Why have different orientations and positions for same point clouds?

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Hello

I would like to get the orientation and position od certain objects from point clouds. So, first I cropped the scene using the PCL CropBox filter like in PCL pcl::CropBox. Then used the PCL Moment of Inertia like in this PCL Tutorial link text. This is my code that compiles without errors. I include the whole point cloud scene and the cropped Box one pictures. Here the C++ ROS code first

typedef pcl::PointXYZRGB PointC;
typedef pcl::PointCloud<pcl::PointXYZRGB> PointCloudC;

void cloud_cb (const sensor_msgs::PointCloud2& msg)
{

  PointCloudC::Ptr cloud(new PointCloudC());
  pcl::fromROSMsg(msg, *cloud);


  //Crop Box
  double min_x, min_y, min_z, max_x, max_y, max_z;
  ros::param::param("crop_min_x", min_x,  0.6416);
  ros::param::param("crop_min_y", min_y, -0.351);
  ros::param::param("crop_min_z", min_z, -0.13453);
  ros::param::param("crop_max_x", max_x, 0.97985);
  ros::param::param("crop_max_y", max_y,  -0.0371288);
  ros::param::param("crop_max_z", max_z,  0.1379478);
  Eigen::Vector4f min_pt(min_x, min_y, min_z, 1);
  Eigen::Vector4f max_pt(max_x, max_y, max_z, 1);
  pcl::CropBox<PointC> crop;
  crop.setInputCloud(cloud);
  crop.setMin(min_pt);
  crop.setMax(max_pt);

  PointCloudC::Ptr cropped_cloud(new PointCloudC());
  crop.filter(*cropped_cloud);

  sensor_msgs::PointCloud2 output;
  pcl::toROSMsg(*cropped_cloud, output);
  pub.publish(output);

  //Moment of Inertia
  pcl::MomentOfInertiaEstimation <pcl::PointXYZRGB> feature_extractor;
  feature_extractor.setInputCloud (cropped_cloud);
  feature_extractor.compute ();

  std::vector <float> moment_of_inertia;
  std::vector <float> eccentricity;

  pcl::PointXYZRGB min_point_OBB;
  pcl::PointXYZRGB max_point_OBB;
  pcl::PointXYZRGB position_OBB;
  Eigen::Matrix3f rotational_matrix_OBB;
  float major_value, middle_value, minor_value;
  Eigen::Vector3f major_vector, middle_vector, minor_vector;
  Eigen::Vector3f mass_center;

  feature_extractor.getMomentOfInertia (moment_of_inertia);
  feature_extractor.getEccentricity (eccentricity);
  feature_extractor.getOBB (min_point_OBB, max_point_OBB, position_OBB, rotational_matrix_OBB);
  feature_extractor.getEigenValues (major_value, middle_value, minor_value);
  feature_extractor.getEigenVectors (major_vector, middle_vector, minor_vector);
  feature_extractor.getMassCenter (mass_center);

  Eigen::Vector3f position (position_OBB.x, position_OBB.y, position_OBB.z);
  Eigen::Quaternionf quat (rotational_matrix_OBB);

  cout << " orientation x  = " << quat.x() <<  endl;
  cout << " orientation y = "  << quat.y() <<  endl;
  cout << " orientation z = "  << quat.z() <<  endl;
  cout << " orientation w = "  << quat.w() <<  endl;
  cout << " postion x  = " << position_OBB.x <<  endl;
  cout << " postion y  = " << position_OBB.y <<  endl;
  cout << " postion z  = " << position_OBB.z <<  endl;


}
int main(int argc, char ** argv)
{
    ros::init(argc, argv, "test_pcl_ros");
    ros::NodeHandle nh;
    ros::Subscriber sub = nh.subscribe("/zed/zed_node/point_cloud/cloud_registered", 10, cloud_cb);
    pub = nh.advertise<sensor_msgs::PointCloud2> ("output", 10);
    ros::spin();
}

Here the point cloud images. First the whole scene. Then the cropped Box. Here the output:

orientation x  = -0.381498
 orientation y = 0.378691
 orientation z = 0.746498
 orientation w = -0.392165
 postion x  = 0.73152
 postion y  = -0.175997
 postion z  = 0.00919689
 orientation x  = -0.385631
 orientation y = 0.426972
 orientation z = 0.682854
 orientation w = -0.450215
 postion x  = 0.74303
 postion y  = -0.180681
 postion z  = 0.000638498
 orientation x  = -0.370232
 orientation y = 0.333529
 orientation z = 0.798585
 orientation w = -0.337563
 postion x  = 0.730808
 postion y  = -0.171258
 postion z  = 0.0171869
 orientation x  = -0.377332
 orientation y = -0.371774
 orientation z = 0.397294
 orientation w = 0.749374

Basically the orientation w change. But why? Or should I do first planar ... (more)