Abstract:
For the purpose of designing watermark to be robust against image modification such as geometric modification (rotating, cutting, enlarging/shrinking, etc.), compression, and blurring, the watermark is embedded in frequency domain after formed as 2 dimensional shape, for example radial or concentric shape. In detecting watermark, it is possible to effectively detect the watermark, by using relation to a generated watermark in case where the peak is detected.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a method for embedding and detecting a digital watermark in and from digital multimedia contents and an apparatus using the same. More particularly, the present invention relates to a digital watermark embedding and detection method and an embedding and detection apparatus using the same for producing a spatially configured watermark, embedding and recording the configured watermark in an image in a frequency domain, and detecting the watermark effectively from the watermark-embedded image.  
         BACKGROUND ART  
         [0002]    Recently, together with the wide spreading of the internet and computers and the rapid distributions of multimedia data, illegal copies (piracy) and distributions are widely prevalent so that an effective protection apparatus for a copyright to multimedia data gets required. Watermarking technology is one that embeds user information (watermark), unrecognizable by a user, in multimedia data, to thereby prevent pirated copies and protect a copyright of a copyright owner.  
           [0003]    The watermark means a mark developed in a step using a frame for pressing wet fibrous material to get rid of water in a process making paper from papyrus in ancient times. Marks embedded in paper in order for paper manufacturers in the middle ages to prove their own goods are the watermarks in the middle ages, and, nowadays, an image is embedded which can be recognized only with light when, in a process of making banknotes, printing on both sides of a sheet of paper after drying the wet sheet on which printing has been done, and the image is referred to as a watermark.  
           [0004]    In these days, together with the increase of digital media, the concept of a digital watermark has appeared. Just as paper in an analog concept is substituted with the concept of digital paper, digitalizing all the analog media in which the past watermarks were embedded has brought into the concept of the digital watermark as a mark hidden in digital images, audio, video, and so on. That is, the watermarking refers to all technical methods hiding and extracting a special form of watermark in multimedia contents in order to protect a series of multimedia contents. At the beginning, researches have been carried out for methods hiding original multimedia contents themselves, but, at present, it is a trend that strong watermarking technologies using lots of technical transform methods are developing.  
           [0005]    The watermarking is classified into a visible watermarking and an invisible watermarking based on the visibility of a watermark, and the invisible watermarking is again classified into a spatial domain watermarking and a frequency domain watermarking based on the methods embedding a watermark.  
           [0006]    The visible watermarking specifies a copyright by embedding in an original image author information which can be recognized with eyes. The visible watermarking can be used with ease but has a drawback in that the originals are damaged.  
           [0007]    Accordingly, the invisible watermarking is primarily used in the image watermarking technology in these days. The invisible watermarking is a technology embedding a watermark not to be visually perceived by using a limit of senses of the human visual system. While the spatial domain watermarking embeds and extracts a watermark with ease, there is a high possibility to lose a watermark by means of signal processing, video processing (non-linear filtering, rotating, cutting, moving, enlarging, and reducing transforms and the like), and compressing.  
           [0008]    However, the frequency domain watermarking employs transform techniques such as Fourier transform, discrete cosine transform, or the like for embedding and extraction, so there exists a drawback in that it has a complicated algorithm and requires lots of arithmetic operations, but it has an advantage in that it is robust on general attacks such as filtering or compressions.  
           [0009]    The invisible embedding of a watermark requires an embedding of the same in a low value on a broad area, which is carried out by the spread spectrum technology of Ingemar J. Cox. In the spread spectrum technology, a pseudo-random sequence is used as a watermark, which is a method that can be effectively used since the sequence has a uniform distribution function and is evenly distributed over the entire bandwidth of frequencies.  
           [0010]    For methods transforming an original image into a frequency domain, the fast Fourier transform (FFT), discrete cosine transform (DCT), and wavelet transform are generally utilized a lot, which takes a method embedding and restoring a watermark into the original state in a transform plane. However, the method has a high possibility to lose a watermark on attacks such as image rotating, cutting, moving, enlarging, reducing, or the like.  
           [0011]    As stated above, the watermarking methods in the spatial domain or frequency domain have advantages and disadvantages in their own ways. For an alternative, a watermarking method using the log-polar mapping and Fourier transform has been developed to compensate for the loss of a watermark, which is the weak point of the frequency domain watermarking method, in rotating, enlarging, or reducing an image. The method converts rotations, enlargements, and reductions into a simple movement forms through the log-polar mapping and detects a watermark by using the characteristics that the amplitudes of the Fourier transform are invariable with movements. However, the method is weak at the video processing such as compressions as well as has a big drawback in that the loss due to the log-polar mapping itself is very high and the implementations become very complicated.  
