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Information sharing in online social networks is a daily practice for billions of users. The sharing process facilitates the maintenance of users' social ties but also entails privacy disclosure in relation to other users and third parties. Depending on the intentions of the latter, this disclosure can become a risk. It is thus important to propose tools that empower the users in their relations to social networks and third parties connected to them. As part of USEMP, a coordinated research effort aimed at user empowerment, we introduce a system that performs privacy-aware classification of images. We show that generic privacy models perform badly with real-life datasets in which images are contributed by individuals because they ignore the subjective nature of privacy. Motivated by this, we develop personalized privacy classification models that, utilizing small amounts of user feedback, provide significantly better performance than generic models. The proposed semi-personalized models lead to performance improvements for the best generic model ranging from 4%, when 5 user-specific examples are provided, to 18% with 35 examples. Furthermore, by using a semantic representation space for these models we manage to provide intuitive explanations of their decisions and to gain novel insights with respect to individuals' privacy concerns stemming from image sharing. We hope that the results reported here will motivate other researchers and practitioners to propose new methods of exploiting user feedback and of explaining privacy classifications to users.
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Personalized Privacy-aware Image Classification
Eleftherios Spyromitros-Xioufis1, Symeon Papadopoulos1, Adrian Popescu2, Yiannis
Kompatsiaris1
1CERTH-ITI, 57001 Thessaloniki, Greece, {espyromi,papadop,ikom}@iti.gr
2CEA, LIST, 91190 Gif-sur-Yvette, France, adrian.popescu@cea.fr
ABSTRACT
Information sharing in online social networks is a daily prac-
tice for billions of users. The sharing process facilitates the
maintenance of users’ social ties but also entails privacy dis-
closure in relation to other users and third parties. Depend-
ing on the intentions of the latter, this disclosure can become
a risk. It is thus important to propose tools that empower
the users in their relations to social networks and third par-
ties connected to them. As part of USEMP, a coordinated
research effort aimed at user empowerment, we introduce a
system that performs privacy-aware classification of images.
We show that generic privacy models perform badly with
real-life datasets in which images are contributed by indi-
viduals because they ignore the subjective nature of privacy.
Motivated by this, we develop personalized privacy classifi-
cation models that, utilizing small amounts of user feedback,
provide significantly better performance than generic mod-
els. The proposed semi-personalized models lead to per-
formance improvements for the best generic model ranging
from 4%, when 5 user-specific examples are provided, to 18%
with 35 examples. Furthermore, by using a semantic repre-
sentation space for these models we manage to provide intu-
itive explanations of their decisions and to gain novel insights
with respect to individuals’ privacy concerns stemming from
image sharing. We hope that the results reported here will
motivate other researchers and practitioners to propose new
methods of exploiting user feedback and of explaining pri-
vacy classifications to users.
1. INTRODUCTION
Uploading and sharing information in Online Social Net-
works (OSNs) is nowadays a frequent activity for the ma-
jority of Internet users. Such shared pieces of information
are aggregated into digital user profiles. These profiles sup-
port a business model based on free access to the OSN ser-
vice and users have little or no control on how their profiles
are exploited by the OSN. Typically, OSNs employ sophisti-
cated algorithms to make sense of the data posted by their
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DOI: http://dx.doi.org/10.1145/2911996.2912018
users in order to create personal profiles that they often use
to perform ad targeting. Different research and industrial
initiatives point out risks related to different aspects of in-
formation sharing. The Sunlight project1enhances trans-
parency by detecting which textual data are used for per-
sonalized advertising. PleaseRobMe2illustrates a straight-
forward implication of explicit location disclosure. Note,
however, that location disclosure can also be implicit, e.g.
through one’s posted images. Our own contribution is part
of USEMP3, a multidisciplinary European project which de-
velops DataBait4, a tool that provides feedback about what
can be inferred from a user’s historical data shared on OSNs.
These above initiatives provide important contributions
to understanding the risks related to information sharing.
However, most of the challenges related to the proposal
of effective privacy preservation tools lie ahead. First, re-
searchers need to operate under the privacy paradox [15]
which causes a discrepancy between what users intend to
share and what they actually disclose. This discrepancy im-
pedes the wide adoption of privacy preservation methods
and tools and an important effort is needed to educate users
toward its reduction. Second, privacy breaches might be
caused by different types of disclosed data, including mul-
timedia documents and behavioral or social links in OSNs.
