IsAtEnd() )
>  {
>   // get the output image index
>   index = outputIt.GetIndex();
>   outputPtr->TransformIndexToPhysicalPoint( index, point );
> 
>   typename DeformationFieldInterpolatorType::ContinuousIndexType defIndex;
>   fieldPtr->TransformPhysicalPointToContinuousIndex(point,defIndex);
>   if(!m_DeformationFieldInterpolator->IsInsideBuffer(defIndex))
>     {
>     itkExceptionMacro(<< "Output location outsde DeformationField"
>                       " Buffer" << point);
>     }
>     // get the required displacement
>     displacement = 
>     m_DeformationFieldInterpolator->EvaluateAtContinuousIndex(defIndex);
>     // stuff removed
>   }
> 
> What I thought would happen (and what the function names seem to imply) is
> that given an IJK index into an image volume, you could get a world
> coordinate system point corresponding to that index.
> 
> And conversely, given a point in the WCS, you can retrieve a corresponding
> continuous index.
> 
> The problem? These functions don't take spacing into account; so the names
> of the functions are false advertising.  So calling
> TransformPhysicalPointToContinuousIndex with physical point in the output
> image doesn't return a continuous index inside the Deformation Field.
> 
> In the particular test I'm working on, the output image has a size of
> 16x16x16 and spacing of 1MM, the deformation field has a size of 8x8x8 and
> spacing of 2MM. 
> 
> Since this behavior has been written in stone for years I don't imagine it
> will be changed.  But it's a bit disappointing to me.
> 
> Is there a way to use the existing ITK physical point/index stuff that takes
> spacing into account?
> 
> 
> 
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