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Deep hashing approaches are widely applied to approximate nearest neighbor search for large-scale image retrieval. We propose Spherical Deep Supervised Hashing (SDSH), a new supervised deep hashing approach to learn compact binary codes. The goal of SDSH is to go beyond learning similarity preserving codes, by encouraging them to also be balanced and to maximize the mean average precision. This is enabled by advocating the use of a different relaxation method, allowing the learning of a spherical embedding, which overcomes the challenge of maintaining the learning problem well-posed without the need to add extra binarizing priors. This allows the formulation of a general triplet loss framework, with the introduction of the spring loss for learning balanced codes, and of the ability to learn an embedding quantization that maximizes the mean average precision. Extensive experiments demonstrate that the approach compares favorably with the state-of-the-art while providing significant performance increase at more compact code sizes.
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Deep Supervised Hashing with Spherical Embedding
Stanislav Pidhorskyi1, Quinn Jones1, Saeid Motiian2, Donald Adjeroh1, and
Gianfranco Doretto1
1Lane Department of Computer Science and Electrical Engineering
West Virginia University, Morgantown, WV 26508, USA
{stpidhorskyi, qjones1, daadjeroh, gidoretto}@mix.wvu.edu
2Adobe Applied Research, San Francisco, CA 94103, USA
motiian@adobe.com
Abstract. Deep hashing approaches are widely applied to approximate nearest
neighbor search for large-scale image retrieval. We propose Spherical Deep Su-
pervised Hashing (SDSH), a new supervised deep hashing approach to learn com-
pact binary codes. The goal of SDSH is to go beyond learning similarity preserv-
ing codes, by encouraging them to also be balanced and to maximize the mean
average precision. This is enabled by advocating the use of a different relaxation
method, allowing the learning of a spherical embedding, which overcomes the
challenge of maintaining the learning problem well-posed without the need to
add extra binarizing priors. This allows the formulation of a general triplet loss
framework, with the introduction of the spring loss for learning balanced codes,
and of the ability to learn an embedding quantization that maximizes the mean
average precision. Extensive experiments demonstrate that the approach com-
pares favorably with the state-of-the-art while providing significant performance
increase at more compact code sizes.
1 Introduction
Indexing and searching large-scale image databases leverage heavily hashing based ap-
proximate nearest neighbor search technology. The goal of hashing is to map high di-
mensional data, such as images, into compact codes in a way that visually, or semanti-
cally similar images are mapped into similar codes, according to the Hamming distance.
Given a query image, hierarchically structured based methods can then be used for the
rapid retrieval of the neighbors within a certain distance from the query [1].
Recent data-dependent methods for hashing images (as opposed to data-independent
methods [12]) leverage deep neural networks due to their ability to integrate the image
feature representation learning with the objective of the hashing task [9, 21, 45, 42, 3],
leading to a superior efficiency and compactness of the codes. However, so far the focus
of these approaches has been on designing architectures only with the similarity pre-
serving goal of mapping similar images to similar codes. On the other hand, hashing
methods based on hand-crafted image features [28], have improved performance also
by requiring codes to have certain properties, for example, to be balanced, uncorrelated,
or to be obtained with a small quantization error [44, 39, 36, 13, 30, 26, 25].
Balanced codes are such that bits partition data in equal portions [44, 39]. It is a
desirable property because it increases the variance of bits since they approach 50%
2 S. Pidhorskyi et al.
chance of being one or zero. Also code bits that are uncorrelated, or even better inde-
pendent, increase their information content. In addition, learning hash functions gen-
erally require solving intractable optimization problems that are made tractable with
the continuous relaxation of the codomain of the function. This means that quantiza-
tion is required to discretize the continuous embedding into codes, and controlling the
quantization error has been shown to improve performance [13].
In this work, we propose a deep supervised hashing approach that goes beyond sim-
ilarity preserving, and that aims at learning hashing functions that map onto codes with
good quality, by encouraging them to be balanced, and to maximize the mean average
precision. We do so by advocating the use of a different continuous relaxation strategy
that has several advantages, incto overcome the challenge of maintaining the learning
problem well-posed, without the addition of costly priors that typically encourage a
binary output, and that have led deep hashing approaches to focus, so far, only on simi-
larity preserving codes. This approach allows learning a hashing function composed of a
spherical embedding followed by an optimal quantization. Specifically, the embedding
is based on a convolutional neural network, learned with a triplet loss framework [34],
that has the advantage of being more general, because it allows the use of different
triplet losses, and it allows introducing a new triplet spring loss that aims at learning
balanced codes. Moreover, the loss framework is rotation invariant in the embedded
space, which allows optimizing the quantization for a rotation that provides the high-
est mean average precision. We call the resulting model Spherical Deep Supervised
Hashing (SDSH), and provides state-of-the-art performance on standard benchmarks,
including a significantly greater performance at more compact code dimensions.
