There is no "middle pixel". The number of columns in the image is even, so there are two "middle pixels". Also, what's with all the weird, esoteric terminology with no definitions given? If you're going to use incredibly specific terminology to the field at hand, at least provide a link to definitions of your terms if not placing the definitions directly in the problem description. ie, what does "structuring element" mean, and what does "applying" it mean, etc, etc..