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How to assemble laser scan lines into a composite point cloud

Description: In this tutorial you will learn how to assemble individual laser scan lines into a composite point cloud. One particular use case is to assemble individual scan lines from a laser on a tilting stage into a single point cloud to form a full 3D laser sweep.

Keywords: laser, scan, assemble, aggregate, accumulate, point cloud, cloud, laser scanner, hokuyo, sick

Tutorial Level: INTERMEDIATE

In the laser pipeline tutorials, you learned how to work with a single scan line. In many cases, however, it would be more useful to have multiple scan lines together as a larger point cloud. For example, if the laser is on a tilting unit, it's useful to group together all of the scans that came from one top-to-bottom tilting cycle. Or it might be useful to accumulate a set of scans over a fixed time period. To aggregate scans we look to the laser_assembler package containing the laser_scan_assembler and the point_cloud_assembler.

Example Laser Data

So that you can try all of the nodes described in this tutorial, we've provided a source of laser data. ROS can record sensor data (actually any ROS system data) into a bag file. You can download a bag file containing laser data here.

The bag contains two topics:

• /tf [tf/tfMessage]

• /tilt_scan [sensor_msgs/LaserScan]

You can verify this using rosbag:

rosbag info laser.bag

The tilt_scan topic was recorded from a planar laser mounted on a tilting stage. The /tf topic contains the robot transforms.

To play back the bag, first run a roscore:

 $ roscore

We're going to be using bagfile data, so we want to make sure that all of our nodes' times are based off of simulation time:

 $ rosparam set /use_sim_time true

Then play back the bag:

 $ rosbag play --clock laser.bag

We need the --clock argument in order to publish simulation time.

Familiarize yourself with the data by viewing it in rviz.

The Laser Scan Assembler

The laser_scan_assembler accumulates laser scans by listening to the appropriate topic and accumulating messages in a ring buffer of a specific size. When the assemble_scans service is called, the contents of the current buffer that fall between two times are converted into a single cloud and returned.

Here is an example launch file for the laser_scan_assembler operating on the scan topic tilt_scan:

<launch>
  <node type="laser_scan_assembler" pkg="laser_assembler" name="my_assembler">
    <remap from="scan" to="tilt_scan"/>
    <param name="max_scans" type="int" value="400" />
    <param name="fixed_frame" type="string" value="base_link" />
  </node>
</launch>

This instantiation of the laser_scan_assembler keeps a rolling buffer of 400 scans, and transforms incoming scans into the base_link frame. The full set of parameters is available in the laser_assembler's ROS Interface section.

The fixed_frame parameter name is not accidental. Each single scan is converted into this frame when it arrives, and no additional transforms are done to the data when the cloud is published. Therefore, you will very likely want to choose a frame that isn't moving for your fixed_frame. If your robot is stationary, the base_link frame might be a good choice. If your robot is moving, choose a frame that is stationary in the world like a map frame.

Try running the laser_scan_assembler on the \tilt_scan topic in laser.bag. It doesn't look like much is happening, right? But behind the scenes, the buffer is filling up...

The Point Cloud Assembler

If your scan lines are already in point cloud message form, you can use the point_cloud_assembler instead. The point_cloud_assembler works very much like the laser_scan_assembler, here's a launch file for it:

<launch>
  <node type="point_cloud_assembler" pkg="laser_assembler" name="my_assembler">
    <remap from="cloud" to="my_cloud_in"/>
    <param name="max_clouds" type="int" value="400" />
    <param name="fixed_frame" type="string" value="base_link" />
  </node>
</launch>

Try out the point_cloud_assembler by first converting the tilt_scan topic into a point cloud and then assembling it. (Forgot how to convert a laser scan into a point cloud? Go back to the introduction to working with laser scanner data.) Once again, nothing visible is happening, but the buffer is filling up...

Using the Assemblers: A Laser Snapshotter

The assemblers will work away dutifully as laser scans (or point clouds) come in, but they won't publish anything until you call the assemble_scans service. The assemble_scans service has two request fields:

• time begin

• time end

All buffer entries between these two times will be grouped into one point cloud and returned in the response field:

• sensor_msgs::PointCloud cloud

How often you call assemble_scans depends on your robot and application, here are some suggestions:

• every x seconds,
• whenever you're ready to process the resulting cloud, or
• when receiving a signal from your system, such as when a laser tilting stage passes through one sweep.

As an example, the laser_assembler package has an example node in the examples folder called periodic_snapshotter which calls the assemble_scans service every 5 seconds and publishes the resulting clouds.

Try running the periodic_snapshotter with the laser_scan_assembler and viewing the resulting point cloud (published on the topic assembled_cloud) in rviz. Instead of a updating line-by-line, the point cloud will change every 5 seconds and display all of the accumulated data at once.

Conclusion

Now you should be able to assemble single scans (or clouds) into full point clouds. By combining this with the tools in laser_geometry and laser_filters that you learned about in previous tutorials, you can convert your raw sensor data into more useful formats.