Cubic interpolation

Nil Goyette <[email protected]>
Newsgroups gmane.comp.python.scientific.user
Message-ID <[email protected]>
Hi all,

I'm trying to port `interpolation.affine_transform` [1] to another 
langage and I have a question about your algorithm. I successfully 
ported order 1 (linear) interpolation from another lib, but I failed on 
order 3 (cubic).

I tried reading the interpolation code (zoom shift) [2] but it's no 
simple task. So, I took vtk implementation [3] and ported it, but I 
can't get your results. I was surprised because it's a mathematical 
formula, it should give the same result wathever the algorithm.

So, my question is: *is there something special with SciPy's cubic 
interpolation?* I don't get the feeling that the code fits the 
description in wikipedia [4]. Also, I saw the `spline_filter` function, 
I ported it and but I still can't get the same results, disabled or not.

Nil Goyette

[1] 
https://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.ndimage.interpolation.affine_transform.html
[2] 
https://github.com/scipy/scipy/blob/master/scipy/ndimage/src/ni_interpolation.c#L573
[3] 
https://gitlab.kitware.com/vtk/vtk/blob/master/Imaging/Core/vtkImageInterpolator.cxx#L325
[4] https://en.wikipedia.org/wiki/Tricubic_interpolation

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