How RTAB-Map find the point cloud gravity direction?

[Jacky Liu]

After installing rtab-map ros and run on real environment, I found that rtab-map could plot the point cloud in ros rviz with correct gravity direction.  How does it do that?  Could someone give me the section of paper or code which describe the related method?

[matlabbe]

Hi,

This is implicit to TF. If TF of the camera is correctly set on the robot, the cloud will be correctly aligned with gravity. In contrast without a robot, if you do the hand-held tutorial and start the camera with an angle, the whole map will be tilted. In that case (without robot), the parameter "Odom/AlignWithGround" can be set to true so that the ground plane is detected to correct the camera angle and height against the ground (code is here).

cheers,
Mathieu