https://www.googblogs.com/announcing-scann-efficient-vector-similarity-search/
Announcing ScaNN: Efficient Vector Similarity Search
Posted by Philip Sun, Software Engineer, Google Research
“Based on this intuition, we more heavily penalize quantization error that is parallel to the original vector. We refer to our novel quantization technique as anisotropic vector quantization due to the directional dependence of its loss function. The ability of this technique to trade increased quantization error of lower inner products in exchange for superior accuracy for high inner products is the key innovation and the source of its performance gains.”