2 Related Work
The existing variety of data-dependent, learning based hashing methods can be catego-
rized into unsupervised and supervised methods [40].
Unsupervised methods [44, 13, 27, 20, 19, 15, 11, 16] use unlabeled data to learn a
hashing function that preserves some metric distance between data points. This work
instead falls into the supervised category, which tries to improve the quality of hashing
by leveraging label information to learn compact codes. Supervised methods can be
divided into those which use off-the-shelf visual features versus those that leverage
deep networks. Representative examples of non-deep methods include Minimal Loss
Hashing (MLH) [31], Supervised Hashing with Kernels (KSH) [26] and Latent Factor
Hashing (LFH) [46].
The initial attempts to utilize deep learning [18, 37, 38, 41] for hashing included
CNNH [45] and DNNH [21]. Deep Hashing Network (DHN) [50] and Deep Super-
vised Hashing (DSH) [24] extend DNNH by performing a continuous relaxation of
the intractable discrete optimization by introducing a quantization error prior which is
controlled by a quantization loss. DHN uses a cross entropy loss to link the pairwise
Hamming distances with the pairwise similarity labels, while DSH uses max-margin
loss. Deep Cauchy Hashing (DCH) [2] improves DHN by utilizing Cauchy distribu-
tion. Deep Pairwise-Supervised Hashing (DPSH) [23] uses pairwise similarity labels
and a loss-function similar to LFH, which maximizes the log-likelihood of the pairwise
Deep Supervised Hashing with Spherical Embedding 3
query
positive
negative
Spherical embedding Rotation optimization
Image feature learning
Shared
weights
Shared
weights
Fig. 1. Overview. Overview of the approach, highlighting the stages of the hash embedding learn-
ing, and the optimal quantization.
similarities. The log-likelihood is modeled as a function of the Hamming distance be-
tween the corresponding data points. Deep Triplet-Supervised Hashing (DTSH) [42]
extends DPSH by using triplet label information.
The learning problem of deep hashing methods (DHN, DSH, DPSH, DTSH, etc.)
turns out to be NP-complete, due to the discrete nature of the codomain of the hash
function being sought, which is the Hamming space. The workaround is to relax the na-
tive space into the continuous Euclidean counterpart. This makes the original learning
problem ill-posed, and a regularizing prior becomes necessary, which is often chosen to
encourage the sought mapping to produce a nearly binary output. While needed, such
prior complicates the training and might lead to performance reduction. Discrepancy
Minimizing Deep Hashing (DMDH) [7] suggests an alternating optimization approach
with a series expansion of the objective. Our work instead, leverages a different relax-
ation, which has the advantage of maintaining a well-posed learning problem, without
the need for extra priors. Besides the obvious computational advantage, the framework
allows to identify a class of triplet losses, and to define new ones, tailored to seeking
good hash functions.
Another line of work, like Deep Quantization Network (DQN) [4], avoid the use of
relaxation by incorporating quantization methods into the approach [10, 48, 43]. DQN
performs a joint learning of image representations and a product quantization for gen-
erating compact binary codes. Deep Visual-Semantic Quantization (DVSQ) [3] extends
DQN by adding semantic knowledge extracted from the label space. In this way, hash
codes are directly optimized. While being an interesting direction, it significantly in-
creases the complexity of the learning process. Indeed, that might be one of the con-
tributing factors that make our approach comparing favorably against those.
Finally, our approach also considers the quantization problem. Indeed, the proposed
relaxation suggests learning a spherical embedding, which is an equivalence class of so-
lutions, because the loss turns out to be rotation invariant with respect to the embedded
spherical space. This allows picking, as a solution, a representative of the class that will
affect the quantization of the spherical embedding in such a way that it directly maxi-
mizes the mean average precision. This is different from previous approaches, and it is
different also from approaches like Iterative Quantization (ITQ) [13], which is unsuper-
4 S. Pidhorskyi et al.
vised, and it aims at minimizing the quantization error. Our comparison with ITQ shows
that linking the quantization directly to the retrieval metric leads to better solutions.