Research should focus on these data individually, as well as
on their interlinking. Third, privacy perception is inherently
subjective and dynamic. Consequently, its modeling should
include a strong personalization component that caters to
each user’s needs. Equally important, the models should
evolve over time. Fourth, effective privacy preservation tools
might be perceived as a threat for current business practices
that are built around the exploitation of user profiles. The
adoption of such tools by OSNs is conditioned by public
demand combined with regulatory requirements. The Pri-
vacy Impact Assessment required by the US E-government
Act5issued in 2002 and the proposed European General
Data Protection Regulation6are examples of how regula-
tion acts upon business practices. Fifth, the proposed pri-
vacy inference methods should work under real-time con-
straints in order to give immediate feedback to the user,
preferably before the information is shared on OSNs. These
1http://columbia.github.io/sunlight
2http://www.pleaserobme.com
3http://www.usemp-project.eu
4https://databait.hwcomms.com
5https://www.gpo.gov/fdsys/pkg/PLAW-107publ347
6http://ec.europa.eu/justice/data-protection/reform/
index en.htm
Figure 1: A hardly comprehensible justification (green
rectangles highlighting the most discriminative local
patches) provided for a private classification by the Pic-
Alert system. Image from [23]
methods should also offer high-quality and understandable
results in order to be adopted by users. Finally, the cre-
ation and sharing of privacy-related evaluation datasets is
difficult due to the very nature of the information included.
However, such datasets are essential to evaluate the mer-
its of proposed methods in a quantifiable way and facilitate
reproducibility.
Here, we tackle the privacy-related risks in the context
of image sharing and try to tackle some of the challenges
mentioned above. Image sharing is such a widely used and
valued service that preventing users from sharing their im-
ages cannot be considered as a viable means of protecting
their online privacy. Instead, having access to a service that
could automatically process one’s images before they are
shared with the OSN, and being alerted in case their con-
tent is found to be sensitive, would be a very practical and
transparent way of safeguarding the online privacy of OSN
users without affecting their image sharing experience.
A first solution to this problem was presented in [24],
where the authors defined the problem of automatically clas-
sifying users’ images as being of private or public nature, and
tested the effectiveness of standard image and text features
in a supervised learning setting for solving the problem. In
that work, the authors focused on developing models that
capture a generic (“community”) notion of privacy, making
the underlying assumption that each user perceives privacy
in the same way. However, OSN users often have wildly
different perceptions and norms regarding privacy [16]. A
further limitation of that solution is that the classification
decision was justified by highlighting the most discrimina-
tive local patches in the image as shown in Figure 1. Such
a justification is hardly comprehensible by non-experts in
computer vision. Providing more intuitive, higher-level, ex-
planations would be clearly more desirable. An example is
given in Figure 2, where a private classification is accompa-
nied by an automatically generated cloud of the most preva-
lent image tags and a projection of those tags into a number
of privacy-related dimensions.
In this paper, we propose a personalized image privacy
scoring and classification system that provides an effective
privacy safeguarding mechanism on top of image sharing
OSN facilities and alleviates the limitations of previous so-
lutions. In particular, we make the following contributions:
Personalized privacy classification: We demon-
strate that by combining feedback from multiple users
with a limited amount of user-specific feedback, we
can obtain significantly more accurate privacy classi-
knitwear
young-back
hand-glass
cigar-smoker
smoker
drinker
Freudian
children
drinking
erotic
relatives
vacations
wedding
semantic representation
1st level 2nd level
Figure 2: A better justification of the classifier’s deci-
sion consisting of a tag-cloud of the most prevalent im-
age tags and a projection of those tags into six privacy-
related dimensions
fications compared to those obtained from a generic
model (Section 4.3).
Real-world dataset: We create a realistic bench-
mark dataset via a user study where participants an-
notate their own photos as private or public according
to their own notion of privacy. Experiments on this
dataset reveal the limitations a generic privacy defini-
tion and highlight the necessity of building personal-
ized privacy classification models (Section 4.2).
Semantic justification: We employ a type of se-
mantic features that facilitate the explanation of im-
age privacy classifications and support the discovery
of valuable insights with respect to users’ privacy con-
cerns (Section 4.4). Importantly, these features are
computed based solely on the visual content of the im-
ages and, therefore, the approach does not require the
existence of manually assigned image tags.
State-of-the-art performance: By using visual fea-
tures extracted from deep convolutional neural net-
works (CNNs) we significantly improve the state-of-
the-art performance on an existing private image clas-
sification benchmark (Section 4.2).
2. RELATED WORK
Most modern OSNs allow users to control the privacy set-
tings of their shared content. Yet, the typical user finds it
difficult to understand and correctly configure the offered
access control policies [12]. As a result, several studies [11,
12] have identified a serious mismatch between the desired
and the actual privacy settings of online shared content.