           [0012]    As mentioned above, the developed watermarking technologies for video have advantages and disadvantages in general in their own ways. Further, the pseudo-random sequence watermark being widely used at present can confirm what key value a watermark embedded in an image has, but has difficulties in embedding and extracting various copyright information.  
           [0013]    Further, in case of firstly casting and then embedding a watermark in an input image, an embedded watermark is changed if the image undergoes rotations, partial cuttings, or the like, causing a problem impairing copyright information.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0014]    It is an object of the present invention to provide a digital watermark embedding and detection method and a apparatus using the same which are robust against image variations such as rotation, enlargement/reduction, cutting, and filtering.  
           [0015]    It is another object of the present invention to provide a digital watermark embedding and detection method and a apparatus using the same which spatially configure a watermark, convert an image signal into a frequency domain, and embed the spatially configured watermark to thereby be robust against image variations.  
           [0016]    It is yet another object of the present invention to provide a watermark detection method and a apparatus using the same which effectively detect a spatially configured digital watermark embedded in an image signal in a frequency domain.  
           [0017]    In order to achieve the above objects, a method for embedding a digital watermark in an image signal according to the present invention comprises steps of:  
           [0018]    using a user key and an inherent key and generating respective pseudo-noise codes thereof;  
           [0019]    adding the pseudo-noise code generated based on the user key and the pseudo-noise code generated based on the inherent key;  
           [0020]    generating a digital watermark including a step of arranging in a two-dimensional form a watermark formed by the addition;  
           [0021]    converting an image signal from a spatial domain to a frequency domain; and  
           [0022]    adding a magnitude component of the image signal converted into the frequency domain and a watermark generated by the watermark generation step.  
           [0023]    Further, a method for detecting a digital watermark according to the present invention comprises steps of:  
           [0024]    strengthening a component of the digital watermark embedded in the image signal;  
           [0025]    converting the digital watermark-strengthened image signal from a spatial domain to a frequency domain and extracting the digital watermark included in the image signal;  
           [0026]    generating a digital watermark for comparison with the extracted digital watermark;  
           [0027]    calculating correlation between the generated digital watermark and the digital watermark extracted from the image signal; and  
           [0028]    detecting the watermark embedded in the image signal based on the correlation.  
           [0029]    As stated above, unlike a method simply embedding a watermark of a certain form in an existing spatial domain or frequency domain, the present invention can embed a watermark not linearly but spatially configured in a frequency domain so that the watermark is not changed due to external variations such as image rotation, cutting, or the like. In particular, the watermark embedded according to the present invention is arranged in a radial form or in a form of plural concentric circles about the center of a block structuring an image signal from a watermark of a stream form.  
           [0030]    Furthermore, the digital image watermarking apparatus and method according to the present invention employ a sharpness degree, a maximum value, and its position in use of the fourth moment (Kurtosis) in the correlation of a user key value and a watermark, featuring maximizing the accuracies of the watermark detections and authentications. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The above objects and other features of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings, in which:  
         [0032]    [0032]FIG. 1 is a block diagram for schematically showing a structure of a digital watermark embedding and detection apparatus according to an embodiment of the present invention;  
         [0033]    [0033]FIG. 2 is a flow chart for schematically showing operations of an image converter of a watermark embedding apparatus in FIG. 1;  
         [0034]    [0034]FIG. 3 is a flow chart for schematically showing operations of a first SF transformer of a watermark embedding apparatus in FIG. 1;  
         [0035]    [0035]FIG. 4 is a view for schematically showing a structure of a watermark generator of a watermark embedding apparatus in FIG. 1;  
         [0036]    [0036]FIG. 5 is a view for showing an example of a two-dimensional watermark implemented by a watermark configurer in FIG. 4;  
         [0037]    [0037]FIG. 6A is a view for showing a process forming another example of a two-dimensional watermark implemented by a watermark configurer in FIG. 4, and FIG. 6B is a view for showing a watermark formed by FIG. 6A;  
         [0038]    [0038]FIG. 7 is a flow chart for schematically showing operations of an FS transformer of a watermark embedding apparatus in FIG. 1;  
         [0039]    [0039]FIG. 8 is a flow chart for showing operations of an image recorder of a watermark embedding apparatus in FIG. 1;  
         [0040]    [0040]FIG. 9 is an exemplary view for showing filters serving watermark detections, wherein FIG. 9A shows a high boost filter, FIG. 9B shows a Laplacian filter, and FIG. 9C shows a DoG (Difference of Gaussian) filter having 7×7 and 9×9 masks;  
         [0041]    [0041]FIG. 10 is a view for showing an example of a mask form employed for an effective watermark detection;  
         [0042]    [0042]FIG. 11 is an exemplary view for showing processed results by filters of FIG. 9, wherein FIG. 11A is a view for showing an example of a watermarked image before filtering, and FIGS. 11B to  11 D respectively show the processed results by a high boost filter, Laplacian filter, and DoG filter;  
         [0043]    [0043]FIG. 12 is a flow chart for showing operations of a second FS transformer of a watermark detection apparatus in FIG. 1;  
         [0044]    [0044]FIG. 13 is a view for showing an example of a peak detection employed for a watermark detection; and  
         [0045]    [0045]FIG. 14 is a flow chart for showing operations of a watermark detector in FIG. 1.  
