[Insight-developers] Coordinate system semantics of ImageIOBase?

[Steve M. Robbins]

On Tue, Jan 13, 2009 at 07:33:32AM -0500, Rupert Brooks wrote:

> >> Just a couple questions:
> >>
> >
> >> MINC and ITK use different ways of specifying the origin / offset of
> >> the coordinate system.  I believe
> >> that MINC uses the xstart, ystart, zstart as the distance _along the
> >> x,y,z direction cosines_, while
> >> ITK places the origin _such that pixel 0,0,0 is at the origin_.
> >
> > If that's true, then it is my misunderstanding.
> >
> > Since ImageIOBase::GetOrigin() is not documented, I asked this list
> > about it:
> >
> > ME:
> >> The method GetOrigin(int i) returns a double; I assume that
> >> GetOrigin(0), GetOrigin(1), GetOrigin(2) return the column,
> >> row, and slice, respectively, of the world-space origin.
> >> Is that correct?
> >>
> >
> > Louis responded "Yes"; c.f.
> > http://www.itk.org/mailman/private/insight-developers/2008-December/011435.html
> >
> > To me, that means that ITK's GetOrigin() is the same notion as MINC's
> > "starts".  If these are not identical, then I need a better description
> > of what GetOrigin() returns.
> 
> I think this conversation is about the ITK origin - which is the location of the
> pixel zero.   However - i think that the wording is funny -


Agreed: my wording is clumsy.  Let me try to phrase it more precisely.

Suppose we have read a 3D image using some subclass of itk::ImageIOBase.

Let T be the voxel-to-world transformation given by the metadata of
the image; i.e. the direction cosines & etc.

Let x be the 3-vector 

    x  = (GetOrigin(0), GetOrigin(1), GetOrigin(2))

My interpretation is that T(x) = (0,0,0).

I believe that your interpretation is that T(0,0,0) = x.


ITK-gurus: which is correct?

Thanks,
-Steve
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