title: Why Sobel operator looks that way?
body: <p>For image derivative computation, Sobel operator looks this way:</p>
  
  <pre><code>[-1 0 1]
  [-2 0 2]
  [-1 0 1]
  </code></pre>
  
  <p>I don't quite understand 2 things about it,</p>
  
  <p>1.Why the centre pixel is <strong>0</strong>? Can't I just use an operator like below,</p>
  
  <pre><code>[-1 1]
  [-1 1]
  [-1 1]
  </code></pre>
  
  <p>2.Why the centre row is 2 times the other rows?</p>
  
  <p>I googled my questions, didn't find any answer which can convince me. Please help me.</p>