     
    
     EMBODIMENT  
       [0046]    Hereinafter, the watermark embedding and detection method and the watermark embedding and detection apparatus using the same according to the present invention are described in detail with reference to the accompanying drawings.  
         [0047]    [0047]FIG. 1 is a block diagram for schematically showing a structure of a digital watermark embedding and detection apparatus according to an embodiment of the present invention.  
         [0048]    The digital watermark embedding and detection apparatus in FIG. 1 comprises a watermark embedding apparatus  100  for embedding a watermark into an inputted image and a watermark detection apparatus  200  for detecting a watermark from a watermark-embedded image. The watermark embedding apparatus  100  includes an image converter  110  converting an inputted image  10  into a certain form based on characteristics thereof, a first Spatial-to-Frequency (FS) transformer  120  transforming an output signal of the image converter  110  from a spatial domain to a frequency domain in consideration of an image form, a watermark generator  130  generating a watermark spatially arranged, an adder  140  adding the watermark generated from the watermark generator  130  and an image signal outputted from the first SF transformer  120 , a Frequency-to-Spatial (FS) transformer  150  transforming the watermark-added signal from the frequency domain to the spatial domain again, and an image recorder  160  recording the watermark-embedded image signal.  
         [0049]    Further, the watermark detection apparatus  200  includes an image converter  210  receiving a reproduced image signal and converting it into a certain form of format, a pre-processor  220  strengthening the characteristics of a watermark included in an output signal of the image converter  210 , a second SF transformer  230  transforming the watermark characteristics-strengthened image signal from the spatial domain to the frequency domain and extracting a watermark region from the corresponding image signal, a watermark generator  240  generating a watermark spatially arranged, a correlation calculator  250  calculating a correlation between a watermark extracted from the second SF transformer  230  and a watermark outputted from the watermark generator  240 , and a watermark detector  260  detecting a watermark included in an image signal based on an output value from the correlation calculator  250 .  
         [0050]    The operations in the watermark embedding and detection apparatus having the above structure are described with respective constituents thereof. First, the operations of the watermark embedding apparatus  100  are described with reference to FIG. 2 to FIG. 8.  
         [0051]    An image  10  is inputted to the image converter  110  to embed a watermark in a digital image signal. Describing the operation flow of the image converter  110  with reference to FIG. 2, the image converter  110  checks whether the inputted image  10  is a 24-bit color image (Step S 100 ). At this time, it is determined by checking the header information of the inputted image signal whether the inputted signal is in 24-bit. If the inputted image  10  is in the 24-bit color, the RGB components of the inputted image are converted into a YIQ format by employing Formula 1 as below (Step S 110 ), wherein Y stands for Luminance, I for In-phase, and Q for Quadrature.  
         [0052]    [Formula 1] 
         [         Y           I           Q         ]     =       [         0.2989       0.587       0.114           0.5959         -   0.2744           -   0.3216             0.2115         -   0.5229         0.3114         ]          [         R           G           B         ]                             
 
         [0053]    The I and Q components in the converted format are separately stored, and only the Y component is extracted (Step S 120 ). The extracted Y component passes over to the first SF transformer  120 .  
         [0054]    In the step S 100 , if the inputted image is not a 24-bit color image, the process directly proceeds with the first SF transformer  120 . That is, if the inputted image is not the 24-bit color image, the image  10  actually inputted corresponds to the same signal as the Y component of the 24-bit color case, so the inputted image  10  is directly transferred to the first SF transformer  120  without any conversion process into the YIQ format. Accordingly, if the inputted signal is not the 24-bit, no separate image converter  110  may be provided. Further, in the above case, an input image is processed into the YIQ format based on the NTSC format, and, in other formats, a watermark can be embedded by directly separating an RGB signal into respective R, G, and B channels for outputs without converting a format of the inputted image.  
         [0055]    A result processed in the image converter  110  is inputted to the first SF transformer  120 . The first SF transformer  120  carries out a transform in order shown in FIG. 3.  
         [0056]    The output signal of the image converter  110  is applied with two-dimensional Fast Fourier Transform (Formula 2) (Step S 200 ). A result of the two-dimensional Fast Fourier Transform is expressed as a form of a complex number which is divided into a real number component (R) and an imaginary number component (I). The general Fourier Transform can be employed in lieu of the above Fast Fourier Transform.  
