Assigning Visual Words to Places for Loop Closure Detection

Place recognition of pre-visited areas, widely known as Loop Closure Detection (LCD), constitutes one of the most important components in robotic applications, where the robot needs to estimate its pose while navigating through the field (e.g., simultaneous localization and mapping). In this paper, we present a novel approach for LCD based on the assignment of Visual Words (VWs) to particular places of the traversed path. The system operates in real time and does not require any pre-training procedure, such as visual vocabulary construction or descriptor-space dimensionality reduction. A place is defined through a dynamic segmentation of the incoming image stream and is assigned with VWs through the usage of an on-line clustering algorithm. At query time, image descriptors are converted into VWs on the map accumulating votes to the corresponding places. By means of a probability function, the mechanism is capable of identifying a loop closing candidate place. A nearest neighbor voting scheme on the descriptors' space allows the system to select the most appropriate image match at the chosen place. Geometrical and temporal consistency checks are applied on the proposed loop closing pair increasing the system's performance. Evaluation took place on several publicly available and challenging datasets offering high precision and recall scores as compared to other state-of-the-art approaches.