[Numpy-discussion] nd_image.affine_transform edge effects
Fri Mar 23 09:48:14 CDT 2007
Stefan van der Walt wrote:
>On Thu, Mar 22, 2007 at 04:33:53PM -0700, Travis Oliphant wrote:
>>>I would rather opt for changing the spline fitting algorithm than for
>>>padding with zeros.
>> From what I understand, the splines used in ndimage have the implicit
>>mirror-symmetric boundary condition which also allows them to be
>>computed rapidly. There may be ways to adapt other boundary conditions
>>and maintain rapid evaluation, but it is not trivial as far as I know.
>>Standard spline-fitting allows multiple boundary conditions because
>>matrix inversion is used. I think the spline-fitting done in ndimage
>>relies on equal-spacing and mirror-symmetry to allow simple IIR filters
>>to be used to compute the spline coefficients very rapidly.
>Thanks, Travis. I wasn't aware of these restrictions.
>Would it be possible to call fitpack to do the spline fitting? I
>noticed that it doesn't exhibit the same mirror-property:
It doesn't. It uses standard spline-fitting techniques (which are also
slower). So, yes you could call fitpack but for a large
multi-dimensional array it's going to take longer.
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