title: Erose/Dilate image with zeros structuring element
body: <p>If I have a structuring element looks like this one (the origin is at the center of SE):</p>
  
  <pre><code>0  0  0
  0  0  0 
  0  0  0
  </code></pre>
  
  <p>If I perform the erosion/dilation on an binary image, the result turns out all 0 or 1. 
  Can someone please explain this one to me? (Check by matlab)
  <br>Thank you very much.</p>