How does rtabmap handle free space in an octomap?

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Thank you for your response! I tried replicating what you expressed running the demo_robot_mapping launch file, changing the parameters, and then running the bag. I can see in the 2D map that ray tracing is working, but when I try to view the free voxels in rviz nothing shows up. I can however view the occupied voxels and when I select "All Voxels" it is the same as "Occupied Voxels". Any idea what I might be missing?

Which ROS and rtabmap version are you using?

ROS Kinetic and Rtabmap 0.19.4. It turns out that even though I cannot visualize the free voxels in RVIZ they are there and the ray tracing is working! Thank you for your help! I exported the octomap and visualized it in octovis and can see the free voxels there. Any ideas why I wouldn't be able to see that in RVIZ? I also have a problem with RVIZ where there are vertical stripes through the 2D map. They are evenly spaced in sets of 8-9 in clusters across the map with the majority of the map being fine.

Build the octomap_rviz_plugins package from source, you will be able to see the free space in rviz. Not sure why we cannot show the colors anymore though, I'll take a look. For the stripes, it is because sometimes the center of the voxels in OctoMap don't match the center of the cells in the 2D grid when we assemble the local occupancy grids into the global one, like more on the edge of the cells, so depending on the floating precision, the points may fall on one or the other side of the edge, creating empty lines.