A Novel Fourier-based Approach for Camera Identification
Vittoria Bruni
a
, Silvia Marconi
b
and Domenico Vitulano
c
Department of Basic and Applied Sciences for Engineering, Sapienza Rome University,
via Antonio Scarpa 14-16, 00161 Rome, Italy
Keywords:
Camera Identification, PRNU, Fourier Transform.
Abstract:
In this paper the source camera identification problem is considered and novel features for PRNU noise are
studied. The regularity of a suitable sampling of the PRNU image is considered and it is measured through
the decay of its Fourier spectrum. This single global feature is independent of image filtering and size, but
it carries significant information concerning the image. The aim is to use this feature in the classification
step. Some preliminary results show that this kind of approach is promising as it is able to reach identification
scores that can be comparable to the reference source camera identification method, without requiring any
image downsampling or cropping.
1 INTRODUCTION
The increasing use of images and videos in sev-
eral multimedia applications has offered new research
challenges and problems. One of the most interest-
ing is Image Forensic, due to the crucial role of im-
ages and videos for several investigation purposes,
such as identification of the sources that took images,
authenticity, robustness of data transmission, storage
and manipulation just to cite a few. Among the
aforementioned problems, probably the most fasci-
nating is camera identification (Al-Ani and Khelifi,
2017; Lukas et al., 2006) as having several implica-
tions in forensics (Chen et al., 2008; Chierchia et al.,
2010; Korus and Huang, 2016). Coarsely speaking,
it mainly consists of looking for a reliable correspon-
dence between a given image and the device that took
it. One of the most adopted (and powerful) tools for
camera identification is PRNU (Photo Response Non
Uniformity noise). It consists of a noise component
in any acquired image that is caused by the CCD im-
perfections, so that particular pixels are susceptible to
giving brighter intensities than others. This peculiar-
ity plays a crucial role as it is a sort of fingerprint of
the device that in principle makes it easy to recognize.
Unfortunately, this is not the case as it is very difficult
to extract PRNU from a given image. That is why a
a
https://orcid.org/0000-0003-3909-7463
b
https://orcid.org/0000-0002-4916-4896
c
https://orcid.org/0000-0001-6088-9743
Figure 1: Block scheme of the whole pipeline for camera
identification problem.
plethora of approaches has been proposed in the last
years concerning this topic.
Coarsely speaking, camera identification problem
can be seen from two different points of view: foren-
sic and scientific. Keeping in mind forensics require-
ments, it is possible to give a coarse taxonomy of the
proposed approaches (Caldelli et al., 2018; Chierchia
et al., 2014; Korus and Huang, 2016; Marra et al.,
2017; Valsesia et al., 2017) by considering the most
frequent cases, briefly described below:
1. given a set of available devices and an image ac-
quired by one of them, the goal consists of looking
for which device took the image;
Bruni, V., Marconi, S. and Vitulano, D.
A Novel Fourier-based Approach for Camera Identification.
DOI: 10.5220/0010437000990106
In Proceedings of the International Conference on Image Processing and Vision Engineering (IMPROVE 2021), pages 99-106
ISBN: 978-989-758-511-1
Copyright
c
2021 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
99
2. given a set of available devices and an image, the
goal consists of determining whether the device
taking that image belongs to such a set;
3. given a set of images, the goal is to group them
according to the corresponding source.
