warning message: nothing to publish, octree is empty

[Nils Loewen]

Hello,

I am quite new to octomap and I a have a few doubts.

I am using a monocular SLAM algorithm in ROS, which outputs the map as a sparse pointcloud, about 400 points at start and I would like to build an occupancy grid map using this point cloud as the input cloud to octomap.

Once I remapped the pointcloud topic in the octomap launch file, I launched it in ROS and I got the following warning: Nothing to publish, octree is empty.

I have seen that octomap has been used  lot with denser pointclouds such as the ones created from laser scans and RGBD cameras. May I be getting this error because the pointcloud from the monocular SLAM algorithm is too sparse? Or would I have to change some of the parameters in order for it to work?

Thank you.

[Armin Hornung]

Hello Nils,

> I am quite new to octomap and I a have a few doubts.
>
> I am using a monocular SLAM algorithm in ROS, which outputs the map as
> a sparse pointcloud, about 400 points at start and I would like to
> build an occupancy grid map using this point cloud as the input cloud
> to octomap.
>
> Once I remapped the pointcloud topic in the octomap launch file, I
> launched it in ROS and I got the following warning: Nothing to
> publish, octree is empty.

octomap_server prints this warning when the octee is empty, that means
there were no sensor readings integrated. Does it constantly appear or
only at the beginning? Are there any other error or debug messages for
octomap_server? (check rqt_console, you might have to change the log level)

Another thing to check are the frame IDs of sensor messages and world
frame, and if there are tf transforms published between them.

Some parameters such as maximum range and pointcloud_[min|max]_z could
also result in an empty map.

>
> I have seen that octomap has been used lot with denser pointclouds
> such as the ones created from laser scans and RGBD cameras. May I be
> getting this error because the pointcloud from the monocular SLAM
> algorithm is too sparse? Or would I have to change some of the
> parameters in order for it to work?

Sparse point clouds should not be a problem in general, but certain
options like ground filtering should be disabled.

Best regards,
Armin

[Nils Loewen]

Hello Dr. Hornung,

Thank you for your reply. I followed your advice and checked rqt_console for mesaages published from octomap. The warning mesaage "Nothing to publish, octree is empty" only appears once, just after launching octomap_server. I kept octomap running and I checked the topics published by octomap with rostopic echo. All topics contain information , except the topic called free_cells_vis_array. I would like to see the built octomap in rviz. I read in the octomap_server documentation that I need to subscribe to the occupied_cells_vis_array, however, this topic is not displayed in the rviz window. Could you please give me some advice on visualizing the octomap in rviz?

Thank you. Best regards,
Nils Loewen