[Insight-developers] ItkImage check for non-orthogonal direction matrix breaks itkExtractImageFilter

[kent williams]

So if I take an image whose direction cosines are:
    {{1, 0, 0}, {0, 0, -1}, {0, -1, 0}}
And I try to use the itk::ExtractImageFilter to extract this region from it:
{
  m_Index = { m_Index = {0, 0, 3} },
  m_Size = { m_Size = {5, 5, 0} }
}

It throws an exception, because it tries to set these 2D direction cosines
on the output image

{{1, 0},{0,0}}

This is the exception thrown:

itk::ExceptionObject (0x76cff0)
Location: "void itk::Image<TPixel, VImageDimension>::SetDirection(typename
itk::ImageBase<VImageDimension>::DirectionType) [with TPixel = unsigned
char, unsigned int VImageDimension = 2u]"
File: 
/scratch/kent/brains2/MACOSX/DEBUG/src/include/InsightToolkit/Common/itkImag
e.h
Line: 364
Description: itk::ERROR: Image(0x76eb50): Bad direction, determinant is 0.
Direction is 1 0
0 0

I can patch the ExtractImageFilter to do something to satisfy the test for
orthogonal directions in itk::Image::SetDirection, as I did the places in
the Insight/Code/IO tests and Insight/Code/IO already.

But the question is this: where does it end?  Do we patch every place that
sets 2D direction cosines from the 3Dimensional direction cosines?

And by patch I mean what I did other places -- fake up nonsense 2D dir
cosines so that their determinant is non-zero.

Before the test of the determinant of the dir cosines was added, a lot of
code was written and apparently passed regression. This was the case by
accident in ITK -- the old tests created images without ever setting the dir
cosines, which left them at identity, and the identity dir cosines are by
happenstance {{1,0,0},{0,1,0},{0,0,1}}.  The identity dir cosines have an
advantage in that lopping off a dimension still left valid 2D Direction
Cosines.



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