This discrepancy motivated the development of mechanisms
that aid users in selecting appropriate privacy settings. In
the work of [14], for instance, the authors focused on Face-
book posts and evaluated prediction models that make use
of users’ previous posts and profile preferences in order to
suggest appropriate privacy settings for new posts. Despite
achieving high performance, the authors noticed differences
in user behaviors and concluded that personalized privacy
models could further improve the results.
Zerr et al. [24], were among the first to consider the prob-
lem of privacy-aware image classification. In their work, a
large-scale user study was conducted asking participants to
annotate a large number of publicly available Flickr photos
as being either “private” or “public”. The study was set up
as a social annotation game where players were instructed
to adopt a common definition of privacy7and were rewarded
for providing annotations that were similar to those of other
players. The resulting dataset, referred to as PicAlert, was
used to train supervised classification models that capture
a generic (“community”) notion of privacy.
Extending that work, [21] experimented with combina-
tions of visual and metadata-derived features and achieved
better prediction accuracy on PicAlert. [21] also attempted
to solve a more complex privacy classification problem where
three types of disclosure were defined for each image (view,
comment, download) and the task was to assign one of five
privacy levels (‘Only You’, ‘Family’, ‘Friends’, ‘SocialNet-
work’, ‘Everyone’) to each type of disclosure. As in [24],
their models captured only a generic perception of privacy.
Differently from the majority of previous works, our pa-
per highlights the limitations of generic image privacy clas-
sification models and proposes an effective personalization
method. To the best of our knowledge, [4] is the only work
that considers privacy classification of personal photos as
we do here. However, [4] evaluates only purely personalized
models, assuming that each user provides sufficient amount
of feedback. In contrast, our method achieves high per-
formance even at the presence of very limited user-specific
feedback by leveraging feedback from other users. Moreover,
while [4] uses only metadata-based (location, time, etc.) and
simple visual features (colors, edges, etc.), we employ state-
of-the-art CNN-based semantic visual features that facilitate
comprehensible explanations of the classification outputs.
Very recently, [22] evaluated the performance of deep fea-
tures on PicAlert (again in the context of a generic privacy
model) and found that they yield remarkable improvements
in performance compared to SIFT, GIST and user-assigned
tag features. Moreover, the authors evaluated the perfor-
mance of ‘deep tag’ features (which are similar to the first
level of semantic features that we extract here) but did not
exploit them for justifying the classifier’s decisions.
3. APPROACH
3.1 Personalized Privacy Models
Privacy classifications based on a generic privacy classifi-
cation model as the one developed in [24] are undoubtedly
useful for preventing users from uploading images that are
considered to be private according to a generic notion of pri-
vacy. However, as the perception of privacy varies greatly
among users depending on factors such as age, social status
and culture, it is expected that a generic model would pro-
vide inaccurate predictions for certain users, thus decreas-
ing the reliability and usefulness of the alerting mechanism.
To overcome this issue, we propose the exploitation of user
feedback in order to build personalized privacy models. Such
feedback could be acquired either explicitly, by asking OSN
users to provide examples of private and public photos, or
implicitly, by exploiting logs of the user’s interaction with
his/her photos (e.g. changes in privacy settings, removal of
previously shared images, etc.).
Provided that sufficient amount of feedback is available
from each user, one could rely only on user-specific exam-
ples for training personalized privacy classification models.
7
“Private are photos which have to do with the private
sphere (like self portraits, family, friends, your home) or
contain objects that you would not share with the entire
world (like a private email). The rest is public.” [24]
This, however, might require considerable effort from the
user and cannot be taken for granted. As a result, user-
specific privacy classification models might not be able to
generalize well. To overcome this problem, we propose the
development of semi-personalized models that are learned
using a combination of user-specific training examples and
examples from other users. The intuition behind such an
expansion of the training set is that, although each person
has a personal notion of privacy, there are also similarities
between different users (since everyone is affected to some
degree by general trends and norms) and the expansion of
the training set is tailored exactly towards an exploitation of
these similarities. Importantly, in order to retain the person-
alized nature of the models, we assign higher weights to the
user-specific examples, effectively increasing their influence
on the resulting model.
More formally, given a set of users U={u1, u2,...,uk}
and assuming that each user uiUhas provided ground
truth annotations for a set of personal images Iui={im1
ui,
im2
ui,...,imn
ui}, a user-specific dataset Dui={(x1
ui, y1
ui),
(x2
ui, y2
ui),...,(xn
ui, yn
ui)}can be constructed where xui=
[x1ui, x2ui,...,xdui] is a vector representation of imuiand
yuiequals 1 if the image is annotated as private, 0 other-
wise. The typical approach is to train a personalized classi-
fier hui:X → Y (where X=Rdand Y={0,1}are the do-
mains of xand yrespectively) using only examples from Dui.
