Learning hybrid ranking representation for person re-identification

•We propose to jointly learn ranking context cues and appearance features to exploit discriminative feature representations for person re-id.•We design a novel two-stream architecture to learn a hybrid ranking representation for more effective person re-id.•Our method achieves superior performance compared with the state-of-the-art alternative methods on four large-scale person re-id benchmarks. Contemporary person re-identification (re-id) methods mostly compute independentlya feature representation of each person image in the query set and the gallery set. This strategy fails to consider any ranking context information of each probe image in the query set represented implicitly by the whole gallery set. Some recent re-ranking re-id methods therefore propose to take a post-processing strategy to exploit such contextual information for improving re-id matching performance. However, post-processing is independent of model training without jointly optimising the re-id feature and the ranking context information for better compatibility. In this work, for the first time, we show that the appearance feature and the ranking context information can be jointly optimised for learning more discriminative representations and achieving superior matching accuracy. Specifically, we propose to learn a hybrid ranking representation for person re-id with a two-stream architecture: (1) In the external stream, we use the ranking list of each probe image to learn plausible visual variations among the top ranks from the gallery as the external ranking information; (2) In the internal stream, we employ the part-based fine-grained feature as the internal ranking information, which mitigates the harm of incorrect matches in the ranking list. Assembling these two streams generates a hybrid ranking representation for person matching. Extensive experiments demonstrate the superiority of our method over the state-of-the-art methods on four large-scale re-id benchmarks (Market-1501, DukeMTMC-ReID, CUHK03 and MSMT17), under both supervised and unsupervised settings.