3 Problem Overview
Given a training set of Nimages I={I1,· · · , IN}, with labels Y={y1,· · · , yN}, we
are interested in learning a hash function hthat maps an image Ionto a compact binary
code b=h(I)∈ {+1,1}Bof length B. The typical approach based on deep learning
assumes that given three images Ii,Ij,Ik, with labels yi,yj,yk, such that yi=yj, and
yi6=yk, then the hash function should be such that the corresponding binary codes bi
and bjshould be close, while biand bkshould be far away in the Hamming space. If
dH(·,·)indicates the Hamming distance, this means that dH(bi,bj)should be as small
as possible, while dH(bi,bk)should be as large as possible.
In addition to that, ideally, we would want to encourage hash codes to be maximally
informative, where bits are independent and balanced, and to ultimately maximize the
mean average precision (mAP). These aspects have been of secondary importance thus
far, because deep learning approaches have to use binarizing priors to regularize loss
functions that are already computationally intensive to optimize.
We overcome the major hurdle of the binarizing prior by advocating the use of
a different relaxation method, which does not require additional priors, and learns a
spherical embedding. This modeling choice has ripple effects. Besides simplifying the
learning by eliminating the binarizing prior, it enables a unified formulation of a class
of triplet losses [34] that are rotation invariant, and it allows to introduce one, which
we name spring loss, that encourages balanced hash codes. In addition, the rotation
invariance allows us to look for a rotation of the embedding hypersphere that leads to
its optimal quantization for producing hash codes that directly maximize the retrieval
mAP. This two-stage approach is depicted in Figure 1.
4 Hash Function Learning
The desired hash function should ensure that the code biof image Iiwould be closer
to all other codes bjof Ijbecause yi=yj, than it would be to any code bkof Iksince
yi6=yk. Therefore, if T={(i, j, k)|yi=yj6=yk}is the set of the allowed triplet
labels, then we certainly desire this condition to be verified
dH(bi,bj)< dH(bi,bk)(i, j, k)∈ T .(1)
A loss function that aims at achieving condition (1) could simply be written as
L(h) = X
(i,j,k)∈T
`(dH(bi,bj)dH(bi,bk)) (2)
where `(·) : R[0,+)is the cost for a triplet that does not satisfy (1). Equation (2)
is a more general version of the well known triplet loss [34].
Deep Supervised Hashing with Spherical Embedding 5
Fig. 2. Embedding distribution. Bimodal dis-
tribution of the hash embedding components
(a) early on during training, and (b) at ad-
vanced training stage, when modes are sepa-
rated.
(a) (b)
Fig. 3. Quantization. (a) Distribution of hash
embeddings on the unit circle for two classes.
The sign quantization assigns different hash
codes to samples in the same class. (b) Ro-
tated distribution of hash embeddings. The
sign quantization assigns same hash codes to
samples in the same class, thus increasing the
mAP.
As pointed out in [42], it is easy to realize that
dH(bi,bj)dH(bi,bk) = 1
2b>
ibk1
2b>
ibj.(3)
In particular, the Hamming space where codes are defined, and through which depends
the estimation of the hash function h, makes the optimization of (2) intractable [23].
Therefore, the typical approach is to relax the domain of bfrom the Hamming space
to the continuous space RB[23, 42]. However, this method has severe drawbacks. The
first and most important one is that optimizing (2) becomes an ill-posed problem, with
trivial solutions corresponding to pulling infinitely apart relaxed codes with label mis-
match. This forces the introduction of a regularizing prior to the loss, which typically is
designed to encourage the relaxed code ˜
bto be also “as binary as possible”, or in other
words, to stay close to one of the vertices of the Hamming space.
Computationally, adding a prior is a major setback because it increases the number
of hyperparameters at best, with all the consequences. In addition, if we look at the
distribution of the values of the components of ˜
b, we have observed experimentally
that as the two main modes around +1 and 1become separated, the corresponding
hash codes, obtained simply by taking b=sgn(˜
b), stop changing during the training
procedure. See Figure 2. This “locking” behavior might prevent from learning a hash
function that could potentially be more efficient if it still had room to adjust the outputs.
Finally, we note that (3) does not hold in the relaxed space, meaning that
˜
bi˜
bj
2
2
˜
bi˜
bk
2
2
6=1
2˜
b>
i˜
bk1
2˜
b>
i˜
bj.(4)
even though there are approaches that rely on the left-hand-side of (4) being approxi-
mately equal to the right-hand-side [42].
The following section addresses the drawbacks outlined above by advocating the
use of a different relaxation for hash function learning.
