…g the current frame with all the previous frames is not always suitable for real-time applications. One solution is to define key frames (a subset of all the previous frames) and compare the current frame with only those key frames. The most common way to filter the loop closure frame candidates is to use a place recognition approach based on the vocabulary tree in which the feature descriptors of the candidate key frames are hierarchically quantized and are represented by a “Bag of Visual Words” (B.O.W).
more meaningf…ting to use in robotics or AR, as well as giving a more meaningful representation to the human eye. Some disadvantages of direct methods are that they cannot handle outliers very well, as they will always try to process them and implement them into the final map. Direct methods are also generally slower than feature-based variants.