Visual Odometry/SLAM Before Map Is Fully Built (Possibly With Rtabmap)

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Hi, I am trying to use visual odometry to do localization. Is there any code/ROS library that enables you to get visual odometry data before the map is built? Or automatically starts when enough locations has been detected?

I'm looking at the Rtabmap library and it asks that a map (with at least 30 locations) is first built before localization is performed, as per this guide. I am open to using other libraries too.