rtabmap_ros/rtabmap_slam/README.md

rtabmap_slam

The SLAM node of RTAB-Map: it takes a pose from odometry and data from the sensors, builds a graph of where the robot has been, and corrects that graph whenever it recognizes a place it has seen before — in the current run or in a previous one, so a map can be extended over several sessions.

Contents

• Nodes
• Conventions
  • Frames and TF
  • The database
  • Update rate and dropped updates
  • Odometry, covariance and new maps
  • Mapping and localization
• License

Nodes

Node	Description
rtabmap	Graph SLAM with appearance- and proximity-based loop closure detection, memory management, map assembly and planning on the graph.

flowchart LR
    SYNC["<div style='text-align:left'><b>synchronized</b><br>RGB-D camera(s)<br>Stereo camera(s)<br>2D LiDAR<br>3D LiDAR<br>Odometry</div>"]
    ASYNC["<div style='text-align:left'><b>asynchronous</b><br>IMU<br>GPS<br>Landmarks (markers, tags, fiducials)</div>"]
    RTAB(["<b>rtabmap</b>"])
    GRAPH["Graph"]
    INFO["Info"]
    MAPS["<div style='text-align:left'><b>maps</b><br>2D occupancy grid<br>OctoMap<br>Elevation map<br>3D point cloud</div>"]
    TF["TF map → odom"]
    SYNC --> RTAB
    ASYNC --> RTAB
    RTAB --> GRAPH
    RTAB --> INFO
    RTAB --> MAPS
    RTAB --> TF

Conventions

Frames and TF

The node publishes map → odom, the correction from the optimized graph; odometry publishes odom → base_link, and the sensors are attached to base_link. See Frames and TF for the parameters.

flowchart TB
    MAP(["map<br><i>map_frame_id</i>"])
    ODOM(["odom<br><i>odometry frame</i>"])
    BASE(["base_link<br><i>frame_id</i>"])
    SENSOR(["camera, lidar, imu..."])
    MAP -->|this node| ODOM
    ODOM -->|odometry| BASE
    BASE -->|static, URDF| SENSOR

The database

The map is stored in database_path, ~/.ros/rtabmap.db by default (under $ROS_HOME if that is set). Set delete_db_on_start, or pass -d as an argument, to start from an empty one. Otherwise restarting on an existing database continues it: the map is reloaded, the next update starts a new session, and a loop closure between the new session and an old one merges the two. That is how a map is extended over several runs.

The database is saved on shutdown. A node that is killed rather than shut down loses whatever had not been written yet.

The database also remembers the parameters it was built with, and reopening it without setting them again brings them back. For example, a map made with ICP registration keeps using ICP, which is what makes the new session compatible with the old ones. Anything set explicitly still wins, and delete_db_on_start forgets them along with the map.

Update rate and dropped updates

Rtabmap/DetectionRate is how many updates per second are processed. It is 1 Hz by default because SLAM does not need more: odometry carries the pose between nodes, and each node costs memory, loop closure detection and optimization time for as long as the map exists.

Warning: Rtabmap/DetectionRate at 0 with sensor updates faster than about 2 Hz makes loop closure detection, graph optimization and map generation intractable fast. Every update then becomes a node, and each node is compared against all the nodes in working memory, adds a pose to optimize and data to assemble into the maps, so the map, and the time each update takes, grow at the sensors' rate until the node cannot keep up. Keep a detection rate of 1 to 2 Hz, or bound working memory with Rtabmap/TimeThr or Rtabmap/MemoryThr.

A robot standing still does not grow the map. An update that moved less than both RGBD/LinearUpdate and RGBD/AngularUpdate since the last node is still used to detect loop closures, and then dropped. Set both to 0 to add a node every time.

An update arriving while the previous one is still being processed is dropped, not queued. SLAM time grows with the map, so a queue would only fall further behind; dropping keeps the node on the newest data. info shows how long each update took (RtabmapROS/TimeTotal/ms), and /diagnostics how many arrived versus how many were processed.

Odometry, covariance and new maps

The link between two consecutive nodes is the odometry between them, weighted by its covariance — its inverse becomes the link's information matrix, so the optimizer knows how far to trust each one.

Which covariance is used:

• The twist covariance, if it is set. It is the uncertainty of the motion since the previous message, which is what a link between two nodes is.
• Otherwise half the pose covariance, for odometry sources that only fill that one. This assumes it is the error of the motion since the previous message, as visual odometry often publishes it, not the unbounded uncertainty of the pose estimated by a filtered odometry.
• Otherwise odom_tf_linear_variance and odom_tf_angular_variance (0.001 by default), for a covariance that is zero, not finite, or exactly 1 — which is what several drivers publish to mean "not set". Many do publish zeros, and taking those at face value would make each link infinitely confident.

Between two nodes, the largest covariance seen is kept, so updates dropped by the rate do not make the link look more certain than any of the motions that made it up.

An odometry reset starts a new map, in the same database, rather than deforming the graph across a jump the robot never made:

Odometry is reset (identity pose or high variance detected). Increment map id!

A reset is an identity pose after a non-identity one, or 9999 on both the pose and the twist covariance diagonals — which is what the odometry nodes publish when they lose track or restart. Odometry read from TF has no covariance, so only the identity pose counts there. The new map is merged back into the old one on the first loop closure between them.

A consequence worth knowing: an odometry that returns to exactly the identity is taken for a reset. Real odometry never does, but a simulator or a test that drives back to the origin will.

staleness_factor treats a long silence the same way. With Rtabmap/DetectionRate at 1 Hz and a factor of 2, an update more than 2 seconds after the previous one starts a new map. It is for odometry sources that go quiet instead of reporting a reset — a gap that long means the motion across it is not known, even if the next pose looks plausible.

Mapping and localization

Mem/IncrementalMemory chooses between the two:

• true, mapping (SLAM), the default. Updates become nodes and the map grows. This is the mode to create a map of the environment.
• false, localization. The map is loaded and not extended: each update is compared against it, localizes the robot if it matches, and is not added to the database. This is the mode to localize in a map already recorded, without increasing CPU and RAM usage, since the map is kept fixed.

set_mode_localization and set_mode_mapping switch at runtime. Going back to mapping starts a new session, since nothing links where the robot is now to where it left the map — until a loop closure does.

See Localization for where the robot starts on the map in localization mode.

License

BSD-3-Clause. See the repository root.