title: Why are the min and max vectors for pcl::CropBox 4-dimensional?
body: <p>The <code>setMin</code> and <code>setMax</code> methods of the <a href="http://docs.pointclouds.org/trunk/classpcl_1_1_crop_box.html" rel="nofollow noreferrer"><code>pcl::CropBox</code></a> filter each take an <code>Eigen::Vector4f</code> as a parameter.</p>
  
  <p>Why 4f and not 3f? What's the fourth dimension for?</p>