6 S. Pidhorskyi et al.
5 Spherical Embedding
The hash function learning problem can be summarized as learning a function ˜
hsuch
that h(I) = sgn[˜
h(I)], where ˜
hoptimizes a relaxed version of (2). Differently from pre-
vious hashing work, we propose to use a relaxation where ˜
his a spherical embedding,
meaning that we constrain the output s=˜
h(I)to be defined on the (B1)-dimensional
unit sphere. This means that, using the previous notation, s.
=˜
b/k˜
bk, and the mean-
ing of Equations (1), (2), and (3) remain valid by simply substituting bwith s, and
dH(bi,bj)with k(sisj)/2k2. Therefore, we advocate the end-to-end learning of a
function ˜
h, given by minimizing the loss
L(˜
h) = X
(i,j,k)∈T
`(s>
isks>
isj).(5)
This approach, also used in [34] to regularize the spreading of the embedding, is
leveraged here to address the limitations of the continuous relaxation described in Sec-
tion 4. Indeed, a spherical embedding makes the optimization of the relaxed version
of (2) (which is (5)) a well-posed problem, and this was the main reason why previous
works required a regularizing prior. In addition, previous priors encouraged the embed-
ding space to be “as binary as possible” by moving ˜
bcloser to a vertex of the Hamming
cube, without direct evidence that this was producing better hash codes. On the other
hand, we observed this practice to encourage a “locking” behavior, wich we do not
have with the spherical embedding because there are no forces pushing towards the
Hamming cube, and sis free to move on the unit sphere. Moreover, if ˜
his an optimal
spherical embedding, so is R˜
h, where Ris a rotation matrix, since it still minimizes (5).
Therefore, since (5) is rotation invariant,˜
his found modulo a rotation, which can be
estimated at a later stage to optimize other hash code properties (Section 6).
In Section 7 we design different triplet loss functions `(·), leading to different spher-
ical embeddings. In practice, the spherical embedding comprises a convolutional neural
network with a number of layers aiming at learning visual features from images, fol-
lowed by fully connected layers with an output dimension equal to the number of bits
Bof the hash code. We adopt the VGG-F architecture [5], and replace the last fully
connected layer, but other architectures can also be used [18,14].
6 Quantization
Given an image I, its hash code is computed with a sign quantization as b=sgn(˜
h(I)).
Since ˜
hminimizes (5), we observed that also a rotated version R˜
hdoes, so it is impor-
tant to analyze the difference between the two solutions. Figure 3(a) shows a case where
the spherical embeddings of two classes along the unit circle are such that the sign quan-
tization assigns different hash codes to samples of the same class. Therefore, a rotation
Rcould be applied to the embeddings as in Figure 3(b), where the sign quantization
would now produce the expected results.
We propose to use the extra degrees of freedom due to the rotation invariance of (5)
for learning a rotation matrix Rthat finds the quantization that produces the best hash
Deep Supervised Hashing with Spherical Embedding 7
function. We do so by estimating the rotation Rthat maximizes the mean average pre-
cision (mAP), which is the metric that we value the most for retrieval
ˆ
R= arg max
RmAP(R).(6)
Since mAP(·)is not a smooth function, and has zero gradient almost everywhere, (6) is
not easy to optimize, even with derivative-free methods. On the other hand, we found
that a standard random search optimization with a linear annealing schedule allows
to achieve good results quickly. At the i-th iteration we apply a random perturbation
Q(i)to the current rotation matrix R(i)to obtain the update R(i+1) =Q(i)R(i), which
we retain if it improves the mAP. Since the perturbation should be random, uniform,
and with a controllable magnitude, we generate it by setting Q(i)=P(i)E(θ)P(i)>,
where P(i)is a random unitary matrix, generated with a simplified approach based on
the SV D decomposition of a matrix with normally sampled elements [29, 32]. E(θ)
instead represents a rotation by θon the plane identified by the first two basis vectors.
The angle θdefines the perturbation magnitude and varies linearly with the iteration
number, starting with θ0= 1.0down to 0when the maximum number of iterations
is reached, which was 800 in our experiments. Algorithm 1 summarizes the steps. We
compute the mAP with a C++ implementation, where we take 1000 samples from the
training set as queries and 16000 samples as database, or a smaller number if the training
set is smaller. With a PC workstation with CPU Core i7 5820K 3.30GHz the running
time for one iteration update is around 0.4s, keeping the time for the random search
optimization very small, if compared with the time for training the deep network of the
spherical embedding.
We now note that if Ris an optimal solution, by swapping two columns of Rwe
obtain a new solution, corresponding to swapping two bits in all hash codes. Since
there are B!of these kind of changes, it means that the order of growth of the solution
space is O(B!). Therefore, as Bincreases, estimating Raccording to (6) becomes less
important because the likelihood that a random Ris not far from an optimal solution
has increased accordingly. The experimental section supports this observation.