Sometimes, the requirements above can be relaxed
and it may be sufficient to solve subproblems such
as to find out only the brand or the model - rather than
the specific device. From a scientific/practical point
of view, the problems above necessarily require two
main steps: PRNU extraction and PRNU comparison
(via a suitable metric). With regard to PRNU extrac-
tion, several approaches adopt denoising techniques
that allow to get the noise (i.e., PRNU) in terms of the
residual image. PRNU comparison represents an even
more difficult task because of PRNU noisy nature
for approaches belonging to this class that mainly fo-
cus on clustering, see for instance (Georgievska et al.,
2017; Huang et al., 2015; Marra et al., 2017). To fur-
ther complicate the problem, distortions introduced
by image upload/download from the social networks
have to be accounted for, as they often represent the
actual available data (Caldelli et al., 2018; Valsesia
et al., 2017). Fig. 1 depicts the whole source camera
identification pipeline. It is evident that each block
in the pipeline gives a contribution to the final result
in terms of error. For instance, PRNU extraction usu-
ally employs a classical denoising process as in
the pioneering work of Lukas et al. (Lukas et al.,
2006). Since denoising is not perfect, as a side effect
one achieves a mix of PRNU and image components
that usually alters the final result. Even though differ-
ent and more sophisticated approaches have been pro-
posed in the literature (Akshatha et al., 2016; Al-Ani
and Khelifi, 2017; Salvi and Vitulano, 2019; Fridrich,
2009; Tiwari and Gupta, 2018; Kumar and Hasse-
brook, 1990; Li et al., 2018; Thaia et al., 2015; Xu
et al., 2016; Zhao et al., 2019), the problem still holds.
On the other hand, PRNU comparison is delicate due
to the impulsive (and quasi random) nature of PRNU.
It is straightforward that in the camera identification
problem a cumulative error occurs and it is difficult to
separate and quantify the contribution of each specific
phase.
In this paper we step back to reformulate the prob-
lem as the following subproblem. Specifically, we
work on flat fields images, i.e. those representing
a uniform and flat background thus mainly con-
taining PRNU. This assumption, although simplis-
tic, allows us to by-pass all problems due to network
uploading/downloading and PRNU extraction. The
(sub)problem we aim to address is: which is the best
way to represent and compare two PRNUs? Although
simpler, it remains a hard problem because two noisy
signals have to be compared. If they have no distor-
tion, the correlation method proposed in (Lukas et al.,
2006) is very performing. Unfortunately, this is not
the real case in practice. Hence, the question is now
if it is possible to differently represent PRNU in order
to use classical image processing tools to get better
results. To this aim, a different PRNU representation
is proposed. It consists of suitably rearranging it in
order to increase its visual regularity. A simple exam-
ple is shown in Fig. 2. The interesting aspect of this
approach is twofold. On the one hand, the resulting
image is more intellegible from a visual point of view;
on the other hand, this image is more ”tractable” by
means of classical signal processing tools. In par-
ticular, the decay of the Fourier spectrum is consid-
ered as representative of the global signal regularity,
and it has been used as single feature in the source
identification process, as described in the first case of
forensics requirements listed at the beginning of this
section. Even if this is a first attempt to give a dif-
ferent representation of a PRNU signal, to the best
of the authors knowledge, it has various advantages:
i) it reaches comparable performance to the classi-
cal correlation approach in (Lukas et al., 2006) when
comparing images having the same size, even if just
one global spectrum parameter rather than the whole
PRNU image is employed; ii) it is independent of the
image size so that no resinzing or cropping is required
for comparing device PRNU and image PRNU (more
realistic scenario).
Preliminary results have been achieved on the
publicly available Dresden database (Gloe and Bhme,
2010) and identification results have been studied in
different conditions. Particular attention has been de-
voted to the ability of the single global feature to cor-
rectly assign each image to the corresponding device;
as it will be shown, the estimation of this parameter
on different regions of the image allows us to define
an array of features that contributes to further increase
identification performance.
The remainder of the paper is the following. Next
section presents the proposed method along with
some theoretical and practical motivations. Section 3
presents some preliminary experimental results, while
the last section draws the conclusions.
2 THE PROPOSED MODEL
In the literature, the observed image is modelled as
J(x, y) = I(x, y)+ I(x, y)K(x, y) + N(x, y),
where J is the acquired image, (x, y) the pixel loca-
tion, I is the original image content, K is the PRNU
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100
Figure 2: Two different PRNU representations: Top: Con-
ventional PRNU matrix, Bottom: PRNU matrix whose val-
ues have been sorted row-wise and then column-wise.
component while N represents other noise sources
that are independent of K (Lukas et al., 2006). K is
a zero-mean noise component that is independent of I
and it is specific for the device that took the image J.
