[Numpy-discussion] nd_image.affine_transform edge effects

Zachary Pincus zpincus@stanford....
Fri Mar 23 11:28:15 CDT 2007


Hello again,

On Mar 23, 2007, at 7:56 AM, Travis Oliphant wrote:
>> I'm not sure what people normally do in computer graphics, since I'm
>> working more with natural band-limited images, but in the case of
>> Stefan's "rotate_artifacts" example, wouldn't it be appropriate to
>> use the 1st- or 0th-order spline instead of the 2nd order? If your
>> real-life data are smooth enough, however, then in theory the
>> ringing with higher orders should go away.
>>
>>
> Yes, that is true.  But, you really shouldn't see much ringing with a
> 3rd order spline, though.

There are a couple of different examples of ringing that we've seen  
(specifically, the first and third attachments to scipy ticket 213:
http://projects.scipy.org/scipy/scipy/ticket/213

The first was on Stefan's artificial data which had sharp edges, and  
got very nasty ringing artifacts even with 3rd order splines. From  
your recollection, is this expected behavior based on splines and the  
nature of Stefan's image, or more likely to be a bug?

The second example is rotated version of Lena -- an image with  
natural and much smoother edges -- where 3rd order spline  
interpolation still introduced strong ringing (or something nasty).  
This looks more like a bug, but (as per this discussion) my insight  
is clearly limited in this case.

Zach


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