CA-Jaccard: Camera-aware Jaccard Distance for Person Re-identification

Person re-identification (re-ID) is a challenging task that aims to learn discriminative features for person retrieval. In person re-ID Jaccard distance is a widely used distance metric especially in re-ranking and clustering scenarios. However we discover that camera variation has a significant negative impact on the reliability of Jaccard distance. In particular Jaccard distance calculates the distance based on the overlap of relevant neighbors. Due to camera variation intra-camera samples dominate the relevant neighbors which reduces the reliability of the neighbors by introducing intra-camera negative samples and excluding inter-camera positive samples. To overcome this problem we propose a novel camera-aware Jaccard (CA-Jaccard) distance that leverages camera information to enhance the reliability of Jaccard distance. Specifically we design camera-aware k-reciprocal nearest neighbors (CKRNNs) to find k-reciprocal nearest neighbors on the intra-camera and inter-camera ranking lists which improves the reliability of relevant neighbors and guarantees the contribution of inter-camera samples in the overlap. Moreover we propose a camera-aware local query expansion (CLQE) to mine reliable samples in relevant neighbors by exploiting camera variation as a strong constraint and assign these samples higher weights in overlap further improving the reliability. Our CA-Jaccard distance is simple yet effective and can serve as a general distance metric for person re-ID methods with high reliability and low computational cost. Extensive experiments demonstrate the effectiveness of our method. Code is available at https://github.com/chen960/CA-Jaccard/.