Merge connectivity resolving helper functions in morphology._util

[lagru]

Right now there are

scikit-image/skimage/morphology/_util.py

Lines 8 to 29 in 9aac849

	def _validate_connectivity(image_dim, connectivity, offset):
	"""Convert any valid connectivity to a structuring element and offset.
	Parameters
	----------
	image_dim : int
	The number of dimensions of the input image.
	connectivity : int, array, or None
	The neighborhood connectivity. An integer is interpreted as in
	``scipy.ndimage.generate_binary_structure``, as the maximum number
	of orthogonal steps to reach a neighbor. An array is directly
	interpreted as a structuring element and its shape is validated against
	the input image shape. ``None`` is interpreted as a connectivity of 1.
	offset : tuple of int, or None
	The coordinates of the center of the structuring element.
	Returns
	-------
	c_connectivity : array of bool
	The structuring element corresponding to the input `connectivity`.
	offset : array of int
	The offset corresponding to the center of the structuring element.

and

scikit-image/skimage/morphology/_util.py

Lines 129 to 156 in 9aac849

	def _resolve_neighborhood(selem, connectivity, ndim):
	"""Validate or create structuring element.
	Depending on the values of `connectivity` and `selem` this function
	either creates a new structuring element (`selem` is None) using
	`connectivity` or validates the given structuring element (`selem` is not
	None).
	Parameters
	----------
	selem : ndarray
	A structuring element used to determine the neighborhood of each
	evaluated pixel (``True`` denotes a connected pixel). It must be a
	boolean array and have the same number of dimensions as `image`. If
	neither `selem` nor `connectivity` are given, all adjacent pixels are
	considered as part of the neighborhood.
	connectivity : int
	A number used to determine the neighborhood of each evaluated pixel.
	Adjacent pixels whose squared distance from the center is less than or
	equal to `connectivity` are considered neighbors. Ignored if
	`selem` is not None.
	ndim : int
	Number of dimensions `selem` ought to have.
	Returns
	-------
	selem : ndarray
	Validated or new structuring element specifying the neighborhood.

which have very much in common and should probably be merged.

[jni]

Ha, love it. They are so similar. Thanks for spotting this @lagru, and I agree that they should be merged. We need to decide on selem vs footprint vs structure vs neighborhood, though. 😂 See #4154.

[jni]

fwiw my preference is "neighborhood".

[stefanv]

Elsewhere we had neighbors, which has now become connectivity, so I guess technically neighborhood is free; but it's not the most intuitive naming to me.

Structuring element is technically a correct naming, but the selem shortening isn't very appealing.

footprint would be consistent with SciPy; but, note that SciPy allows for a combination of footprint (which values are taken into consideration) + structure (which can be grey level). I think we only allow 1s and 0s in our structuring elements, so in that sense structure is appealing as the "combination" of footprint+structure.