As it can be observed, the original image plays
a crucial role in the equation above and then the ex-
traction of K is a very delicate operation and active
research field. However, if an almost constant image
is acquired, we get a flat field (FF) image that can be
briefly written as
J(x, y) = B + BK(x, y) + N(x, y),
where B denotes the constant background. Despite
the presence of the noise component N, FF image bet-
ter reveals K. That is why, whenever possible, flat
field images are used for estimating device PRNU,
i.e. the reference pattern. The common procedure
for K estimation is described below and it also char-
acterizes the most famous and pioneering approach
for source camera identification presented in (Lukas
et al., 2006).
A set of m images having almost constant back-
ground is acquired, i.e.
J
i
(x, y) = B
i
+ B
i
K
i
(x, y) + N
i
(x, y), i = 1, .., m;
a denoiser F is applied to each J
i
and the residual im-
age is extracted, i.e. R
i
= J
i
F(J
i
). Since the im-
ages J
i
, i = 1, ..., m have been acquired by the same
device and the noise sources N
i
are iid, then K can
be estimated either as a simple pixelwise mean of the
estimated residuals R
i
or through the maximum like-
lihood estimator, i.e. K =
m
i=1
R
i
J
i
m
i=1
J
2
i
. The latter method
is more robust and it allows for a more accurate esti-
mation of the reference pattern.
The same denoising procedure is applied when-
ever the PRNU from a single image has to be ex-
tracted. In order to assess if an image W has been
taken by a given device, the normalized correla-
tion between the estimated reference pattern K and
the residual R for the image W is computed, i.e.
ρ(K, R) =
σ
KR
σ
K
σ
R
, where σ
KR
is the cross correlation
between K and R, while σ
K
and σ
R
are the standard
deviations of K and R respectively. The closer to 1
ρ, the higher and more reliable the match between the
image W and the device having K as reference pattern.
Even though this kind of approach provides sat-
isfying results, the use of correlation implies a per-
fect match between K and R dimension; if it is not
the case, some resizing/cropping is required but this
would cause the modification of noise component or
misalignments between the comparing noise compo-
nents. In addition, correlation values are in general
close to zero as the denoising process cannot perfectly
separate PRNU from the remaining noise sources. Fi-
nally, the accuracy of correlation decreases whenever
the original image has been altered by some opera-
tions, as for example, the upload or download from
the web or social networks. That is why in this pa-
per we are interested in contributing to solve the first
problem. i.e. to find novel features for K image that
can be used independently of the image size. For the
same reason, flat fields images will be analysed.
The decreasing/increasing rearrangement is a well
known concept in mathematics (Duff, 1967) but it is
also used in signal processing as rank ordering for
continuous-time signals (Ferreira, 2001). It consists
of a sorted version of the original signal. In a more
formal setting, the decreasing rearrangment f
of f
is defined as the inverse of the cumulative distribu-
tion of the function f . As a result, f
is a function
having some nice properties and relations with the
original one; more precisely, it preserves some in-
teresting properties of f , such as mean, energy, dis-
tribution function and global regularity see (Duff,
1967; Ferreira, 2001) for details. In the studied case,
the function f is the image K and we are interested
in studying and exploiting the properties of its sorted
counterpart K
. In particular, we empirically ob-
served that K
, that has been defined by rearrang-
ing the values of K in increasing order along the x
(rows) and y (columns) direction, shows some geo-
metrical structures that seem to characterize the sin-
gle device. An example is shown in Fig. 3, where
K
of distinct devices are depicted. As it can be ob-
served, these sorted images present a peculiar pattern
that allows for discriminating between different de-
A Novel Fourier-based Approach for Camera Identification
101
Figure 3: Top: Device. Bottom: corresponding rearranged
PRNU matrix K
for three different devices belonging to
three different brands.
vices. In fact, for each considered device, a differ-
ent pattern is observable, while for the same device,
the same pattern is provided by distinct flat field im-
ages, as Fig. 4 shows. We then conclude that the
characterization of the observed pattern enables the
characterization of the device and, in particular, it can
contribute to discriminating images acquired by dif-
ferent devices. This observation motivates our work
whose aim is then to define features for the observed
behaviour.
