Oriented object detection is a practical and challenging task in remote sensing image interpretation. Nowadays, oriented detectors mostly use horizontal boxes as intermedium to derive oriented boxes from them. However, the horizontal boxes are inclined to get small Intersection-over-Unions (IoUs) with ground truths, which may have some undesirable effects, such as introducing redundant noise, mismatching with ground truths, and detracting from the robustness of detectors. In this article, we propose a novel anchor-free oriented proposal generator (AOPG) that abandons horizontal box-related operations from the network architecture. AOPG first produces coarse oriented boxes by a coarse location module (CLM) in an anchor-free manner and then refines them into high-quality oriented proposals. After AOPG, we apply a Fast Region-based Convolutional Neural Network (R-CNN) head to produce the final detection results. Furthermore, the shortage of large-scale datasets is also a hindrance to the development of oriented object detection. To alleviate the data insufficiency, we release a new dataset on the basis of our DIOR dataset and name it DIOR-R. Massive experiments demonstrate the effectiveness of AOPG. Particularly, without bells and whistles, we achieve the accuracy of 64.41%, 75.24%, and 96.22% mAP on the DIOR-R, DOTA, and HRSC2016 datasets, respectively. Code and models are available at https://github.com/jbwang1997/AOPG .

In the past few years, object detection in remote sensing images has achieved remarkable progress. However, the detection of oriented and densely packed objects are still unsatisfactory due to the following spatial and feature misalignments. 1) Most two-stage oriented detectors only introduce an orientation regression branch in the detection head, while still leverage horizontal proposals for classification and regression. This inevitably results in the spatial misalignment problem between horizontal proposals and oriented objects. 2) The features used for classification are in fact extracted from the region proposals which have shifted to the final predictions via the regression branch. This leads to the feature misalignment problem between the classification and the localization tasks. In this article, we present a two-stage oriented object detection method, termed dual-aligned oriented detector (DODet), toward evading the aforementioned problems of spatial and feature misalignments. In DODet, the first stage is an oriented proposal network (OPN), which generates high-quality oriented proposals via a novel representation scheme of oriented objects. The second stage is a localization-guided detection head (LDH) that aims at alleviating the feature misalignment between classification and localization. Comprehensive and extensive evaluations on three benchmarks, including DIOR-R, DOTA, and HRSC2016, indicate that our method could obtain consistent and substantial gains compared with the baseline method. The source code is publicly available at https://github.com/yanqingyao1994/DODet .

Detecting objects in remote sensing images using oriented bounding boxes is flourishing but challenging, wherein the design of oriented bounding box representations is the key to achieving accurate detection. In this paper, we focus on two issues that hinder the performance of two-stage oriented detectors: (i) the notorious boundary discontinuity problem, which would result in significant loss increases in boundary conditions, and (ii) the inconsistency in regression schemes between the two stages. We propose a simple and effective bounding box representation by drawing inspiration from the polar coordinate system and integrate it into both two detection stages to circumvent the two issues. The first stage specifically initializes four quadrant points as the starting points of the regression for producing high-quality oriented candidates without any post-processing. In the second stage, the final localization results are refined using the proposed novel bounding box representation, which can fully release the capabilities of the oriented detectors. Such consistency brings a good trade-off between accuracy and speed. With only flipping augmentation and single-scale training and testing, our approach with ResNet-50-FPN harvests 76.25% mAP on the DOTA dataset with a speed of up to 16.5 FPS, achieving the best accuracy and the fastest speed among mainstream two-stage oriented detectors. Additional results on the DIOR-R and HRSC2016 datasets also demonstrate the effectiveness and robustness of our method. The source code will be publicly available at https://github.com/yanqingyao1994/QPDet.

Fine-grained object detection (FGOD) in remote sensing images is an emerging and challenging task in the field of image intelligent interpretation. It aims to localize objects while classifying them into different fine-grained categories. Modern FGOD methods are mainly derived from well-developed detectors and have made compelling progress. Despite this, these methods struggle to perform well in classifying objects at the subordinate level due to the limitations of their representation manners. In this paper, we propose a network capable of learning discriminative representation (DR) for fine-grained object detection in remote sensing images, named DRNet. First, a fine-grained branch that works in parallel with other task branches is introduced, where objects’ features are re-encoded with dual refinement to generate discriminative representation, enabling accurate fine-grained classification. Second, we design a confusion-minimized loss that automatically scales loss contributions according to the separability of samples to train the fine-grained branch, further boosting discriminative ability of the representation and better addressing hard-to-distinguish objects. Moreover, we devise an interaction verification strategy that empowers the network to fully utilize the results of fine-grained classification and coarse classification for achieving robust inference. On large-scale FAIR1M-1.0 and FAIR1M-2.0 datasets, our DRNet with ResNet50 and 1× training schedule obtains 40.87% mAP and 47.04% mAP, respectively, establishing new state-of-the-arts for fine-grained object detection in remote sensing images. The source code is available at https://github.com//54wb//DRNet.

Object detection in remote sensing images has garnered significant attention due to its wide applications in real-world scenarios. However, most existing oriented object detectors still suffer from complex backgrounds and varying angles , limiting their performance to further improvement. In this paper, we propose a novel oriented detector with Hierarchical mask prompting and Robust integrated regression, termed HRDet. Specifically, to cope with the first issue, we construct a hierarchical mask prompting module consisting of a semantic mask prediction branch and hierarchical Softmax technique. The former aims to isolate object instances from cluttered interferences guided by coarse box-wise masks, while the latter propagates differentiated features for adjacent layers using hierarchical attentive weights. To deal with the second issue, we strive for robust integrated regression and formulate an efficient oriented IoU loss, explicitly measuring the discrepancies of three geometric factors in oriented regression, i.e ., the central point distance, side length, and angle. This innovative loss intends to overcome the problem that existing IoU-based losses are invariant during the regression of varying angles. We applied these two strategies to a simple one-stage detection pipeline, achieving a new level of trade-off between speed and accuracy. Extensive experiments on four large aerial imagery datasets, DOTA-v1.0, DOTA-v2.0, DIOR-R, and HRSC2016, demonstrate that our HRDet significantly improves the accuracy of the one-stage detector over refine-stage counter-parts while maintaining the efficiency advantage. The source code will be available at https://github.com/yanqingyao1994/HRDet.

In object detection, particularly within remote sensing images, the quality of selected samples is crucial for the accuracy and robustness of detection models. However, current sampling strategies demonstrate inherent limitations. They empirically define positive sample sets using fixed thresholds or preset areas, ignoring the actual shapes of the objects and failing to distinguish the intrinsic value of each sample point. To address these critical issues, this article proposes a novel centric probability-based sample selection approach that includes centering probability mapping (CPM), Expectation-Maximization-based boundary optimization (EBO), and probabilistic random sampling (PRS) technologies. Specifically, the CPM is constructed to assign various confidence levels for all sample points based on their proximity to the center of bounding box, effectively discerning the value of individual samples. Then, the EBO is utilized to dynamically optimize the boundaries for positive and negative samples based on the EM algorithm, thus avoiding the sample imbalance problem associated with empirical thresholds. Finally, the PRS strategy is proposed to select training samples from the sample space constructed by CPM and EBO in a manner of random probability sampling, which could improve the diversity of samples while guaranteeing their quality. Experimental validation on three remote sensing image datasets, including DOTA-v1.0, DOTA-v2.0, and DIOR-R, demonstrates that our method achieves robust performance improvements over baseline and significantly surpasses the advanced sample selection methods. The source code will be available at https://github.com/yanqingyao1994/CPSS.