I see that in RTAB-Map preferences an RGB camera can be set for source. Can RTAB-Map be used with monocular vision only for odometry and loop-closures without map building? I calibrated my USB camera using the "Calibrate" procedure inside RTAB-Map, but when I start the acquisition in localization mode (no mapping) the odometry cannot be computed, all feature points are yellow and the camera pose is invalid.
Can this be done, or the odometry is computed only from 3D points coming from RGBD point cloud?
No, the RGB source is only for experimental tests with OdometryMono (tracking the camera on a small workspace like for virtual reality purpose over a simple desktop for example, another usage is to track a RGB camera over a prebuilt map from a RGB-D sensor). While there is no documentation and that it works only a very small scale (the solution degenerates quickly when extending the map, and the loop closure approach doesn't deal with the scale drift), I don't recommend to use it.