Instead of that, we propose that each classifier huiis trained
on Sk
i=1 Dui, i.e. the union of all user-specific datasets, and
personalization is achieved by assigning a higher weight w
to the examples of Dui. Example weights are directly han-
dled by some learning algorithms (e.g. decision trees) while
other learning algorithms can be “forced” to take weights
into account by including duplicates of specific examples in
the training set. The effect of weighting is that the classi-
fier is biased towards correct prediction of higher weighted
examples and is commonly used in supervised learning tech-
niques, e.g. cost-sensitive learning [6] and boosting [8].
We note that our approach resembles techniques from the
domains of transfer and multi-task learning [17, 5], com-
monly referred to as instance sharing or instance pooling.
In fact, if we consider the privacy classification of the im-
ages of each user as a different learning task, the problem of
personalized image privacy classification can be considered
as an instance of multi-task learning. These methods are
known to work better than methods that treat each learn-
ing task independently whenever the tasks are related and
there is lack of training data for some of the tasks [1], two
conditions that hold in the problem that we tackle here.
3.2 A Realistic Image Privacy Benchmark
The PicAlert dataset is certainly useful for training mod-
els that capture a generic notion of privacy. However, there
are two limitations that make PicAlert unsuitable as a real-
istic image privacy classification benchmark: a) it consists
of publicly available images with few of them being of re-
ally private nature, b) the ground truth collection process
makes the unrealistic assumption that all OSN users have
common privacy preferences. As a result, a privacy classi-
fication model trained on this dataset may practically fail
to provide accurate classifications (as shown in Section 4.2).
Moreover, the variability of privacy preferences among users
is not taken into account when evaluating the accuracy of
privacy classifications on PicAlert, resulting to overly opti-
mistic performance estimates.
To overcome these limitations, we created a new privacy-
oriented image dataset with two goals: a) the development
of personalized image privacy models, and b) the realistic
evaluation of both generic and personalized image privacy
models. To this end, we conducted a realistic user study
where we asked users to provide privacy annotations for
photos of their personal collections. A call for contributions
that described our research goals and the potential benefits
for OSN users was distributed within our workplaces and
through our OSN accounts. To reduce the concerns associ-
ated with sharing personal images (especially private ones),
we provided users with software that automatically extracts
the above visual features from the images and helps them
share the features and the corresponding annotations (in-
stead of the original images). To provide loose guidance
and let users develop their own notion of privacy, we briefly
described as public “images that they would share with all
their OSN friends or even make them publicly visible”and as
private “images that they would share only with close OSN
friends or not share them at all”. To ensure a representation
of both classes we asked each user to provide (if possible) at
least 10 private and 30 public images.
In total, we received feedback from 27 users (22 males
and 5 females), with ages ranging from 25 to 39 years. Each
user contributed approximately 16.4 private and 39.5 public
photos (on average) for a total of 1511 photos. The result-
ing dataset (features and privacy annotations), named Your-
Alert, is made publicly available8for future benchmarks.
3.3 Visual and Semantic Features
In our experiments we focus on privacy classification based
on the visual content - a piece of information that is always
available in contrast to metadata and manually assigned tags
- and extract the following state-of-the-art visual features:
vlad: We used the implementation of [20] to extract d=
24,576-dimensional VLAD+CSURF vectors from a 128-di-
mensional visual vocabulary and then performend PCA and
whitening to project the vectors to d0= 512 dimensions (a
projection size that led to near optimal results in preliminary
experiments).
cnn: standard convolutional neural network features us-
ing the VGG-16 model [19] that includes 16 layers and is
learned with the training set of the ImageNet ILSVRC 2014
dataset [18]. VGG-16 was chosen because it obtained one
of the top results during the ImageNet 2014 challenge but
also because it is publicly available and thus facilitates re-
producibility. This dataset includes 1,000 specific classes
and approximately 1,2 million images. These classes cover
a wide range of domains and the obtained model has thus
good performance in transfer learning tasks as attested by
[9]. We use the output of the last fully connected layer (fc7 ),
which consists of 4,096 dimensions.
semfeat: semantic image features obtained by exploiting
the outputs of a large array of classifiers, learned with low-
level features [2]. We use the VGG-16 features described
above as basic features for the semantic features. Here, we
compute a slightly modified version of the semfeat descrip-
tor that was introduced in [9]. Only concepts that have at
least 100 associated images are retained and the total size
of the descriptor is 17,462. Concept models are learned in-
8https://github.com/MKLab-ITI/image-privacy
Table 1: Privacy-related latent topics along with the
top-5 semfeat concepts assigned to each topic
Topic Top-5 semfeat concepts assigned to each topic
children dribbler child godson wimp niece
drinking drinker drunk tippler thinker drunkard
erotic slattern erotic cover-girl maillot back
relatives g-aunt s-cousin grandfather mother g-grandchild
vacations seaside vacationer surf-casting casting sandbank
wedding groom bride celebrant wedding costume
Table 2: Dataset statistics
Dataset # examples (private/public) Source
PicAlert 26458 (3651/22807) [24]
YourAlert 1511 (444/1067) This paper
dependently as binary classifiers but with a ratio of 1:100
between positive and negative examples instead of a fixed
number of negatives. The negative class includes images
that illustrate ImageNet concepts that were not modeled.