7 Triplet Spherical Loss
In this section we give three examples of rotation invariant triplet loss `that can be
used in (5), namely the margin loss, the label likelihood loss, and the new spring loss.
To shorten the notation, we define the quantity di,j,k .
=s>
isks>
isj.
7.1 Margin Loss
The first loss that we consider is well known, and stems from requiring condition (1) to
be verified with a certain margin α, in combination with using the standard hinge loss.
This translates into the following margin loss
`(di,j,k) = max{0, di,j,k +α}.(7)
8 S. Pidhorskyi et al.
2
d
Fig. 4. Spring loss. Unit sphere where two points with different class labels are pulled apart by
an elastic force proportional to the displacement 2d, while constrained to remain on the sphere.
7.2 Label Likelihood Loss
The second loss has been originally proposed in [42], where it was used with the Ham-
ming space relaxed into the continuous space RBfor learning a hashing function. Here
we extend it for learning a spherical embedding. The loss is derived from a proba-
bilistic formulation of the likelihood of the triplet labels T, where triplets that verify
condition (1) by bigger margins have a bigger likelihood, and where the margin param-
eter αcan affect the speed of the training process. This label likelihood loss, adapted to
our framework becomes
`(di,j,k) = di,j,k +α+ log(1 + edi,j,kα).(8)
7.3 Spring Loss
While both the margin loss and the label likelihood loss produce remarkable results,
none of them make explicit efforts towards clustering samples in the spherical embed-
ding space, according to their classes, and in a way that classes cover the sphere in a
spatially uniform manner. This last property is very important because if we assume an
equal number of samples per class, for each bit b, balanced codes satisfy the property
N
X
i=1
hb(Ii)=0, b = 1,· · · , B . (9)
Therefore, we note that condition (9) is satisfied whenever the spherical embedding
distributes the classes uniformly on the unit sphere, thus producing balanced codes,
where code bits have higher variance and are more informative. This has motivated the
design of the loss that we introduce.
Let us consider two points siand skon the (B1)-dimensional sphere of unit
radius, and let us assume that a spring is connecting them. The Euclidean distance
d=ksiskk.
=pdi,k, between the points varies in the range [0,2]. At distance 2, we
consider the spring unstretched, while at distance d < 2, the spring will have accumu-
lated an elastic potential energy proportional to (2d)2. See Figure 4. This suggests that
we could train the hash embedding by minimizing (5), with `(di,k) = (2 pdi,k)2,
Deep Supervised Hashing with Spherical Embedding 9
Result: Returns the optimal matrix Raccording to a random search
R(0) =I;
i= 0 ;
while i < number of iterations do
// f(i) is a linear annealing schedule to update the
rotation magnitude
θ(i)f(i);E(i)E(θ(i));P(i)random unitary matrix ;
Q(i)P(i)E(i)P(i)>;R0Q(i)R(i);
if mAP(R0)>mAP(R(i))then
R(i+1) R0;
else
R(i+1) R(i);
end
ii+ 1;
end
return Ri
Algorithm 1: Random search for the optimal rotation R.
where we would limit the summation to the pairs (i, k)∈ Q ={(i, j)|yi6=yj}. In
this way, the training would aim at minimizing the total elastic potential energy of the
system of springs. The effect is that samples from different classes would be mapped
on the sphere, but as far apart as possible. We note that minimizing this loss is equiva-
lent to solving a first order linear approximation to the Thomson’s Problem [35], which
concerns the determination of the minimum electrostatic potential energy configura-
tion of Nelectrons, constrained to the surface of a unit sphere. The solution has been
rigorously identified in only a handful of cases, which mostly correspond to spatial
configurations forming regular polyhedrons, which we have observed also in our simu-
lations using the spring loss described before, as it can be seen in Figure 5. If we were
to perform a Voronoi tessellation on the sphere based on the class centroids, we clearly
would obtain a pretty uniform partition of the sphere, which is our main goal. For com-
parison, the right-most image in Figure 5 is obtained with the margin loss, which stops
pulling apart query and negative samples once they are more far apart than the margin.
This leads to a less uniform distribution of the classes.
We have experimented with the loss described above and indeed, it provides fairly
good results. However, we argue that it can be improved because it does not explicitly
pull closer samples that belong to the same class, besides pulling apart samples with
different labels. We address that issue with the triplet spring loss, which we define as
follows
`(di,j,k) = (2 p2di,j,k)2.(10)
Note that di,j,k varies in the range [2,2], thus the square root varies in the range [0,2],
and the loss varies in the range [0,4]. The loss is minimized when di,j,k approaches 2.
