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Wed, 22 Mar 2017 19:45:35 -0700

OK, this is embarrassing.
I had in fact made the kernel only almost symmetric, it was slightly offset to one side.
This caused it. Only a wetware bug.
Sorry for having wasted your time!


On 03/23/2017 03:38 AM, josef.p...@gmail.com wrote:


On Wed, Mar 22, 2017 at 10:06 PM, Th?ger Emil Rivera-Thorsen <thoger.e...@gmail.com <mailto:thoger.e...@gmail.com>> wrote:

    Dear list;

    I am honestly not certain whether this, or the SciPy list, is the
    appropriate place to post this; please let me know if I got it wrong.

    I am convolving a 1D data set containing a relatively narrow peak,
    with a relatively narrow Gaussian kernel, in order to emulate the
    effect of atmospheric seeing on astrophysical observations.

    I have a 1D data array 45 pixels long, and a Gaussian kernel, and
    run np.convolve(data, kernel, mode='same') on the two arrays, the
    resulting array's peak is shifted relative to the origin. I have
    attached a plot to illustrate.

    The original data is shown in blue. When I convolve it with a
    symmetric kernel (black), I get an offset resulting peak
    (magenta). If I flip the kernel -- even though it is perfectly
    symmetric -- the resulting curve is offset in the opposite
    direction (yellow). However, if I offset the kernel so it is
    centered exactly one pixel below the central value, the output
    array gets centered correct (red), even if I flip the (now no
    longer symmetric) kernel.

    This is using Numpy 1.11.3, python 2.7.13, on Anaconda 4.3.0
    64-bit on Ubuntu 16.10

    Using astropy.convolution, reproduces the correct red curve, so I
    can use that for now, but it seems to me this is either a bug or,
    if it is indeed the intended behavior, a word of caution would be
    merited in the docstring.

    Cheers,

    Emil Rivera-Thorsen



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Can you provide an example to replicate?

I haven't seen this behavior, it looks centered to me, at least for odd window length.. AFAIR, I had to try a bit in the past for how to set the window and location with even window length.

>>> np.__version__
'1.11.2'

>>> x = np.linspace(-1, 1, 21)
>>> w = stats.norm.pdf(np.linspace(-3, 3, 5))
>>> np.column_stack((x, np.convolve(x, w, mode='same')))[8:13]
array([[ -2.00000000e-01,  -1.33368234e-01],
       [ -1.00000000e-01,  -6.66841169e-02],
       [  0.00000000e+00, 1.51788304e-17],
       [  1.00000000e-01, 6.66841169e-02],
       [  2.00000000e-01, 1.33368234e-01]])


>>> x = np.abs(np.linspace(-1, 1, 21))
>>> w = stats.norm.pdf(np.linspace(-3, 3, 4))
>>> np.column_stack((x, np.convolve(x, w, mode='same')))[8:13]
array([[ 0.2       ,  0.12320129],
       [ 0.1       ,  0.07392077],
       [ 0.        ,  0.02552663],
       [ 0.1       ,  0.02552663],
       [ 0.2       ,  0.07392077]])


>>> w = stats.norm.pdf(np.linspace(-3, 3, 5))
>>> np.column_stack((x, np.convolve(x, w, mode='same')))[8:13]
array([[ 0.2       ,  0.13336823],
       [ 0.1       ,  0.06757049],
       [ 0.        ,  0.02767626],
       [ 0.1       ,  0.06757049],
       [ 0.2       ,  0.13336823]])

Josef


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