[vtk-developers] vtkImageData, C versus Fortran ordering

[Marcus D. Hanwell]

On Wed, Jul 19, 2017 at 12:35 PM, David Thompson <david.thompson at kitware.com
> wrote:

> Hi Marcus,
>
> > We work on Tomviz, and one of its primary goals is to work with volumes
> in an intuitive way making heavy use of the volume rendering code. It also
> needs to interact with several libraries that operate on volumes, ideally
> using our Python NumPy integration and zero copy approaches to process data
> in place.
> >
> > One of the challenges has been preserving the Fortran ordering of the
> single component scalar data. We will also be adding support for
> multi-component data soon. What are people's thoughts on potentially adding
> some API to vtkImageData to mark it as C ordered?
> >
> > That would largely just geometrically transform how the volume is
> rendered, but ensure a contour is correctly extracted and aligned. It would
> then let us use the default C ordering, simplifying our application code,
> and the current order could remain the default. I would suggest considering
> the same for the vtk.js work too.
> >
> > It is feasible to make these changes in the application logic too, but
> this feels like a fairly common problem that others would also hit from
> time to time. Maybe I am missing more corner cases, so I am posting this in
> hopes of feedback before writing any code (or asking someone else to).
>
> I think a common problem would be that many image filters choose a
> particular order to process data because access is much faster in one plane
> than others. Are you thinking that those filters would also adjust their
> algorithms/traverse order to accommodate FORTRAN ordering when it is
> present on the input (and presumably output)?


My ideal scenario is to mark a vtkImageData as C ordered (default it to
Fortran/what it is now). It may be easier to manage more of that at an
application level though too, I was more thinking out loud and wondering if
other people had explored this or solved it in other ways. I get the
historical perspective, and possibly that what I want may not be needed by
the wider VTK community.

I would like to leave as much of VTK alone as possible, and so let most VTK
stuff continue as it always has, but be able to pass a vtkImageData to a 3D
NumPy array as C ordered, operate on it, and then render the result. We
currently go to great pains to ensure if comes back as Fortran ordered, and
there is no way to set a default. I also find that most stuff assumes the C
ordering, so it can require some reshuffling of the data which can involve
large volumes.
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