Since di,j,k is proportional to ksisjk2− ksiskk2, convergence is approached by
maximally pulling closer siand sj, while maximally pushing apart siand sk. Note that,
differently than before, now even when siand skhave reached a distance of 2, the loss
still works to pull siand sjcloser.
10 S. Pidhorskyi et al.
Fig. 5. Regular polyhedrons. Three left-most images: Three unit spheres with 5×npoints at
minimum elastic potential, where nis the number of classes. From left to right nis equal to 4,
12, 24. The 5 points per class coincide with the class centroid at equilibrium. Right-most image:
For n= 12, the points at minimum margin loss do not reach a uniform distribution on the sphere.
8 Experiments
We tested our approach on the most relevant datasets for deep hashing applications,
namely CIFAR-10 [17], NUS WIDE [8], and we also tested on MNIST [22] to compare
with some older methods. For each experimental setting, we report the average mAP
score over 5 runs for comparison against the previous works for hashes of size as low
as 4 and up to 48 bits.
8.1 Experimental setup
Similar to other deep hashing methods we use raw image pixels as input. Following [42,
49, 23], for the spherical embedding we adopt VGG-F [6] pre-trained on ImageNet. We
replace the last layer with our own, initialized with normal distribution. The output layer
doesn’t have activation function and the number of outputs matches the needed number
of bits - B. The input layer of VGG-F is 224x224, so we crop and resize images of the
NUS WIDE dataset and upsample images of the CIFAR-10 dataset to match the input
size.
CIFAR-10: CIFAR-10 [17] is a publicly available dataset of small images of size
32x32 which have each been labeled to one of ten classes. Each class is represented
by 6,000 images for a total of 60,000 available samples. In terms of evaluation in the
CIFAR domain, two images are counted as relevant to each other if their labels match.
In order for our experiments to be comparable to as many works as possible, includ-
ing [42] and [3], we use two different experimental settings, which are labeled “Full”
and “Reduced”.
“Full” setting: For this setting, 1,000 images are first selected randomly from each
class of the dataset to make up the test images. Which, by extension, results in 10,000
query images. The remaining 50,000 images are used as the database images and as the
images used in training.
“Reduced” setting: For this setting, 100 images are selected randomly from each
class for use as 1,000 total test images. From the remaining 59,000 samples we ran-
domly sample 500 images per category to form the reduced training set with only 5,000
images. The database is composed of all 59,000 samples which were not selected for
testing.
Deep Supervised Hashing with Spherical Embedding 11
Table 1. Mean Average Precision (MAP) Results for Different Number of Bits of CIFAR-10: In
the case of DTSH and DVSQ we have filled some additional results which were not presented
by the original papers by using the authors’ respective released source code to replicate their
experiments such that we may compare with them across more hash sizes.
CIFAR-10 Full setting: Number of Bits CIFAR-10 Reduced setting: Number of Bits
Method 4 8 12 16 24 32 48 4 8 12 16 24 32 48
DQN[4] - - - - - - - - - 0.554 - 0.558 0.564 0.580
DSH[24] - - 0.616 - 0.651 0.661 0.676 - - - - - - -
DPSH[23] - - 0.763 - 0.781 0.795 0.807 - - 0.763 - 0.727 0.744 0.757
DMDH [7] - - - - - - - - - - 0.704 - 0.719 0.732
DTSH[42] - 0.814 0.859 0.915 0.923 0.925 0.926 - 0.641 0.710 0.723 0.750 0.765 0.774
DVSQ[3] - 0.839 - 0.839 0.843 0.840 0.842 - 0.715 - 0.727 0.730 0.733 0.764
BL[33] 0.870 -
SDSH-ML - 0.839 0.882 0.886 0.939 0.880 0.878 - 0.657 0.712 0.756 0.747 0.765 0.764
SDSH-LL 0.481 0.763 0.854 0.942 0.945 0.944 0.947 0.407 0.673 0.757 0.782 0.799 0.815 0.822
SDSH-S 0.755 0.911 0.939 0.938 0.939 0.939 0.934 0.569 0.697 0.723 0.783 0.801 0.810 0.813
The mAP for CIFAR-10, full and reduced setting, is computed based on all sam-
ples from the database set. We have used for training purposes a system with only one
NVIDIA Titan X GPU, in this configuration training takes about four hours for the Cifar
Full setting.
NUS WIDE: NUS WIDE [8] is another publicly available dataset, but unlike CIFAR-
10 each sample image is multi-labeled from a set of 81 possible labels across all 269,643.
