[Insight-developers] Documentation on Spacing and Orientation.

[Williams, Norman K]

This came up in some work I was doing last week, which was causing me some
confusion.

Namely, the Origin of an image is a point in physical space, corresponding
to the voxel at index (0,0,[0,0,...]).

This was confusing me because if I need to use an individual component of
the origin to do something (unavoidably) in Index space, I needed to know
the mapping of XYZ to IJK as it were, which is dependent on the voxel
organization, which is dependent on the direction cosines.

So, a similar issue arises with Image Spacing.  Reading the source code,
I'm confused whether the spacing is in Index or Physical space.  Which is
it?

My confusion might indicate some shortcomings in my mathematical education
and comprehension, but I'd like to make these points clear in the Doxygen
comments, for people who might want to use ITK but are even more confused
than I am.