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How to show nav2 costmaps from a rosbag2 replay next to robot poses with Dalaran

Replay a rosbag2 with nav2 costmap topics and inspect the layers on one timeline.

Who it is forA ROS 2 navigation engineer debugging why a robot took a path, using a recorded rosbag2 from a nav2 run.

The problemCostmaps flattened into plain images hide which cells are lethal obstacles and which are inflated, making path failures hard to explain.

The outcomeThe global and local costmaps appear as grid map entities with origin and resolution handled, with cost categories drawn distinctly.

What you need first

  • Python with pip, or Rust with cargo, to install the Dalaran SDK or viewer
  • A terminal to run the dalaran command
  • An existing .rrd, .dlr, or MCAP recording, or a ROS 2 setup with rosbag2 files, for the replay use cases

Steps

  1. Locate a rosbag2 or MCAP file that contains /global_costmap or /local_costmap topics from your nav2 run.
  2. Replay the rosbag2 into Dalaran with the ROS 2 bridge described in the README, saving the output as a .dlr file. (inferred, check the README)
  3. Run dalaran session.dlr to open the saved recording, replacing session.dlr with your file name.
  4. Step through the timeline and check that the costmap layers stack with their own opacity alongside the robot pose.

Prompt for your agent

In the Flaminis/Dalaran repository, replay <bag_path> into a .dlr file using the ROS 2 bridge and open it with the dalaran command. Expected output is a viewer showing nav2 costmap grids on the timeline. Verify that the global and local costmap entities appear and that lethal and inflated obstacle cells are shown as separate categories.

Caveat. The README does not give the exact ROS 2 bridge command, so this use case needs the BUILD.md, docs, or examples to confirm the replay steps.

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