https://en.wikipedia.org/wiki/Structuring_element
Structuring element - Wikipedia
structuring elementwikipedia
https://www.mathworks.com/help/images/ref/offsetstrel.html
offsetstrel - Morphological offset structuring element - MATLAB
An offsetstrel object represents a nonflat morphological structuring element, which is an essential part of morphological dilation and erosion operations.
structuring elementmorphologicaloffsetmatlab