rtabmap_ros/rtabmap_util/doc/lidar_deskewing.md

lidar_deskewing

Removes the motion distortion from a lidar scan.

A spinning lidar takes tens of milliseconds to complete a sweep, and on a moving robot every point in that sweep is measured from a slightly different pose. The result is a skewed cloud: straight walls come out bent, and registration against it drifts.

This node uses TF to find where the sensor actually was when each point was taken, and moves every point into the pose at the start of the sweep. A straight wall comes back straight.

Contents

• Usage
• Subscribed Topics
• Published Topics
• Required Transforms
• Parameters
• Requirements
• Behavior when TF is missing
• Notes

Usage

ros2 run rtabmap_util lidar_deskewing --ros-args \
  -p fixed_frame_id:=odom \
  -r input_cloud:=/velodyne_points

ComposableNode(
    package='rtabmap_util',
    plugin='rtabmap_util::LidarDeskewing',
    name='lidar_deskewing',
    parameters=[{'fixed_frame_id': 'odom'}],
    remappings=[('input_cloud', '/velodyne_points')])

Subscribed Topics

Connect one of the two.

Topic	Type	Description
input_cloud	sensor_msgs/msg/PointCloud2	Must carry a per-point time channel, see Requirements.
input_scan	sensor_msgs/msg/LaserScan	Per-point times come from time_increment.

Published Topics

Output names are derived from the resolved input names, so remapping the input moves the output with it. With input_cloud remapped to /velodyne_points the output is /velodyne_points/deskewed.

Topic	Type	Description
<input_cloud>/deskewed	sensor_msgs/msg/PointCloud2	The deskewed cloud, same frame and stamp as the input.
<input_scan>/deskewed	sensor_msgs/msg/PointCloud2	A LaserScan cannot represent a deskewed sweep — the points no longer lie on a regular angular grid — so the scan input also produces a cloud.

Required Transforms

Transform	Description
fixed_frame_id → sensor frame, across the sweep	Must be available for the whole span of the sweep, at both its first and last stamp.

Parameters

Parameter	Type	Default	Description
fixed_frame_id	string	""	Required. Frame the motion is measured against, usually odom.
wait_for_transform	double	0.01	Seconds to wait for the transforms spanning the sweep. Raise it if odometry lags the lidar.
slerp	bool	false	Interpolate between the poses at the start and end of the sweep instead of looking up TF per point. Much cheaper, and accurate enough at constant velocity.
queue_size	int	5	Queue depth of the input subscriptions.
output_queue_size	int	1	History depth of the deskewed cloud publishers.
qos	int	0	Reliability of the input subscriptions: 0 system default, 1 reliable, 2 best effort.

Requirements

For input_cloud, the cloud must have a per-point time field. Without one the node cannot know when each point was taken and cannot deskew.

The field has to be named t, time, stamps or timestamp — anything else is not recognized, whatever it contains. Its type decides how the value is read:

Type	Meaning
uint32	nanoseconds since the cloud's own stamp
float32	seconds since the cloud's own stamp
float64	an absolute timestamp; seconds, milliseconds, microseconds and nanoseconds are told apart by magnitude

Common drivers that satisfy this out of the box: Ouster (t), Velodyne (time), RoboSense (timestamp) and Livox (timestamp). Livox needs its PointCloud2 output rather than the default CustomMsg format, which this node cannot subscribe to at all.

To check what your driver actually publishes:

ros2 topic echo /your/points --field fields --once

If none of the four names is in that list, look for a driver option to add per-point timestamps before anything else.

The fixed_frame_id → sensor transform must cover the whole sweep, which means odometry has to be at least as recent as the lidar. If it lags, raise wait_for_transform.

Behavior when TF is missing

The two inputs deliberately differ:

• A cloud is republished unchanged with a warning. Deskewing is an improvement, not a precondition, and dropping frames would break the pipeline behind it.
• A scan is dropped, because converting it to a cloud is only worth doing as part of deskewing.

Notes

Deskewing matters most when rotating: at 1 rad/s a 100 ms sweep spans nearly 6°, and the far end of the scan is badly misplaced. Pure translation at walking speed is a few centimeters, which matters at close range.

Put this node before ICP odometry or point_cloud_assembler, not after. Anything registering against a skewed cloud has already paid for the distortion.