title: Why does the focal length in the camera intrinsics matrix have two dimensions?
body: <p>In the <a href="http://en.wikipedia.org/wiki/Pinhole_camera_model" rel="noreferrer">pinhole camera model</a> there is only one focal length which is between the principal point and the camera center.</p>
  
  <p>However, after calculating the camera's intrinsic parameters, the matrix contains </p>
  
  <pre><code>(fx,  0,  offsetx,  0,
   0,  fy,  offsety,  0,
   0,   0,  1,        0)
  </code></pre>
  
  <p>Is this because the pixels of the image sensor are not square in x and y?</p>
  
  <p>Thank you.</p>