[Numpy-discussion] nd_image.affine_transform edge effects

[Travis Oliphant]

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. 

-Travis