In particular, if we design an ideal straight line
from the topleft corner to the bottommost right one,
i.e. the diagonal of the rectangular image domain, we
get a signal that captures most of the observed geo-
metrical structure, as Fig. 5 shows. Hence, the aim
Figure 4: Rearranged flat field images acquired by the same
device. The first device in Fig. 3 has been considered.
is to provide a feature of the resulting signal and to
evaluate to what extent it is able to represent/identify
the whole reference PRNU. To this aim we made the
following observations:
the geometrical patterns are mainly characterized
by a different frequency of the repeated struc-
tures (sort of pseudospherical waves having spe-
cific wavelength). We then expect that this kind
of feature is mainly emphasized and measurable
in the signal spectral components;
one of the main feature of a function is its regular-
ity that can be measured through its global Lips-
chitz exponent;
the decay of the Fourier spectrum is stricly corre-
lated to signal global Lipschitz exponent.
It turns out that the decay of the modulus of the
coefficient of the Fourier transform of the diagonal of
the rank sorted matrix K
of the PRNU image is the
feature that is worth to investigate. In the sequel f (y)
will identify the intensity signal corresponding to the
diagonal of the rectangular K
domain.
2.1 Fourier Spectrum Decay as PRNU
Feature
One of the main features of a function is its regularity.
The latter can be referred to the whole function, i.e.
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102
Figure 5: Intensity signal corresponding to the diagonal of
the rectangular domain of K
. Plots are referred to the blue
color channel of the images depicted in Fig. 3.
global regularity, or it can be considered locally (lo-
cal regularity). Independently of its kind, regularity
can be measured as the Lipschitz regularity (Mallat,
1998). We recall that a function f is uniformly Lips-
chitz over an interval [a, b] of the real axis if for each
y [a, b] there exist a positive constant C independent
of y and a polynomial p
y
of degree bαc such that
v [a, b] | f (v) p(v)| C|y v|
α
. (1)
If α < 1, then α characterizes the singularity type.
Furthermore, if f (y) is the signal under study,
ˆ
f (ω)
its Fourier transform, ω the frequency, and if
|
ˆ
f (ω)|
C
1 + ω
1+q+δ
, (2)
Figure 6: Loglog plot of the Fourier spectrum of the Inten-
sity signals in Fig. 5 (gray line). The estimated straight line,
as in eq. (3), using the first 10% frequencies, except for the
DC term, is also depicted (black line).
with C and δ two positive constants, then f is smooth
and q is the order of smoothness. The same result
holds whenever the smoothness is measured in terms
of Lipschitz exponent α. As a result, by measuring
the asymptotic decay of the modulus of the Fourier
transform of the signal f , we get some information
concerning its global regularity.
Based on these results, we consider the global Lip-
schitz exponent α of f and, without loss of generality,
we relax the relation in eq. (2) as |
ˆ
f (ω)|
C
|ω|
1+α+δ
.
By taking the logarithm to both members, we get
log(|
ˆ
f (ω)|) log(C) (1 + α + δ)log(|ω|) (3)
so that α regulates the slope of a straight line that
A Novel Fourier-based Approach for Camera Identification
103
bounds the logarithm of the absolute value of the
Fourier transform. Hence, a least squares regression
on log(|
ˆ
f (ω)|) allows for the estimation of the slope
of the straight line, and then α — see Fig. 6.
It is worth observing that the regularity measured
in the sorted signal is not independent of the regularity
of the original signal so that the estimated regularity
is strictly related, in some sense, to the regularity of
the original PRNU (Ferreira, 2001). This observation
further motivates the choice of focusing on the prop-
erties of the rank sorted matrix. On the other hand,
α is a global feature, but eq. (1) also holds locally.
Hence, in order to better characterize the analyzed
PRNU image, the same Lipschitz exponent estima-
tion procedure can be applied on subregions of the
whole image. In this way a more local information is
embedded in the estimated Lipschitz coefficients and
a vector of features can be defined for the image under
study. The components of this features vector are the
Lipschitz coefficients estimated on each image subre-
gion
j
, j = 1, ..., M. In this paper, rectangular subre-
gions have been considered by partitioning the image
into sub-blocks having the same size.
