Re: Possibly reduce duplication between `signal` and `ndimage` filtering routines

On Tue, Feb 21, 2023 at 8:12 PM Evgeni Burovski <[email protected]>
wrote:

> Hi,
>
> In `scipy.signal` and `scipy.ndimage`, we ship a set of
> almost-if-not-completely overlapping functionality. For instance,
> - `signal.order_filter` has a counterpart, `ndimage.rank_filter`;
> - `signal.median_filter` can be implemented in terms of the rank filter
>
> In fact, applying the patch below (the patch is based on the CuPy
> implementation of `cupyx.signal`, see
>
> https://github.com/cupy/cupy/blob/v11.5.0/cupyx/scipy/signal/_signaltools.py#L490
> ),
> and running `$ python dev.py test -s signal`, I only get three
> failures in `medfilt`, related to
> - object arrays of `Decimal` instances
> - `longdouble` arrays.
>
> So, assuming we can deprecate longdoubles and decimals in these
> filters, we could potentially remove some 700 lines of obscure C code
> in https://github.com/scipy/scipy/blob/main/scipy/signal/_lfilter.c.in
>
> Thoughts?
>

+1 this sounds like a healthy idea. There's a lot of duplication in these
types of routines, and it's quite under-maintained code. And these
deprecations sound perfectly fine.

Cheers,
Ralf



> Evgeni
>
>
> P.S. The patch to check the mapping between
> signal.{order,median}_filter and ndimage.rank_filter:
>
>
> ```diff
> diff --git a/scipy/signal/_signaltools.py b/scipy/signal/_signaltools.py
> index 31baba2421..2f00ead3a2 100644
> --- a/scipy/signal/_signaltools.py
> +++ b/scipy/signal/_signaltools.py
> @@ -1492,7 +1492,15 @@ def order_filter(a, domain, rank):
>          if (dimsize % 2) != 1:
>              raise ValueError("Each dimension of domain argument "
>                               "should have an odd number of elements.")
> -    return _sigtools._order_filterND(a, domain, rank)
> +
> +    result = _sigtools._order_filterND(a, domain, rank)
> +
> +    import scipy.ndimage as ndimage
> +    import numpy.testing as testing
> +    res_ndi = ndimage.rank_filter(a, rank, footprint=domain,
> mode='constant')
> +    testing.assert_equal(result, res_ndi)
> +
> +    return result
>
>
>  def medfilt(volume, kernel_size=None):
> @@ -1555,7 +1563,17 @@ def medfilt(volume, kernel_size=None):
>
>      numels = np.prod(kernel_size, axis=0)
>      order = numels // 2
> -    return _sigtools._order_filterND(volume, domain, order)
> +    result = _sigtools._order_filterND(volume, domain, order)
> +
> +
> +    import scipy.ndimage as ndimage
> +    import numpy.testing as testing
> +    size = np.prod(kernel_size)
> +    res_ndi = ndimage.rank_filter(volume, size // 2, size=kernel_size,
> +                                mode='constant')
> +    testing.assert_equal(result, res_ndi)
> +
> +    return result
>
>
>  def wiener(im, mysize=None, noise=None):
> ```
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