[Insight-developers] Image::DirectionType should be Nx3 rather than NxN?

[Steve M. Robbins]

Howdy,

In the DICOM view of things, the world coordinate system is always 3
dimensional.  Therefore a 2D DICOM image has two 3D direction vectors
(a.k.a. the direction cosines).

The itk::ImageBase::DirectionType, however, is defined as

  /** Direction typedef support.  The Direction is a matix of
   * direction cosines that specify the direction between samples. */
  typedef Matrix<double, VImageDimension, VImageDimension> DirectionType;

So a 2D image is provided with two 2D direction vectors.  That's a bit
odd, I think.  Is it a bug or is there some subtlety I don't understand.
How does one interpret a 4D direction cosine for volumetric time series
data?

I have read 

     http://www.itk.org/Wiki/Proposals:Orientation
and  http://www.insight-journal.org/dspace/handle/1926/145

without gaining any enlightenment.

Thanks,
-Steve
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