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How to read the x,y,z coordinates of point cloud from the LIDAR SICK LMS 200.

asked 2012-08-13 17:31:41 -0500

metal gravatar image

Goal: To employ obstacle avoidance by interpreting the data from the LIDAR using PCL

Description: Hi everyone, I am working with a LIDAR device for the first time. I used the sick toolbox ros wrapper for reading the values from the LIDAR. I converted the "scan" topic given out by the LIDAR device to point-cloud 2 format. here is the program:

#include <ros/ros.h>
#include <sensor_msgs/PointCloud2.h>
#include <pcl/point_types.h>
#include <pcl/filters/voxel_grid.h>
#include <pcl_ros/point_cloud.h>
#include <boost/foreach.hpp>
#include <pcl/io/pcd_io.h>
#include <tf/transform_listener.h>
#include <laser_geometry/laser_geometry.h>

 ros::Publisher pub;

class My_Filter {
        void scanCallback(const sensor_msgs::LaserScan::ConstPtr& scan);
        ros::NodeHandle node_;
        laser_geometry::LaserProjection projector_;
        tf::TransformListener tfListener_;

        ros::Publisher point_cloud_publisher_;
        ros::Subscriber scan_sub_;

        scan_sub_ = node_.subscribe<sensor_msgs::LaserScan> ("/scan", 100, &My_Filter::scanCallback, this);
        point_cloud_publisher_ = node_.advertise<sensor_msgs::PointCloud2> ("/cloud", 100, false);

void My_Filter::scanCallback(const sensor_msgs::LaserScan::ConstPtr& scan){
    sensor_msgs::PointCloud2 cloud;

    projector_.transformLaserScanToPointCloud("/laser", *scan, cloud, tfListener_);

int main(int argc, char** argv)
    ros::init(argc, argv, "my_filter");

    My_Filter filter;


    return 0;

This above node gives out the point cloud 2 topic "cloud". I subscribed to this topic in the following program. I was successful in reading the widht and height of the point clouds,but I need some help reading the X,Y,Z coordinates. I also need to know how to access the other objects which might be useful in data processing.

#include <ros/ros.h>
#include <sensor_msgs/PointCloud2.h>
#include <pcl/point_types.h>
#include <pcl/filters/voxel_grid.h>
#include <pcl_ros/point_cloud.h>
#include <boost/foreach.hpp>
#include <pcl/io/pcd_io.h>
#include <tf/transform_listener.h>
#include <laser_geometry/laser_geometry.h>
#include <pcl/ros/conversions.h> 
ros::Publisher pub;
void cloud_cb (const sensor_msgs::PointCloud2ConstPtr& input)
  // ... do data processing
 printf ("Cloud: width = %d, height = %d\n", input->width,input->height);

  BOOST_FOREACH (const pcl::PointXYZ& pt, input->points)
  printf ("\t(%f, %f, %f)\n", pt.x, pt.y, pt.z);
  // Publish the data
 // pub.publish (output);

int main (int argc, char** argv)
  // Initialize ROS
  ros::init (argc, argv, "pcs200");
  ros::NodeHandle nh;

  // Create a ROS subscriber for the input point cloud
  ros::Subscriber sub = nh.subscribe ("/cloud", 100, cloud_cb);

  // Create a ROS publisher for the output point cloud
  pub = nh.advertise<sensor_msgs::PointCloud2> ("output", 100,false);

  // Spin
  ros::spin ();

Here is the output for that:

[  0%] Built target rospack_genmsg_libexe
  [  0%] Built target rosbuild_precompile
  Scanning dependencies of target pcs200
  [100%] Building CXX object CMakeFiles/pcs200.dir/src/pcs200.o
  /home/karthik/ros_workspace/pcs200/src/pcs200.cpp: In function ‘void cloud_cb(const sensor_msgs::PointCloud2ConstPtr&)’:
  /home/karthik/ros_workspace/pcs200/src/pcs200.cpp:17: error: ‘const struct sensor_msgs::PointCloud2_<std::allocator<void> >’ has no member named ‘points’
  /home/karthik/ros_workspace/pcs200/src/pcs200.cpp:17: error: ‘const struct sensor_msgs::PointCloud2_<std::allocator<void> >’ has no member named ‘points’
  /home/karthik/ros_workspace/pcs200/src/pcs200.cpp:17: error: ‘const struct sensor_msgs::PointCloud2_<std::allocator<void> >’ has ...
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1 Answer

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answered 2012-08-13 17:58:07 -0500

updated 2012-08-13 18:00:44 -0500

Hi Metal!

The error message

`const struct sensor_msgs::PointCloud2_<std::allocator<void> >’ has no member named ‘points’

is telling the truth, the variable input has type sensor_msgs::PointCloud2 and has no member called points, you can see its definition here.

You can use input->data or transform input to a pcl::PointCloud<pcl::pointxyz> which is somewhat easier to use.

You can do it like this

pcl::PointCloud<pcl::PointXYZ> input_;
pcl::fromROSMsg(*input, input_);

I hope this is helpful for you. Martin.

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It did rectify the error,thank you. But the z is always showing as zero. Also could you please recommend the approach to implement obstacle avoidance using sick lidar?. Is PCL a good approach ?

metal gravatar image metal  ( 2012-08-15 09:21:34 -0500 )edit

I might be mistaken, I have never worked with LIDAR, but as far as I know the readings of a LIDAR sensor are in 2D, usually in the plane X,Y, unless you are artificially tilting it with a motor. So to me it makes sense that the Z is always zero.

Martin Peris gravatar image Martin Peris  ( 2012-08-15 15:05:15 -0500 )edit

As for the best approach to implement obstacle avoidance using sick lidar, I would start a new question, so people with more experience than me can guide you. Also, you should check out the navigation stack lots of stuff already implemented there

Martin Peris gravatar image Martin Peris  ( 2012-08-15 15:07:12 -0500 )edit

Is there a way to use fromROSMsg in pcl_conversions or pcl_ros with Python? I have a Python node that subscribes to LaserScan data, converts the LaserScan to a PointCloud2 message, and need to extract the x,y data from this point cloud.

Ajay Jain gravatar image Ajay Jain  ( 2015-02-09 19:46:22 -0500 )edit

I made a separate question:

Ajay Jain gravatar image Ajay Jain  ( 2015-02-09 19:57:28 -0500 )edit

What are the CmakeLists.txt and package codes for this project?

Tooght gravatar image Tooght  ( 2016-12-02 16:47:36 -0500 )edit this is my full code,,

please have a look

the image view is crashing but i am getting values though,, also,, there is nothing in x y and z !!

zubair gravatar image zubair  ( 2017-04-20 05:15:35 -0500 )edit

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Asked: 2012-08-13 17:31:41 -0500

Seen: 6,974 times

Last updated: Aug 13 '12