1 Introduction
Autonomous navigation inside buildings is a key ability of robotic intelligent systems [27], [45]. Successful navigation requires both to localize a robot and to determine a path to its goal. One approach to solve the localization problem is to build a 3D map of the building and then use a camera to estimate the current position and orientation of the robot (Fig. 1). Imagine also the benefit of an intelligent indoor navigation system that helps you find your way, for example, at Chicago airport, Tokyo Metropolitan station or a convention center. Besides intelligent systems, the visual localization problem is also highly relevant for any type of Mixed Reality applications, including Augmented Reality [19], [52], [88].