Image analysis – Applications
Reexamination Certificate
2001-01-09
2004-04-06
Patel, Jayanti K. (Department: 2625)
Image analysis
Applications
C380S051000, C380S054000, C713S176000
Reexamination Certificate
active
06718045
ABSTRACT:
TECHNICAL FIELD
The present invention lies in general terms within the technical field of watermarking of digital images, more particularly fixed images.
The present invention concerns in particular a method of inserting a watermarking signal in a set of coefficients representing a digital image, in which at least one subset of coefficients is modified by said watermarking signal.
The invention also concerns a device able to implement such a method.
BACKGROUND OF THE INVENTION
Watermarking digital data makes it possible to protect these data, for example by associating copyright information with them.
In its general principle, watermarking consists of inserting an indelible mark in digital data, similar to the encoding of additional information in the data.
The decoding of this additional information makes it possible to check the copyright information inserted.
This inserted watermark must consequently be imperceptible, robust to certain distortions applied to the digital image and capable of reliable detection.
Conventionally, the insertion of a watermarking signal in an image is obtained by modifying coefficients representing the image to be watermarked.
For example, a usual technique of inserting a watermarking signal in a digital image consists of using a linear modulation model in which at least one subset of spectral coefficients representing the digital image is modulated according to this linear model using a weighting coefficient.
Denoting X={X
i
, 1≦i≦N} a set of coefficients representing a digital image and w={w
j
, 1≦j≦P} a watermarking signal of size P less than N, the linear insertion formula is:
X*
j
=X
j(i)
+&agr;
j
w
j
in which X
j(i)
designates a spectral coefficient amongst a subset chosen from the set X of spectral coefficients, and &agr;
j
is a weighting coefficient, also referred to as the modulation amplitude.
The detection of the watermarking signal then consists of detecting whether or not a modulation sequence has been inserted in a set of coefficients. This detection is carried out without using the original watermarked image and is based on a correlation calculation or on a standardised statistical test which makes it possible to theoretically calculate a probability of correct detection.
Such an insertion technique makes it possible, through the insertion of a watermarking signal, to insert a single information bit since the response of the detector is binary.
To insert a larger number of information bits in the digital image, in particular when a code of C bits indicating for example the name or address of the owner or author of the image is required, it is necessary to reiterate the insertion method described previously as many times as there are bits of information to be inserted.
Each weighting coefficient &agr;
j
must be greater than a minimum value, which can be termed the detection amplitude, so as to permit detection of the inserted signal with a correct level of probability.
This minimum value or detection amplitude depends notably on the length of the watermarking signal and on the level of detection probability required.
Moreover, each coefficient &agr;
j
must be less than a maximum value which can be termed the visual amplitude, denoted generally JND (Just Noticeable Difference) so as to meet the imperceptibility criterion of the watermarking signal.
It is known from the state of the art that the local content of the image to be watermarked has a visual masking effect which results in a drop in visual sensitivity making it possible to locally increase the modulation amplitude in the addition of the watermarking signal. Typically, luminance masking and contrast masking are spoken of.
It is then a case of applying psychovisual models in the domain of the image processing, and in particular in the compression of images.
The use of the results of psychovisual studies for adjusting the watermarking weighting in the domain of the watermarking of digital images is known from the state of the art.
For example, in the article entitled “
A DWT—based technique for spatio
-
frequency masking of digital signatures
” by M. Barni, F. Bartolini, V. Cappellini, A. Lippi and A. Piva, which appeared in Proc. SPIE, pages 31-39, January 1999, a method of inserting a watermarking signal in the domain of the digital wavelet transformation (DWT) of a digital image is described.
This method uses a masking model previously proposed for image compression, in order to adapt the force of the watermarking to the characteristics of the human visual system. The model used makes it possible in particular to calculate the maximum weighting amplitude (&agr;
j
) allowed with respect to each representative coefficient (X
j
) modified.
According to this model, this maximum weighting amplitude &agr;
j
is calculated from the energy of coefficients representing the neighbourhood of the representative coefficient X
j
under consideration.
However, this method has the drawback of being of an analytical type, which entails many calculations for determining the parameters adapted to a given image. In addition, these parameters have to be recalculated for each image to be watermarked.
Moreover, it is known that the analytical masking models offer only an approximation of the masking effected by the human visual system.
SUMMARY OF THE INVENTION
The present invention aims to remedy the aforementioned drawbacks.
In this regard, the aim of the present invention is to propose a method of inserting a watermarking signal in an image by modifying coefficients representing the image, the modification of a coefficient under consideration taking into account the visual masking effect of the modification by the coefficients situated in a neighbourhood of the coefficient under consideration. The use of this method being very simple from the calculation point of view, and adaptive with respect to the type of images to be watermarked.
To this end, the present invention concerns a method of inserting a watermarking signal (w) in a set (X) of coefficients (X
i
) representing a digital image (I), in which at least one subset of coefficients is modified by the watermarking signal (w). This method comprises, for each representative coefficient (X
i
) to be modified, the following steps:
determining a neighbourhood (V(X
i
)) of the representative coefficient (X
i
) to be modified, in the image (I);
selecting a neighbourhood (V
sim
d
) in a dictionary of neighbourhoods representing coefficients representing the image (I), according to a predetermined criterion of similarity with the neighbourhood (V(X
i
)) of the representative coefficient (X
i
); and
modifying the representative coefficient (X
i
) as a function of the watermarking signal (w) and a predetermined masking data item (M(V
sim
d
)) representing the masking effect on a watermarking signal of the neighbourhood (V
sim
d
) selected from the dictionary.
Taking into account a masking data item in the modification of a coefficient representing the image, it is possible to increase the degree of modification whilst meeting the invisibility criterion for the inserted signal. This makes it possible to obtain a better detectability of the watermarking signal in the watermarked image, or, at a fixed detectability level, to have a greater watermarking capacity, that is to say to be able to insert a larger number of watermarking bits. As this masking data item is obtained from a representative neighbourhood selected from a dictionary or stored table, the multiplication to be applied to the relevant coefficient of the image is calculated very rapidly.
According to a preferred embodiment of the invention, the method of inserting a watermarking signal comprises the following prior steps:
creating the said dictionary of neighbourhoods (V
d
) representing the coefficients representing the image (I);
generating, for each neighbourhood (V
d
) in the dictionary, a masking data item (M(V
d
)) representing the masking effect of the neighbourhood on a watermarking signal.
Generating such a masking dat
Donescu Ioana
Nguyen Eric
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Tabatabai Abolfazl
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