The implementation of the B-spline filter for image
scaling in the ILU source (1.6.8-RC1) is faulty,
because the cubic B-spline kernel is used as a
convolution kernel for convolution with the original
image data, when it should operate on 'control points'
generated from the image.
This leads to a strong blurring effect as you can also
see in the examples in the manual
and renders the B-splines unusable.
To implement real B-splines, the image data has to be
prefiltered to create so called control points. You
could have a look at the following paper and code:
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