This is reduced slightly, as [42] and [3] have also done in their experiments, by first re-
moving every image which does not have any of the 21 most common labels associated
with it. This is done as many of the less common labels have very few samples asso-
ciated with them, but prepared in this way each of the 21 labels are represented by at
least 5,000 samples. In terms of evaluation in the NUS WIDE domain two images are
counted as relevant to each other if any of their labels match. Note, that despite the
usage of samples associated with the 21 most frequent labels, all 81 labels are used for
determining similarity between samples. To compare with previous work we use three
different settings, labeled “Full”, “Reduced A”, and “Reduced B”.
“Full” setting: For this setting, 100 samples from each of the 21 most frequent labels
are reserved for the test set. And the remaining images are used both as the database and
as the training set. The mAP is computed based on the top 50000 returned neighbors.
“Reduced A” setting: For this setting, the 2100 test samples are selected as in the
Full setting. From the remaining samples, 500 were sampled from the 21 most frequent
labels to compose the training set. The remaining were used for the database. The mAP
is computed based on top 5000 returned neighbors.
“Reduced B” setting: For this setting, the training set was chosen by sampling the
available images uniformly 5,000 times. From the remaining samples the training set
was uniformly sampled 10,000 times. All of the remaining samples from these two
operations were used as the database. The mAP is computed based on top 5000 returned
neighbors.
8.2 Results
Our method is abbreviated as SDSH, and the combination with the margin loss, label
likelihood loss, and spring loss are indicated as SDSH-ML, SDSH-LL, and SDSH-S
12 S. Pidhorskyi et al.
Table 2. Mean Average Precision (MAP) Results for Different Number of Bits on NUS WIDE
Number of Bits
Method 16 24 32 48
DTSH[42] 0.756 0.776 0.785 0.799
DPSH[23] 0.715 0.722 0.736 0.741
SDSH-ML 0.794 0.800 0.797 0.805
SDSH-LL 0.452 0.808 0.810 0.812
SDSH-S 0.812 0.817 0.821 0.821
(a) Full settings
Number of Bits
Method 8 12 16 24 32 48
DTSH[42] - 0.773 - 0.808 0.812 0.824
SDSH-ML 0.758 0.770 0.784 0.798 0.802 0.810
SDSH-LL 0.751 0.780 0.792 0.805 0.810 0.817
SDSH-S 0.774 0.789 0.796 0.807 0.812 0.820
(b) Reduced A settings
Number of Bits
Method 8 16 24 32
DVSQ[3] 0.780 0.790 0.792 0.797
DMDH [7] - 0.751 - 0.781
SDSH-ML 0.739 0.771 0.785 0.791
SDSH-LL 0.750 0.771 0.782 0.789
SDSH-S 0.755 0.783 0.786 0.790
(c) Reduced B settings
Table 3. Mean Average Precision (MAP) Results for Different Number of Bits of MNIST
Number of Bits
Method 16 24 32 48
CNNH[45] 0.957 0.963 0.956 0.960
CNNH+[45] 0.969 0.975 0.971 0.975
DSCH[47] 0.965 0.966 0.972 0.975
DRSCH[47] 0.969 0.974 0.979 0.979
SDSH-ML 0.993 0.995 0.994 0.995
SDSH-LL 0.994 0.994 0.995 0.996
SDSH-S 0.994 0.995 0.995 0.995
respectively. All reported results are from our publicly available implementation3. The
average mAP scores for all methods on CIFAR-10 are listed in Table 1, which includes
also the baseline (BL) classification accuracy. As pointed out in [33], the BL value can
be interpreted as mAP attainable by a supervised system retrieving samples, one class
at a time, with classes ranked according to the class probability of the query. Therefore,
a retrieval approach should surpass the BL threshold to be effective. Table 1 shows that
the proposed SDSH-S is above BL starting from 8-bits, and it always outperforms the
state-of-the-art methods. In full setting and B= 8, and 12, SDSH-S shows large im-
provements, and outperforms DVSQ by 7.2% and 5.7%. SDSH-LL performs slightly
better than SDSH-S for B > 12, but has worse performance for lower number of bits.
For reduced setting, SDSH-S and SDSH-LL perform about the same, and always out-
performs the state-of-the-art methods with an exception for the 8-bit case. Figure 6
presents the data of Table 1 in the form of plots.
Figure 7 shows the comparison of the precision-recall curves of SDSH-S with those
produced by two state-of-the-art approaches, namely DTSH [42] and DVSQ [3], high-
lighting the promising performance of the proposed approach.
