Quarter-point codeword expansion for product quantization

Due to its low storage cost and high query accuracy, Product Quantization (PQ) has been widely used for approximate nearest neighbor (ANN) search. However, almost all existing PQ-based methods use the nearest clustering center as the codeword, which might not fully utilize the information of distances from data points to clustering centers. In this paper, we propose a novel codeword expansion method for PQ-based methods, called Quarter-point Codeword Expansion (QCE), by estimating the distances from the query points to the database points using the quarter points instead of the clustering centers. The distances can be computed more precisely and it will result in a lower distortion using QCE, which is also a general method could be used to improve all PQ-based methods. Extensive experiments on approximate nearest neighbor search show that PQ-based methods with QCE can outperform the state-of-the-art.