These images are sorted in order to provide a conceptually
diversified sample of negatives for each modeled concept.
Following the conclusions of [9] concerning the positive ef-
fect of sparsity, only the top n= 100 classifier outputs are
retained for each image.
Compared to vlad and cnn,semfeat have the advan-
tage that they enable result explainability: users can obtain
human-understandable feedback about why an image was
classified as private or not, in the form of top concepts as-
sociated to it. A limitation of this approach is that, having
been constructed for general purpose concept detection, the
semfeat vocabulary contains many concepts that are too
specific and unrelated to privacy (e.g. osteocyte: ‘mature
bone cell’). As a result, many of the top nconcepts of each
image can not be easily linked to privacy.
To address this limitation, we developed a privacy aspect
modeling approach that projects the detected semfeat con-
cepts into a number of privacy-related latent topics using
Latent Dirichlet Allocation (LDA) [3]. More specifically,
each image is treated as a document consisting of its top
n= 10 semfeat concepts and a private image corpus is cre-
ated by combining the private images of the PicAlert and
YourAlert datasets. LDA (the Mallet implementation [13])
is then applied on this corpus to create a topic model with
30 topics. Among the detected topics, 6 privacy-related ones
are identified: children,drinking ,erotic,relatives,va-
cations,wedding (Table 1). Given such a topic model, the
topics of each image are inferred (using Gibbs sampling in-
ference) from its semfeat concepts and the assignments to
the privacy-related topics are used as a means of justifica-
tion of the classifier’s decision (as shown in Figure 2). We
refer to this representation as semfeat-lda.
4. EXPERIMENTS
4.1 Experimental Setup
In our experiments we used the PicAlert9and YourAlert
datasets, of which the statistics are provided in Table 2.
To measure the accuracy of a classification model, we use
9Since some images of PicAlert are no longer available in
Flickr, the version that we use here contains about 18% less
images than the original one.
the area under the ROC curve (AUC). This was preferred
over other evaluation measures due to the fact that it is
unaffected by class imbalance and it is independent of the
threshold applied to transform the confidence (or proba-
bility) scores of a classification model into hard 1/0 (pri-
vate/public) decisions. Moreover, AUC has an intuitive in-
terpretation: it is equal to the probability that the classifi-
cation model will assign a higher score to a randomly chosen
private image than a randomly chosen public image. Thus,
a random classifier has an expected AUC score of 0.5 while
a perfect classifier has an AUC score of 1.
Throughout the experiments, we use an L2-regularized lo-
gistic regression classifier (the LibLinear implementation [7])
as it provided a good trade-off between efficiency and ac-
curacy compared to other state-of-the-art classifiers in pre-
liminary experiments. Moreover, the coefficients of a regu-
larized logistic regression model are suitable for identifying
features that are strongly correlated with the class variable
[10], thus facilitating explanation of the privacy classifica-
tions when features with a semantic interpretation such as
semfeat are used. The regularization parameter was tuned
by applying internal 3-fold cross-validation and choosing the
value (among 10r:r∈ {−2,...,2}) that leads to the high-
est AUC. Finally, all feature vectors were normalized to unit
length before being fed to the classifier, as suggested in [7].
To facilitate reproducibility of our experimental results we
have created a GitHub project10 where we make available
the experimental testbed and the datasets that we used.
4.2 Limitations of Generic Privacy Models
In this section, we evaluate the performance of generic
image privacy classification models when applied in a real-
istic setting where different users have different perceptions
of image privacy. To this end, we conduct the following ex-
periment: A generic privacy classification model is trained
using a randomly chosen 60% of the PicAlert dataset and
then tested on: a) the remaining 40% of PicAlert and b) the
YourAlert dataset. In the first case, we have an idealized
evaluation setting (similar to the one adopted in [24]), while
in the second case we have an evaluation setting that bet-
ter resembles the test conditions that a privacy classification
model will encounter in practice. To ensure reliability of the
performance estimates, we repeat the above evaluation pro-
cedure five times (using different random splits of PicAlert)
and take the average of the individual estimates.