Tables 3a, 3b, and 3c show average mAP scores on NUS WIDE for Full, Reduced
A, and Reduced B settings respectively. Unfortunately, the protocol of this experiment
does not allow to compare against the BL value. Although [33] describes two additional
protocols, since they are suitable for tasks other than supervised hashing, for compar-
ison with the state-of-the-art, here we follow protocols that have been in use by the
widest majority of the literature. In particular, SDSH-S, SDSH-LL, and SDSH-ML al-
ways outperform the state-of-the-art methods on Full setting except for SDSH-LL at 8-
bit case, where it converged poorly. On full setting, SDSH-S outperforms other type of
losses for all bit numbers. On Reduced A setting, SDSH-S outperforms the state-of-the-
art methods for B < 24 and for higher Bit stays about the same as DTSH and slightly
3https://github.com/podgorskiy/SDSH
Deep Supervised Hashing with Spherical Embedding 13
Fig. 6. Average mAP scores. Comparison of mAP values w.r.t. bit number for our method
(SDSH-ML, SDSH-LL, SDSH-S) with DPSH [23], DTSH[42] and DVSQ[3].
below for B= 48. On Reduced B setting our method is outperformed by DVSQ. It is
important to note that NUS WIDE has 81 labels but only 500 samples from the 21 most
frequent labels are used for training. Therefore, even though the training set for NUS-
WIDE’s reduced setting is still about twice as large as the training set for CIFAR-10’s
reduced setting, the ratio of samples per label for CIFAR is 500, while the ratio for this
NUS WIDE setting is on average 129.6 per label. Therefore, for NUS WIDE reduced
setting, the network is more prone to overfitting. As for CIFAR-10, the bottom row of
Figure 6 presents the data of Tables 3a, 3b, and 3c in the form of plots.
In Table 3 we show a comparison between CNNH, CNNH+ [45], and DSCH,
DRSCH [47] on the MNIST [22] dataset, noticing that all the three losses provide a
performance increase.
8.3 Ablation Study
The proposed approach requires two steps for learning a hash function. The first step
learns a spherical embedding that identifies an equivalence class of solutions because
of the rotation invariance of the loss, and then a rotation needs to be identified to pick
a representative of the equivalence class. Here we analyze what happens if use the
identity as rotation, versus using the proposed method, versus using an off-the-shelve
method like ITQ [13]. The summary of the results is shown in Figure 9, where the three
approaches have been applied to SDSH-S, which has been tested on CIFAR-10, and
NUS WIDE. The first observation is that estimating the optimal rotation becomes more
important at lower number of bits. As we suggested in Section 6, when Bgrows, the
solution space grows significantly, so a random solution is more likely to do well. In
addition, we note that ITQ tends to underperform our approach, and often decreases the
performance of the identity solution. We note that ITQ differs from the proposed ap-
proach at least in two important aspects. First, ITQ is an unsupervised method, whereas
our random search leverages the label information. In addition, ITQ aims at minimiz-
14 S. Pidhorskyi et al.
Fig. 7. Precision-Recall Curves Comparison
of P-R curves from our method, DVSQ [3] and
DTSH [42] on CIFAR-10 reduced, top 5000
samples @ 32 bits.
Fig. 8. Effect of quantization step. Contribu-
tion of quantization step for different losses on
NUS-WIDE Full @ 24 bits, 32bits.
Fig. 9. Effect of learning rotation Comparison of mAP values for a range of bit number for three
scenarios: ITQ [13] and random search optimization and no rotation optimization.
ing the quantization error, whereas the proposed method looks for the best quantization
that maximizes the mAP. Finally, Figure 8 shows the performance improvement that
adds rotation optimisation for different loss types, highlighting, as expected, that each
of them can benefit from that step.
9 Conclusions
We have introduced SDSH, a novel deep hashing method that moves beyond the sole
goal of similarity preserving, and explicitly learns a hashing function that produces
quality codes. This is achieved by leveraging a different relaxation method that elim-
inates the need for regularizing priors, and it enables the design of loss functions for
learning balanced codes, and it allows to optimize the quantized hash function to max-
imize the mAP. Extensive experiments on three standard benchmark datasets demon-
strated the strength of the approach. In particular, addressing the issue of quality in deep
hashing approaches has revealed to be valuable, because the performance has increased
particularly for more compact codes, which is very important for building efficient re-
trieval systems.
Acknowledgments
This material is based upon work supported in part by the Center for Identification
Technology Research and the National Science Foundation under Grant No. 1650474.
Deep Supervised Hashing with Spherical Embedding 15
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