2.2 The Algorithm
Before presenting the proposed source identification
algorithm in details, it is worth observing that, each
color channel can be processed separately and the co-
efficients estimated for each color channel, respec-
tively α
R
, α
G
, α
B
, define the feature vector associated
to PRNU image. The second observation is the fol-
lowing. K regularity can be estimated directly from
flat field images by excluding the highest spectrum
frequencies, where the contribution of the noise com-
ponent N cannot be considered negligible. In this way
we do not need to estimate the reference pattern K for
each device, but the corresponding features vector can
be directly estimated as
α
R
=
1
m
m
i=1
α
R,i
, α
G
=
1
m
m
i=1
α
G,i
, α
B
=
1
m
m
i=1
α
B,i
,
(4)
where α
,i
is the Lipschitz exponent of the * color
channel of the flat field image J
i
. In other words, for
each color channel, the average of the regularity coef-
ficients estimated from each flat field image acquired
by the same device is considered. As a result, it is not
necessary to adopt any denoising or averaging method
for K estimation.
The source identification algorithm is then sket-
ched as below.
Let W be the image to be classified and let J
i,d
,
i = 1, ..., m the set of FF images acquired by the d-th
device.
1. For each device d
compute the feature vector α
i
=
(α
R,i
, α
G,i
, α
B,i
) for each flat field image
J
i,d
by linear regression according to eq. (3),
where f is the diagonal of J
i,d
domain
set α
d
= (α
R
, α
G
, α
B
), with α
computed as in
eq. (4). α
d
is the reference feature vector for
the device d
2. Compute the feature vector α for the image W
3. For each device d, compute the distance D be-
tween α and α
d
, i.e.
D(α, α
d
) = kα α
d
k
2
4. Assign the image W to the device
¯
d realizing the
minimum distance value, i.e.
¯
d = argmin
d
D(α, α
d
).
3 EXPERIMENTAL RESULTS
The proposed feature has been tested on the pub-
licly available Dresden database (Gloe and Bhme,
2010) that includes hundred images (natural and flat
field) captured by several camera models and devices.
For comparative studies, the method in (Lukas et al.,
2006) has been considered as the reference pioneering
one. The objective of the presented tests is to quantify
the ability of the proposed (single) feature in assign-
ing each image in the database to its source device.
In all tests W is a flat field image and all the flat field
images in the database have been considered in the
source identification task. Hence, if W = J
i,d
, with
fixed i, is the image to be classified, then it is com-
pared with all the devices in the database but the fea-
ture vector for the device d, i.e. α
d
, is estimated using
all the flat field images taken by d except for the one
assigned to W . In addition, the range of frequencies
used for Fourier spectrum decay has been empirically
set equal to 10% of the whole spectrum range.
Table 1 refers to results achieved on the whole
dataset, where the success rate is provided. The latter
is measured as the percentage of correct assignments
with respect to the number of considered candidate
images. As it can be observed, the success rate of the
proposed method is not negligible, especially if one
takes into account that just one global image feature
(one parameter for each color channel) has been used
in the matching phase. In fact, one of the main advan-
tage of the proposed approach is the use of a single
and easy to compute image feature in the identifica-
tion process; the adopted feature is independent of
image size and spatial correspondences between the
two comparing images.
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104
Table 1: Success rate provided by the proposed method on
Dresden database (first column) and the ones provided by
the same method applied to the images partitioned into M
equally size subblocks.
Proposed Proposed Proposed
(M = 4) (M = 16)
Device 51.4 85.3 92.8
Model 64.6 92.9 96.9
Brand 67.2 94.7 98.1
In order to further evaluate the advantages and/or
limits of the proposed method, brand and model clas-
sification rates have also been considered. As it can be
observed in the same table, the success rate increases
as the reference set enlarges. This means that many of
the incorrect device assignments are within the same
device model or, more in general, within the same de-
vice brand. As a result, the proposed feature is able to
capture the characteristics of the ”device family”.
