  In this paper, a degeneracy avoidance method for a point and line based
visual SLAM algorithm is proposed. Visual SLAM predominantly uses point
features. However, point features lack robustness in low texture and
illuminance variant environments. Therefore, line features are used to
compensate the weaknesses of point features. In addition, point features are
poor in representing discernable features for the naked eye, meaning mapped
point features cannot be recognized. To overcome the limitations above, line
features were actively employed in previous studies. However, since degeneracy
arises in the process of using line features, this paper attempts to solve this
problem. First, a simple method to identify degenerate lines is presented. In
addition, a novel structural constraint is proposed to avoid the degeneracy
problem. At last, a point and line based monocular SLAM system using a robust
optical-flow based lien tracking method is implemented. The results are
verified using experiments with the EuRoC dataset and compared with other
state-of-the-art algorithms. It is proven that our method yields more accurate
localization as well as mapping results.
