[Insight-users] VectorImage Orientation from NRRD

[Daniel Betz]

Hello Gordon,

VectorImage works in relation to OrientedImage the same as Image.
Both, Image and VectorImage, inherit from ImageBase.

To decree orientation to be LPS has for sure a lot of benefits
for a format like DICOM.

I read itkNrrdImageIO.cxx and you could be right, even though I
never read about such an decree for itk::Image to be in LPS space.

In opposite to make such an decree to a format like DICOM, I think
it doesn't make sense for a class like itk::Image. The reason is
is simply that there are also formats without prescribed orientation.

In my case, I have some world coordinates which I need to process
further in the itk::Image. Then itkNrrdImageIO.cxx flips some signes
around and my point in world coordinates point to another place of
the object.

I can't even use the MetaDataDictionary to get the initial coordinate
system. So I need to get the initial world coordinate system from
outside and then must flip back the signes on origin, direction and
measurement frame.

itkNrrdImageIO.cxx does this "correction" only for RAS and LAS
coordinates, because, quoting an comment from itkNrrdImageIO.cxx,
only for that the conversion to LAS is well-defined. Whatever
"well-defined" means.

Is this intended to prevent wrong-side surgery?
I think it does the opposite.

Any clarification is really appreciated.

> As for the spacing- I'd say its right: the spacings as far as ITK is  
> concerned are supposed to be (I think!) positive, and its the job of  
> the (unit-length) direction cosines to indicate how the samples are  
> arranged along the axes.

Yes thats right. itk::OrientedImage has a transformation matrix named
m_IndexToPhysicalPoint which contains direction * diag(spacing). Image
and VectorImage ignore direction at the moment.

Regards,
Daniel Betz
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