“…Mur-Artal and Tards proposed ORB-SLAM [1], [2], which is one of visual SLAM systems with full sensor support and best performance, with applying ORB features in parallel tracking, mapping, and loop closure detection, and using pose graph optimization and bundle adjustment [13] based optimization. Another kind of visual SLAM systems, unlike feature-based methods mentioned above, directly uses images as input without any abstraction with descriptors or handcrafted feature detectors, called direct methods [14]. DTAM [15], in which tracking is implemented by associating the input image with synthetic view images generated from the reconstructed map, and LSD-SLAM [16], which follows the idea from semidense VO [17], are the leading strategies in direct methods.…”