Table 1 also contains the results achieved by the
proposed method whenever images are partioned in
4 and 16, respectively, rectangular subregions hav-
ing the same size and the vector of coefficients es-
timated in each subregion is used for classification
purposes. This application mode allows for better
capturing some local image features. As it can be
observed, identification success rate significantly in-
creases thanks to the use of a more local information.
In order to perform comparative studies, the pro-
posed method using 16 sub-blocks has been consid-
ered and two different subsets of devices have been
extracted from Dresden database. The first testing set
is composed of 23 devices (belonging to 7 brands)
whose flat field images have the same size (see Ta-
ble 2); the total number of images in this set is 950.
The second testing set is composed of 21 devices (be-
longing to 8 brands) whose flat field images can have
different dimension (see Table 3); the number of im-
ages in the whole subset is 900. For the second test-
ing set, the method in (Lukas et al., 2006) requires
cropping or subsampling in order to compare the nor-
malized correlation coefficient. As Table 4 shows, al-
though the use of a single global feature, the proposed
method allows to reach moderately high success rates.
Even in this case, the success rate increases whenever
model and brand assignments are considered. The
same table contains the results achieved by the ref-
erence method in (Lukas et al., 2006) whenever the
denoising step is omitted in the estimation of device
reference PRNU. In this way the two methods have
comparable computational complexity. As it can be
observed, the proposed method achieves results that
are close to the basic version of the method in (Lukas
et al., 2006), i.e. 100%, but it outperforms it whenever
denoising is not applied. These results prove a cer-
Table 2: Testing set I: the images have the same size, i.e.
2736 × 3648. For each model, the number of devices and
the number of images for each device is provided.
Model
no. Devices no. Images
Olympus MJU 5 50
Pentax Optio A40 1 50
Samsung NV15 3 50
Sony Cyber-shot 4 50
DSC-T77
Sony Cyber-shot 2 50
DSC-W170
Panasonic Lumix 2 25
DMC-FZ50
Ricoh Caplio 5 25
GX100
Table 3: Testing set II: the images have different size.
Model Image size no. no.
Dev. Imgs
CanonIXUS55 1944 × 2592 1 50
CanonIXUS70 2304 × 3072 3 50
NikonS710 3264 × 4352 5 50
NikonD200 2592 × 3872 2 25
NikonD70 2000 × 3008 2 25
NikonD70S 2000 × 3008 2 25
SamsungL74 2304 × 3072 3 50
SamsungNV15 2736 × 3648 3 50
tain robustness of the proposed feature to background
noise.
4 CONCLUSIONS
In this paper a novel feature for PRNU image has been
proposed and its contribution to source camera iden-
tification has been evaluated. It consists of the decay
of the Fourier spectrum of a particular sampling of a
sorted version of PRNU matrix. This decay is strictly
related to the regularity of the signal under study and
then it represents one of its representative features. As
the paper presented a feasibility study, flat field im-
Table 4: Success rate on two different testing sets: com-
parisons between the proposed method (Prop.) using image
partitioning into 16 subblocks and the method (Lukas et al.,
2006) (L06) when denoising filter is not applied before FF
averaging in the estimation of the reference pattern.
Testing set I Testing set II
Prop. L06 Prop. L06
Device 99.5 99,2 92.9 94.7
Model 99.9 99.9 92.9 99.9
Brand 99.9 99.9 92.9 99.9
A Novel Fourier-based Approach for Camera Identification
105
ages have been considered. Preliminary and extensive
tests performed on a publicly available database have
shown that the proposed feature has some potential in
contributing to the solution of the source identifica-
tion problem and it is able to reach high success rates
whenever properly estimated. On the other hand, it
has the advantage of being independent of image size,
so that artificial operations are useless whenever two
PRNU images have to be compared; finally, due to
its nature, it can show some robustness to background
noise sources. Future research will be then devoted to
refine the proposed feature, making it able to capture
both global and local image characteristics; to extend
it to natural images and to evaluate its performance on
natural images and different datasets.
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