Figure 3 shows the AUC scores obtained on PicAlert (light
blue bars) and YourAlert (orange bars) when each of the vi-
sual features described in Section 3.3 is used. On PicAlert,
we also evaluate the performance with quantized SIFT (bow)
and edge-direction coherence (edch) features, the best per-
forming of the visual features used in [24]11.
The performance on PicAlert indicates that vlad,semfeat
and cnn lead to significantly better results than edch and
bow. With semfeat and cnn, in particular, we obtain a near-
perfect 0.95 AUC score which is about 20% better than the
AUC score obtained with bow (the best visual feature among
those used in [24]). semfeat have very similar performance
with cnn, a fact that makes them a very appealing choice,
given their sparsity and interpretability properties.
10https://github.com/MKLab-ITI/image-privacy
11edch and bow were kindly provided by the authors of [24].
13edch and bow could not be tested on YourAlert because we
did not have access to their exact implementations.
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
edch bow vlad semfeat cnn
AUC
Features
PicAlert YourAlert
Figure 3: Performance of generic models on PicAlert
and YourAlert13
0.50
0.55
0.60
0.65
0.70
0.75
50 100 500 1000 5000 15875
AUC
# training examples
cnn
semfeat
vlad
Figure 4: Performance of generic models on YourAlert
as a function of the number of training examples
However, the performance of all models drops significantly
when they are applied on YourAlert. cnn and semfeat, for
instance, have about 24% lower performance in terms of
AUC. As we see, all models perform similarly, which sug-
gests that the accuracy is not expected to improve consider-
ably if better features are used. Moreover, to check whether
the performance on YourAlert could improve by using addi-
tional training examples from PicAlert, we studied the per-
formance of the generic privacy models as a function of the
number of examples. More specifically, for each type of fea-
tures, we built six generic privacy estimation models using
{50,100,500,1000,5000,15875}training examples from Pic-
Alert and applied them on YourAlert. As above, to ensure
reliability of the performance estimates, the evaluation of
each model was repeated five times, i.e. five models were
built (each trained on a different random subset of PicAlert)
for each combination of features and number of training
examples, and the averages of the individual performance
estimates were taken. The results of this experiment are
shown in Figure 4. We observe that for all types of fea-
tures, the AUC performance reaches a plateau and does not
change significantly after 5000 examples. Interestingly, the
generic models that use cnn and semfeat features obtain
96% of their maximum performance with only 50 training
examples, while the generic model that uses vlad features
seems to require about 5000 training examples in order to
approach its maximum performance. Clearly, the use of ad-
ditional generic training examples is not expected to help in
attaining better performance on YourAlert.
Figure 5 presents a per-user performance breakdown for
generic models based on vlad,semfeat and cnn features (i.e.
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
u9
u23
u16
u14
u2
u20
u26
u7
u22
u19
u12
u25
u5
u4
u24
u13
u18
u21
u10
u15
u11
u3
u17
u6
u27
u8
u1
AUC
Users
cnn
semfeat
vlad
Figure 5: Per-user performance of generic models based
on vlad,semfeat and cnn features
a separate AUC score is calculated for each user based on
his/her own images). We note that there is a large vari-
ability in performance across users. For instance, using
semfeat features, near-perfect AUC scores are obtained for
users {u1, u8, u27}while the AUC scores are worse than ran-
dom for users {u9, u23, u16 , u14}suggesting that the privacy
perceptions of these users deviate strongly from the average
notion of privacy. For this type of users, as well as for those
for whom the performance of the generic models is close to
random (about 40% of users), building personalized privacy
classification models is essential to develop a useful alerting
mechanism.
4.3 Personalized Privacy Models
This subsection compares the performance of generic pri-
vacy classification models to that of models employing user
feedback in order to adapt to specific users. Specifically, we
evaluate two types of personalized models on YourAlert.
user: Purely personalized models that use only user-spe-
cific training examples, i.e. a specific model is built for each
YourAlert user from his examples only.
hybrid: Semi-personalized models that use a mixture of
user-specific and generic training examples, with user-spe-
cific examples being assigned a higher weight to achieve
personalization. We experimented with treating as generic
examples: a) examples from PicAlert (hybrid-g variant)
and b) examples from YourAlert that belong to other users
(hybrid-o variant). Since the two choices lead to similar
results, we report results only for hybrid-o.
As discussed in Subsection 3.1, user models are expected
to perform better when a sufficient amount of user-specific
examples are available, while hybrid-o models are expected
to be advantageous with a limited amount of user feedback.
