Ship detection with polarimetric synthetic aperture radar (PolSAR) has received increasing attention for its wide usage in maritime applications. However, extracting discriminative features to implement ship detection is still a challenging problem. In this paper, we propose a novel ship detection method for PolSAR images via task-driven discriminative dictionary learning (TDDDL). An assumption that ship and clutter information are sparsely coded under two separate dictionaries is made. Contextual information is considered by imposing superpixel-level joint sparsity constraints. In order to amplify the discrimination of the ship and clutter, we impose incoherence constraints between the two sub-dictionaries in the objective of feature coding. The discriminative dictionary is trained jointly with a linear classifier in task-driven dictionary learning (TDDL) framework. Based on the learnt dictionary and classifier, we extract discriminative features by sparse coding, and obtain robust detection results through binary classification. Different from previous methods, our ship detection cue is obtained through active learning strategies rather than artificially designed rules, and thus, is more adaptive, effective and robust. Experiments performed on synthetic images and two RADARSAT-2 images demonstrate that our method outperforms other comparative methods. In addition, the proposed method yields better shape-preserving ability and lower computation cost.Remote Sens. 2019, 11, 769 2 of 20 two types. The first is designed by enhancing the contrast between the interested targets and clutter. Novak et al. proposed the polarimetric whitening filter (PWF) to produce a speckle-reduced image by optimally combining all the elements of the scattering matrix [3]. Yang et al. presented the generalized optimization of polarimetric contrast enhancement (GOPCE) to maximize the signal-to-clutter ratio (SCR) in the image [4]. These methods work well in high SCR condition. However, when the SCR decreased, these methods may suffer from severe performance deterioration. The other type is designed by analyzing polarimetric scattering mechanism and introducing polarimetric para-meters. Yeremy et al. implemented ship detection by using Cameron decomposition [5], while the symmetric scattering characterization method (SSCM) was developed by Touzi et al. [6]. Since the methods applied to single-look scattering matrix are generally more susceptible to the speckle and increase the probability of false alarms (PFAs) of small ship, multi-look covariance or coherency matrix-based methods have been explored further. Chen et al. introduced polarization cross entropy (PCE) based on the eigen-decomposition of polarimetric coherence matrix [7]. Moreover, degree of polarization [8] was fully investigated for ship detection. It is true that speckle is greatly reduced by spatial ensemble averaging in these methods. Nevertheless, although the scalar feature implicitly includes the contributions of all polarimetric channels, an explicit consideration of a...