Revisiting Feature Alignment for One stage Object Detection
Recently, one stage object detectors gain much attention due to their simplicity in practice. Its fully convolutional nature greatly reduces the difficulty of training and deployment compared with two stage detectors which require NMS and sorting for the proposal stage. However, a fundamental issue lies in all one stage detectors is the misalignment between anchor boxes and convolutional features, which significantly hinders the performance of one stage detectors. In this work, we first reveal the deep connection between the widely used im2col operator and the RoIAlign operator. Guided by this illuminating observation, we propose a RoIConv operator which aligns the features and its corresponding anchors in one stage detection in a principled way. We then design a fully convolutional AlignDet architecture which combines the flexibility of learned anchors and the preciseness of aligned features. Specifically, our AlignDet achieves a state of the art mAP of 44.1 on the COCO test dev with ResNeXt 101 backbone.
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