title: Hough Line gives negative rhos
body: <p>I'm trying to generate some Hough Lines using <code>HoughLines()</code>,l but some of the lines have negative <code>rho</code> values like:</p>
  
  <p><code>array([[-58.,   3.07177949]], dtype=float32))</code></p>
  
  <p><code>array([[-56.,   3.05432606]], dtype=float32))</code></p>
  
  <p>Since <code>rho = sqrt(x^2 + y^2)</code>, how come <code>rho</code> can ever be negative? Is the origin the centre of the image?</p>