Laser Scan Processing

name: laser-scan-processing description: 'Process LaserScan data in ROS 2. Use when filtering, segmenting, or extracting features from 2D lidar scans.'

LaserScan Processing

Message Structure

sensor_msgs/msg/LaserScan fields:

The angle for the i-th reading is angle_min + i * angle_increment. Valid readings satisfy range_min <= ranges[i] <= range_max. Values of inf mean the beam did not return (open space beyond max range). Values of NaN indicate a sensor error for that ray.

Polar to Cartesian Conversion

Each valid range reading converts to a 2D point in the sensor frame:

angle = msg.angle_min + i * msg.angle_increment
x = msg.ranges[i] * math.cos(angle)
y = msg.ranges[i] * math.sin(angle)

Always check math.isfinite(msg.ranges[i]) before conversion. The resulting points are in the LIDAR's own frame (typically laser_frame or base_scan).

Filtering Techniques

Min/Max Range Filtering: Discard readings outside a working range. Removes ground returns (very short range) and unreliable far readings:

if msg.range_min <= r <= msg.range_max and math.isfinite(r):
    # valid

Angular Sector Selection: Extract only a forward-facing wedge or exclude the rear arc. Useful when rear sensors are occluded by the chassis.

Median Filtering: Replace each reading with the median of a sliding window (e.g., 5 readings). Suppresses impulse noise without blurring edges as much as mean filtering.

laser_filters Package

The laser_filters package provides a composable filter chain applied to LaserScan messages. Each filter is a plugin loaded at runtime.

Key Filter Plugins

Filter Chain YAML

scan_filter_chain:
- name: chassis_exclusion
  type: laser_filters/LaserScanAngularBoundsFilter
  params:
    lower_angle: -2.35619    # -135 degrees
    upper_angle: -1.91986    # -110 degrees — chassis visible here

- name: range_limit
  type: laser_filters/LaserScanRangeFilter
  params:
    use_message_range_limits: false
    lower_threshold: 0.15
    upper_threshold: 12.0
    lower_replacement_value: .nan
    upper_replacement_value: .inf

- name: speckle
  type: laser_filters/LaserScanSpeckleFilter
  params:
    filter_type: 0           # 0 = distance-based
    max_range: 2.0
    max_range_difference: 0.1
    filter_window: 2

- name: shadows
  type: laser_filters/LaserScanShadowsFilter
  params:
    min_angle: 10.0          # degrees
    max_angle: 170.0
    neighbors: 1
    window: 1

Launch Integration

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='laser_filters',
            executable='scan_to_scan_filter_chain',
            name='laser_filter_chain',
            parameters=[{'use_sim_time': False}],
            remappings=[
                ('scan', '/scan_raw'),
                ('scan_filtered', '/scan'),
            ],
            # Load filter chain from YAML
            arguments=['--ros-args', '--params-file', 'laser_filters.yaml'],
        ),
    ])

Costmap Integration

Nav2 costmaps consume LaserScan on the observation_sources topic. Publish the filtered scan to /scan and configure the costmap:

observation_sources: scan
scan:
  topic: /scan
  data_type: LaserScan
  marking: true
  clearing: true
  max_obstacle_height: 2.0
  min_obstacle_height: 0.0
  obstacle_max_range: 10.0
  obstacle_min_range: 0.15
  raytrace_max_range: 12.0
  raytrace_min_range: 0.0

Common Issue: Chassis Self-Hits

If the LIDAR is mounted where it can see the robot's own body, those ranges appear as permanent close obstacles. Diagnose by visualizing /scan in RViz and rotating the robot — fixed points in the base_link frame are chassis hits. Fix with LaserScanAngularBoundsFilter excluding the affected angular range, or LaserScanBoxFilter excluding a box around the robot footprint.