title: How to set the "pcl::PointIndices" with the PCD file (existing point cloud sets)
body: <p>I'm trying to implement a pointcloud background subtraction. 
  (eg. background.pcd = input.pcd - object.pcd)</p>
  
  <p>I found following code </p>
  
  <pre><code>  pcl::PointCloud&lt;pcl::PointXYZ&gt;::Ptr p_obstacles(new pcl::PointCloud&lt;pcl::PointXYZ&gt;);
    pcl::PointIndices::Ptr inliers(new pcl::PointIndices());
    pcl::ExtractIndices&lt;pcl::PointXYZ&gt; extract;
  
  // Insert something. 
  
    extract.setInputCloud(p_obstacles);
    extract.setIndices(inliers);
    extract.setNegative(true);
    extract.filter(*p_obstacles);
  </code></pre>
  
  <p><a href="https://stackoverflow.com/questions/44921987/removing-points-from-a-pclpointcloudpclpointxyzrgb#">Removing points from a pcl::PointCloud&lt;pcl::PointXYZRGB&gt;</a></p>
  
  <p>In my understating, the <code>inliers</code> is subtracted from the input.pcd 
  (inliers = <code>object.pcd</code> ??)</p>
  
  <p>I have no idea how to set the inliers value with the pre-existing x,y,z values(pcd)</p>
  
  <p>Thank you!</p>