Hierarchical Link and Code: Efficient Similarity Search for Billion-Scale Image Sets

Similarity search is an indispensable component in many computer vision applications. To index billions of images on a single commodity server, Douze et al. introduced L&C that works on operating points considering 64–128 bytes per vector. While the idea is inspiring, we observe that L&C still suffers the accuracy saturation problem, which it is aimed to solve. To this end, we propose a simple yet effective two-layer graph index structure, together with dual residual encoding, to attain higher accuracy. Particularly, we partition vectors into multiple clusters and build the top-layer graph using the corresponding centroids. For each cluster, a subgraph is created with compact codes of the first-level vector residuals. Such an index structure provides better graph search precision as well as saves quite a few bytes for compression. We employ the second-level residual quantization to re-rank the candidates obtained through graph traversal, which is more efficient than regression-from-neighbors adopted by L&C. Comprehensive experiments show that our proposal obtains over 30% higher recall@1 than the state-of-thearts, and achieves up to 7.7x and 6.1x speedup over L&C on Deep1B and Sift1B, respectively. CCS Concepts • Information systems → Top-k retrieval in databases;