title: Down sampling point clouds with open3d
body: <p>I'm using the python bindings of <a href="http://www.open3d.org/docs/release/" rel="nofollow noreferrer">open3d</a> to down sample a point cloud. The library offers two methods to do so using voxels: <a href="http://www.open3d.org/docs/release/python_api/open3d.geometry.voxel_down_sample.html" rel="nofollow noreferrer">voxel_down_sample</a> and <a href="http://www.open3d.org/docs/release/python_api/open3d.geometry.voxel_down_sample_and_trace.html" rel="nofollow noreferrer">voxel_down_sample_with_trace</a></p>
  
  <p>While the first returns only a down sampled point cloud, the latter returns a tuple of the down sampled point cloud and a matrix.</p>
  
  <p>Per the documentation, the matrix is supposed to be a mapping between points from the original point cloud to down sampled one. However, the matrices I get are always of size <code>n-by-8</code>, with <code>n</code>being the number of points in the down sampled version. 
  My question is: how do I interpret that matrix in order to retrieve the mapping between the original and down sampled point clouds?</p>