In order to evaluate this type of models ensuring reliable,
out-of-sample estimates for all examples of each user, we
use a modified k-fold cross-validation procedure (k= 10)
that works as follows. The examples contributed by each
user are randomly partitioned into kfolds of approximately
equal size, respecting the original class distribution (as in
stratified cross-validation). Out of these, a single fold is re-
tained as the test set and used to test the model, and from
the remaining k1 folds we randomly select a specified
number of examples (again respecting the original class dis-
tribution) and use them as training data either alone (user
models) or together with generic examples (hybrid-o mod-
els). This process is repeated ktimes, with each of the k
subsets being used exactly once as the test set. All predic-
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0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35
semfeat cnn
AUC
# user-specific examples / features
generic other user hybrid-o w=1
hybrid-o w=10 hybrid-o w=100 hybrid-o w=1000
Figure 6: Performance of personalized models as a func-
tion of user-specific training examples
tions concerning each user are then aggregated into a single
bag to calculate a per-user AUC score, or predictions for
all users are combined together to calculate an overall AUC
score for the examples of the YourAlert dataset.
Figure 6 plots the AUC scores on YourAlert by user and
hybrid-o models trained on {5,10,15,20,25,30,35}user-
specific examples using semfeat and cnn features. We eval-
uate four variations of hybrid-o models, each one using a
different weight (w={1,10,100,1000}) for the user-specific
examples to facilitate a study of the impact of the weight
parameter. In addition to the performance of these person-
alized models, the figure also shows the performance of two
types of generic models to allow a direct comparison: a)
generic: a model trained on a random subset of PicAlert
(containing 5000 examples) and b) other: a model trained
using only examples from other YourAlert users, i.e. a dif-
ferent generic model is build for each user, using the same
generic examples as the corresponding hybrid-o model.
With respect to the generic models, we see that the per-
formance of other is similar to that of generic with cnn
features and better with semfeat features. These results
suggest that although the examples of YourAlert come from
users that adopt a personal, potentially different, notion of
privacy, they are equally useful as the PicAlert examples for
learning a generic privacy model.
With respect to the personalized models, we see that the
performance of user models increases sharply as more user-
specific training examples become available. When semfeat
features are used we see that user models obtain similar
performance with the generic models (generic and other)
with as few as about 30 examples. The situation is even
better when cnn features are used as we see that the perfor-
mance of user models catches up with the performance of
the generic models with as few as 15 examples and improves
it by about 15% when 35 user-specific examples are used.
With regard to the semi-personalized, hybrid-o models
we observe that they outperform significantly the purely
personalized user models (with both types of features), es-
pecially for smaller numbers of user-specific training exam-
ples. As expected by the analysis of Subsection 3.1, the
gap closes as more user-specific training examples become
available. However, we see that for all values of user-specific
examples (up to at least 35) hybrid-o models provide sig-
nificantly better performance than both user and generic
models. Importantly, we see that assigning a higher weight
to user-specific examples is crucial for obtaining better per-
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1.00
u9
u16
u23
u14
u2
u26
u20
u7
u19
u22
u12
u25
u5
u4
u24
u13
u18
u10
u21
u15
u17
u6
u11
u3
u27
u1
u8
AUC
Users
hybrid-o w=1000
user
generic
Figure 7: Per-user performance of generic,user and
hybrid-o (w= 1000) models based on cnn features
formance. Specifically, results improve significantly as we
increase wup to 100 but a saturation is observed with higher
values.
Overall, the best personalized model (hybrid-o with cnn
features) boosts the performance of the best generic model
(other with semfeat features) by about 4% when the user
provides feedback for 5 images to about 18% when the feed-
back increases to 35 images. Figure 7 presents a per-user
performance breakdown for generic,user and hybrid-o
(w= 1000) models based on cnn features (user and hybrid-
ouse 35 user-specific examples). hybrid-o and user pro-
vide better performance than generic for the majority of
users, particularly for those that are poorly predicted by
the generic model. Moreover, we see that in most cases
hybrid-o is equally good or better than user.
4.4 Image Privacy Insights via SemFeat
Besides facilitating easily comprehensible explanations of
privacy classifications (as shown in Figure 2), semfeat fea-
tures can help in identifying users whose privacy concerns
deviate strongly from the average perception of privacy. To
this end, we build a single generic privacy detection model
using the whole YourAlert dataset as well as 27 personalized
privacy models trained using only the examples contributed
by each user. For each model, we identify the concepts that
are assigned the top-50 positive (associated with the pri-
vate class) and top-50 negative (associated with the public
class) coefficients and search for concepts that are strongly
correlated to privacy according to the generic model and
negatively correlated to privacy according to a personalized
model (or vise versa). Despite the fact that less than 1% of
the total semfeat features are considered in these compar-
isons, we can still gain valuable insights. For instance, ac-
cording to the generic model, concepts related to family and
relatives, such as child,mate and son are highly correlated
to private images, while concepts related to natural scenes,
such as uphill,waterside and lakefront are correlated to
public images. In addition, we found some interesting de-
viations from the generic model, e.g. alcoholic is strongly
correlated with privacy according to the generic model while
it is negatively correlated with privacy for users u12 and u22.
