[SciPy-Dev] Splines
denis
denis-bz-py@t-online...
Mon Mar 18 12:17:20 CDT 2013
Charles R Harris <charlesr.harris <at> gmail.com> writes:
> On Wed, Mar 13, 2013 at 12:54 PM, denis <denis-bz-py <at> t-online.de> wrote:
> Chuck,
> is "prefilter=True will filter out high frequencies in the data"
> not correct ?
>
> No, I suspect it increases the high frequencies ;) For cubic splines
convolving the coefficients with [1/6, 2/3, 1/6] will reproduce the sample
values, so to get the spline coefficients you need to deconvolve what is
essentially a low pass filter.
Chuck,
you're right; could spline_filter1d be buggy ?
f .5 x [ 100 -100 100 -100 100 -100 100 -100 100 -100]
np.convolve: [ 50 -33 33 -33 33 -33 33 -33 33 -50]
ndimage.convolve: [ 50 -33 33 -33 33 -33 33 -33 33 -50]
ndimage.spline order 2: [ 200 -200 200 -200 200 -200 200 -200 200 -200]
ndimage.spline order 3: [ 300 -300 300 -300 300 -300 300 -300 300 -300]
ndimage.spline order 4: [ 480 -480 480 -480 480 -480 480 -480 480 -480]
NI_SplineFilter1D in $scipysrc/ndimage/src/ni_interpolation.c has
case 2:
npoles = 1;
pole[0] = sqrt(8.0) - 3.0;
break;
case 3:
npoles = 1;
pole[0] = sqrt(3.0) - 2.0;
beyond me,
spline_filter1d( ones ) is ok though.
cheers
-- denis
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