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

[kent williams]

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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