On the other hand, concept tourist is private for user u11
and public according to the generic model.
Another practical use of semfeat is in creating user pri-
vacy profiles. To this end we employ the semfeat-lda repre-
sentation that was described in subsection 3.3 and construct
a privacy profile for each user by computing the centroid of
the semfeat-lda vectors of his/her private images. This vec-
tor facilitates a summary of the user’s concerns with respect
to the six privacy-related topics that were identified by the
LDA analysis. Given such a representation for each user,
cluster analysis can be performed to identify recurring pri-
vacy themes among users. To illustrate this, we performed
k-means (k= 5) clustering on the users of YourAlert and
present the clustering results in Figure 8. We see that each
cluster captures a different privacy theme. Users clustered
at c0, for instance, are primarily concerned about preserv-
ing the privacy of their vacations while users clustered at
c2 are mainly concerned about the privacy of children and
of photos related to drinking.
5. CONCLUSION AND FUTURE WORK
We presented a framework for personalized privacy-aware
image classification. Our main immediate contribution is the
creation of personalized privacy classification models that,
as verified by experiments on a newly introduced image pri-
vacy dataset, exhibit significantly better performance than
generic ones. Experimenting with different strategies of uti-
lizing user feedback we found that combining user-specific
with generic examples during training yields better perfor-
mance than relying on either the user-specific or the generic
examples alone. Furthermore, we exploited a new type of
semantic features that, in addition to having an impressive
performance, allow the discovery of interesting insights re-
garding the privacy perceptions of individuals.
There are several interesting directions for future work.
First, the current system is limited to binary classification
of images. We would like to develop models able to classify
a user’s photos into finer-grained privacy classes (e.g. close-
friends, all-friends, friends-of-friends, public), correspond-
ing to the different OSN audiences photos can be shared
with. Second, we intend to design more sophisticated in-
stance sharing strategies (e.g. assigning different weights to
the examples of other users based on inter-user similarities)
and make a comparison with well-established methods from
the multi-task learning domain. Third, a limitation of the
current study is that cnn features are obtained with the stan-
dard 1,000 classes from the ImageNet challenge that are, in
a large majority of cases, not linked to privacy. As an al-
ternative, we will explore: 1) the direct training of a neural
network with private/public examples, and 2) the training
of a network with an increased number of privacy-oriented
concepts, based on an analysis similar to the one of subsec-
tion 4.4. Such a privacy-oriented set of concepts can also
facilitate more meaningful semantic justifications compared
to the semfeat vocabulary.
In a larger context, we will work toward the structuring
of an active research community working on users’ privacy
preservation. This effort is necessary because the tackling
of the challenges presented in the introduction is only possi-
ble through the collaboration of a hefty number of research
groups with expertise in different areas related to privacy.
While important among the data shared in OSNs, images
are just a piece of the puzzle. The mining of other types
of data (texts, videos, audio content, cookies, etc.) should
be combined with social network analysis in order to best
serve users. As a first step, we release our implementation
under an open source license and also share the features and
annotations associated with our dataset to encourage repro-
ducibility. Sharing the dataset itself would be very useful
0.00
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c0: {2,3,19,23,25,26,27} c1: {1,5,6,11,12,13,14,20,21} c2: {8,10,17,24} c3: {4,16} c4: {7,9,15,18,22}
Factor Loadings
children
drinking
erotic
relatives
vacations
wedding
Figure 8: Clustering of YourAlert users based on privacy-related latent topics
but it is challenging due to the highly private nature of its
content. The creation of dataset was not straightforward
and we are currently investigating ways to enlarge it. One
possible way is to provide incentives to users in exchange
for the possibility to include their images in the dataset and
to share them with the community. Another possibility is
to liaise with other research groups from the area in or-
der to gather data in a collaborative manner. The insights
gained from this exploration will be shared with the research
community in order to facilitate the creation and sharing of
datasets for other types of data.
Yet another direction that we will explore is the organi-
zation of events to disseminate the topic in the community.
Toward this direction, we have already identified the Me-
diaEval Benchmarking Initiative14 as a relevant venue that
could host a task on privacy classification for multimedia
documents.
Beyond computer science, privacy research should be in-
formed by results from the legal and social sciences domains.
A small interdisciplinary European group is already consti-
tuted as part of the USEMP project. We will work toward its
extension with relevant research groups from other countries
in order to include and confront different takes at privacy.
6. ACKNOWLEDGMENT
This work is supported by the USEMP FP7 project, par-
tially funded by the EC under contract number 611596.
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