A Rotational Libra R-CNN Method for Ship Detection

Recently, ship detection methods based on deep learning have attracted significant attention due to their superior accuracy over traditional methods. However, there still exist two problems affecting its robustness in practical application. 1) The size of ships in one image varies greatly, i.e., different sizes; 2) Numerous ships gather in limited field-of-view, i.e., dense distribution. To address these problems, we propose a rotational Libra R-convolutional neural network (CNN) method. Our idea is to balance the three levels of neural networks for predicting the location of ships with rotational angle information, which refers to the feature level, sample level, and objective level. First, to extract a discriminative feature and improve its robustness against the impact of different sizes of ships, the concept of balanced feature pyramid is introduced. Second, to generate reliable proposals for feature pyramid and efficiently mine hard negative samples, we employ intersection over union (IoU)-balanced sampling. Finally, to eliminate the redundant background and detect densely distributed ships, we bring in a rotational region detection branch with balanced L1 loss. In general, we develop the balanced learning with rotational region detection to achieve consistent improvement on accuracy and visualization. Experimental results on DOTA data set show that the proposed method achieves the state-of-the-art accuracy.