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Title: Spreading vectors for similarity search

Paper content:
Discretizing floating-point vectors is a fundamental step of modern indexing methods.
State-of-the-art techniques learn parameters of the quantizers on training data for optimal performance, thus adapting quantizers to the data.
In this work, we propose to reverse this paradigm and adapt the data to the quantizer: we train a neural net whose last layers form a fixed parameter-free quantizer, such as pre-defined points of a sphere.
As a proxy objective, we design and train a neural network that favors uniformity in the spherical latent space, while preserving the neighborhood structure after the mapping.
For this purpose, we propose a new regularizer derived from the Kozachenko-Leonenko differential entropy estimator and combine it with a locality-aware triplet loss.
Experiments show that our end-to-end approach outperforms most learned quantization methods, and is competitive with the state of the art on widely adopted benchmarks.
Further more, we show that training without the quantization step results in almost no difference in accuracy, but yields a generic catalyser that can be applied with any subsequent quantization technique.
Recent work BID27 proposed to leverage the pattern-matching ability of machine learning algorithms to improve traditional index structures such as B-trees or Bloom filters, with encouraging results.
In their one-dimensional case, an optimal B-Tree can be constructed if the cumulative density function (CDF) of the indexed value is known, and thus they approximate this CDF using a neural network.
We emphasize that the CDF itself is a mapping between the indexed value and a uniform distribution in [0, 1] .
In this work, we wish to generalize such an approach to multi-dimensional spaces.
More precisely, as illustrated by FIG0 , we aim at learning a function that maps real-valued vectors to a uniform distribution over a d-dimensional sphere, such that a fixed discretizing structure, for example a fixed binary encoding (sign of components) or a regular lattice quantizer, offers competitive coding performance.Our approach is evaluated in the context of similarity search, where methods often rely on various forms of learning machinery BID12 BID45 ; in particular there is a substantial body of literature on methods producing compact codes BID20 ).
Yet the problem of jointly optimizing a coding stage and a neural network remains essentially unsolved, partly because .
It is learned end-to-end, yet the part of the network in charge of the discretization operation is fixed in advance, thereby avoiding optimization problems.
The learnable function f , namely the "catalyzer", is optimized to increase the quality of the subsequent coding stage.
input λ = 0 λ = 0.01 λ = 0.1 λ → ∞ FIG1 : Illustration of our method, which takes as input a set of samples from an unknown distribution.
We learn a neural network that aims at preserving the neighborhood structure in the input space while best covering the output space (uniformly).
This trade-off is controlled by a parameter λ.
The case λ = 0 keeps the locality of the neighbors but does not cover the output space.
On the opposite, when the loss degenerates to the differential entropic regularizer (λ → ∞), the neighbors are not maintained by the mapping.
Intermediate values offer different trade-offs between neighbor fidelity and uniformity, which is proper input for an efficient lattice quantizer (depicted here by the hexagonal lattice A 2 ).it
is difficult to optimize through a discretization function. For
this reason, most efforts have been devoted to networks producing binary codes, for which optimization tricks exist, such as soft binarization or stochastic relaxation, which are used in conjunction with neural networks BID28 BID18 . However
it is difficult to improve over more powerful codes such as those produced by product quantization BID20 , and recent solutions addressing product quantization require complex optimization procedures BID24 BID34 .In order
to circumvent this problem, we propose a drastic simplification of learning algorithms for indexing. We learn
a mapping such that the output follows the distribution under which the subsequent discretization method, either binary or a more general quantizer, performs better. In other
terms, instead of trying to adapt an indexing structure to the data, we adapt the data to the index.Our technique requires to jointly optimize two antithetical criteria. First, we
need to ensure that neighbors are preserved by the mapping, using a vanilla ranking loss BID40 BID6 BID44 . Second, the
training must favor a uniform output. This suggests
a regularization similar to maximum entropy BID36 , except that in our case we consider a continuous output space. We therefore
propose to cast an existing differential entropy estimator into a regularization term, which plays the same "distribution-matching" role as the Kullback-Leiber term of variational auto-encoders BID9 .As a side note
, many similarity search methods are implicitly designed for the range search problem (or near neighbor, as opposed to nearest neighbor BID15 BID0 ), that aims at finding all vectors whose distance to the query vector is below a fixed threshold. For real-world
high-dimensional data, range search usually returns either no neighbors or too many. The discrepancy
between near-and nearest-neighbors is significantly reduced by our technique, see Section 3.3 and Appendix C for details.Our method is illustrated by FIG1 . We summarize our
contributions as follows:• We introduce an approach for multi-dimensional indexing that maps the input data to an output space in which indexing is easier. It learns a neural
network that plays the role of an adapter for subsequent similarity search methods.• For this purpose
we introduce a loss derived from the Kozachenko-Leonenko differential entropy estimator to favor uniformity in the spherical output space.• Our learned mapping
makes it possible to leverage spherical lattice quantizers with competitive quantization properties and efficient algebraic encoding.• Our ablation study shows
that our network can be trained without the quantization layer and used as a plug-in for processing features before using standard quantizers. We show quantitatively that
our catalyzer improves performance by a significant margin for quantization-based (OPQ BID10 ) and binary (LSH BID5 ) method.This paper is organized as follows. Section 2 discusses related
works. Section 3 introduces our neural
network model and the optimization scheme. Section 4 details how we combine
this strategy with lattice assignment to produce compact codes. The experimental section 5 evaluates
our approach.
Choice of λ.
The marginal distributions for these two views are much more uniform with our KoLeo regularizer, which is a consequence of the higher uniformity in the high-dimensional latent space.Qualitative evaluation of the uniformity.
Figure 3 shows the histogram of the distance to the nearest (resp. 100 th nearest) neighbor, before applying the catalyzer (left) and after (right).
The overlap between the two distributions is significantly reduced by the catalyzer.
We evaluate this quantitatively by measuring the probability that the distance between a point and its nearest neighbor is larger than the distance between another point and its 100 th nearest neighbor.
In a very imbalanced space, this value is 50%, whereas in a uniform space it should approach 0%.
In the input space, this probability is 20.8%, and it goes down to 5.0% in the output space thanks to our catalyzer.Visualization of the output distribution.
While FIG1 illustrates our method with the 2D disk as an output space, we are interested in mapping input samples to a higher dimensional hyper-sphere.
FIG2 proposes a visualization of the high-dimensional density from a different viewpoint, with the Deep1M dataset mapped in 8 dimensions.
We sample 2 planes randomly in R dout and project the dataset points (f (x 1 ), ..., f (x n )) on them.
For each column, the 2 figures are the angular histograms of the points with a polar parametrization of this plane.
The area inside the curve is constant and proportional to the number of samples n.
A uniform angular distribution produces a centered disk, and less uniform distributions look like unbalanced potatoes.The densities we represent are marginalized, so if the distribution looks non-uniform then it is non-uniform in d out -dimensional space, but the reverse is not true.
Yet one can compare the results obtained for different regularization coefficients, which shows that our regularizer has a strong uniformizing effect on the mapping, ultimately resembling that of a uniform distribution for λ = 1.

assistant: text: We learn a neural network that uniformizes the input distribution, which leads to competitive indexing performance in high-dimensional space