[Numpy-discussion] Is there a defined way to "unpad" an array, and if not, should there be?

Evgeni Burovski evgeny.burovskiy at gmail.com
Tue Apr 13 04:39:26 EDT 2021


Prior art data point: scikit-image.util.crop :

https://scikit-image.org/docs/dev/api/skimage.util.html#skimage.util.crop



вт, 13 апр. 2021 г., 11:37 Eric Wieser <wieser.eric+numpy at gmail.com>:

> Some other options here that avoid the need for a new function:
>
> * Add a `return_view` argument to `pad`, such that for `padded, orig =
> np.pad(arr, ..., return_view=True)`, `orig == arr` and `orig.base is
> padded`. This is useful if `padded` is modified in place, but less useful
> otherwise. It has the advantage of not having to recompute the slices, as
> pad already has them.
> * Accept a `slice` object directly in `np.pad`; for `sl = np.s_[2:-20,
> 4:-40]`, `padded = np.pad(array, sl)`, we have `padded[sl] == array`.
>
> The second idea seems promising to me, but perhaps there are corner cases
> I haven't thought of that it wouldn't help with.
>
> Eric
>
> On Tue, 13 Apr 2021 at 09:26, Ralf Gommers <ralf.gommers at gmail.com> wrote:
>
>>
>>
>> On Tue, Apr 13, 2021 at 3:37 AM Jeff Gostick <jgostick at gmail.com> wrote:
>>
>>> It is great to hear that this might be useful.  I would LOVE to create a
>>> PR on this idea and contribute back to numpy...but let's not get ahead of
>>> ourselves :-)
>>>
>>> Regarding the name, I kinda like "unpad" since it relates directly to
>>> "pad", analogous to "ravel" and "unravel" for instance.  Or maybe "depad".
>>> Although, it's possible to use this on any array, not just a previously
>>> padded one, so maybe tying it too directly to "pad" is not right, in which
>>> case "trim" and "crop" are both perfect.  I must admit that I find it odd
>>> that these functions are not in numpy already.  I just searched the docs
>>> and they show up as keyword args for a few functions but are otherwise
>>> conspicuously absent.  Also, funnily, there is a link to "padding arrays"
>>> but it is basically empty:
>>> https://numpy.org/doc/stable/reference/routines.padding.html.
>>>
>>> Alternatively, I don't hate the idea of passing negative pad widths into
>>> "pad".  I actually tried this at one point to see if there was a hidden
>>> functionality there, to no avail.
>>>
>>> BTW, we just adding a custom "unpad" function to our PoreSpy package for
>>> this purpose:
>>> https://github.com/PMEAL/porespy/blob/dev/porespy/tools/_unpadfunc.py
>>>
>>>
>>>
>>> On Mon, Apr 12, 2021 at 9:15 PM Stephan Hoyer <shoyer at gmail.com> wrote:
>>>
>>>> On Mon, Apr 12, 2021 at 5:12 PM Jeff Gostick <jgostick at gmail.com>
>>>> wrote:
>>>>
>>>>> I guess I should have clarified that I was inquiring about proposing a
>>>>> 'feature request'.  The github site suggested I open a discussion on this
>>>>> list first.  There are several ways to effectively unpad an array as has
>>>>> been pointed out, but they all require more than a little bit of thought
>>>>> and care, are dependent on array shape, and honestly error prone.  It would
>>>>> be very valuable to me to have such a 'predefined' function, so I was
>>>>> wondering if (a) I was unaware of some function that already does this and
>>>>> (b) if I'm alone in thinking this would be useful.
>>>>>
>>>>
>>>> Indeed, this is a fair question.
>>>>
>>>> Given that this is not entirely trivial to write correctly, I think it
>>>> would be reasonable to add the inverse operation for pad() into NumPy. This
>>>> is generally better than encouraging users to write their own thing.
>>>>
>>>> From a naming perspective, here are some possibilities:
>>>> unpad
>>>> trim
>>>> crop
>>>>
>>>> I think "trim" would be pretty descriptive, probably slightly better
>>>> than "unpad."
>>>>
>>>
>> I'm not a fan of `trim`. We already have `clip` which sounds similar.
>>
>> `unpad` looks like the only one that's completely unambiguous.
>>
>> `crop` sounds like an image processing function, and what we don't want
>> is something like Pillow's `crop(left, top, right, bottom)`.
>>
>> Cheers,
>> Ralf
>>
>>
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