         [0057]    [Formula 2] 
           1   )                     F        (       n   1     ,     n   2       )         =       ∑       k   2     =   0         N   2     -   1                         ∑       k   1     =   0         N   1     -   1                         exp        (       2      π                                    k   2          n   2         N   2       )            exp        (       2      π                   k   1          n   1         N   1       )            f        (       k   1     ,     k   2       )                                   
 2) M ={square root}{square root over (( R   2   +I   2 ))} 
           3   )                   Θ     =     arc                   tan   (     I   R     )                             
 
         [0058]    In here, f denotes an image signal, F a frequency coefficient obtained after Fourier Transform, M a magnitude of values obtained after a frequency transform, and θ a phase, respectively.  
         [0059]    Since a complex number form appears by the Fourier Transform, the frequency coefficient is divided into real number and imaginary number components. A magnitude and a phase are calculated and separated, respectively, by using Formula 2 from these components (Step S 210 ). In the step S 210 , the image signal is separated into a magnitude and a phase respectively, and the magnitude component is FFT-shifted to be converted into a form for embedding a watermark (Step S 220 ). The FFT shift shifts magnitude components to a center portion in order for a to-be-later-embedded watermark to be embedded about the center. Further, phase components are transferred to the FS transformer  150  in order to transform an image signal into the spatial domain again after embedding a watermark.  
         [0060]    Using magnitude components for embedding a watermark is because, when Fourier-Transformed, magnitude components have the characteristics shown in Formula 3.  
         [0061]    [Formula 3] 
               f        (       x   +   a     ,     y   +   b       )       ↔       M        (     u   ,   v     )                   -   j                     (     au   +   bv     )                         f        (       ρ                 x     ,     ρ                 y       )       ↔       1   ρ          M        (       u     ρ   ,            v   ρ       )                       f        (         x                 cos                 θ     -     y                 sin                 θ       ,       x                 sin                 θ     +     y                 cos                 θ         )       ↔     M        (         u                 cos                 θ     -     v                 sin                 θ       ,       u                 sin                 θ     +     v                 cos                 θ         )                                   
 
         [0062]    Wherein, f denotes an image signal, M a magnitude of values obtained after frequency conversions, ρ a constant multiplied upon resizing (a resizing multiple), and θ a phase, respectively.  
         [0063]    As seen in a result of Formula 3, the characteristics are used that a magnitude component is the same even though an image is shifted, and an absolute position does not vary even though a image scale changes. By configuring a watermark into a circular form in addition to the characteristics, the watermark is prevented from damages even in variations of an image by rotations.  
         [0064]    The image signal processed by the first SF transformer  120  is added to a watermark generated from the watermark generator  130  and then embedded to the inputted image.  
         [0065]    The watermark generator  130  generates a watermark by a structure shown in FIG. 4. First, a user key is inputted to a pseudo-noise code generator  122 , which generates a pseudo-noise code by using the user key as a seed value. In the meantime, an inherent key generated to facilitate a watermark detection, separately from the user key, is inputted into a pseudo-noise code generator  124 , and a pseudo-noise code is generated in the same manner as in the user key.  
         [0066]    The two pseudo-noise codes so generated are added in an adder  126 . An added pseudo-noise code is inputted to a watermark configurer  128 . The watermark configurer  128  newly configures a watermark of a one-dimensional stream format into a two-dimensional format. Viewing a format shown in FIG. 5 as an example, a watermark is configured to be arranged in a two-dimensional radial format while rotating 360 degrees about a first value of a watermark of a certain length. In the case of configuring a watermark in the two-dimensional radial format as above, a watermark-embedded image is not affected by external attacks since a watermark format does not changes even though the watermark-embedded image is varied by external attacks such as rotations and so on.  
         [0067]    Further, another example configuring a watermark in the two-dimensional format is described with reference to FIG. 6. FIG. 6A is a view for showing a process for configuring one watermark sequence into plural concentric circles, and FIG. 6B is a view for showing a watermark in plural concentric circles consequently generated.  
         [0068]    As in (a) of FIG. 6A, after configuring a watermark Wseq formed in a sequence of 1 and −1, the watermark Wseq is resized in various lengths as in (b) to configure a watermark as in (c). FIG. 6B shows a watermark implemented by the watermark configurer through the process of FIG. 6A.  
         [0069]    As stated above, when a watermark is configured in a circular shape, a watermark is arranged in a concentric format about the center of a block. When arranged in the concentric format, there exists a radial difference between an inner watermark and an outer watermark, so a difference occurs between bit lengths configuring watermarks. Accordingly, watermarks are sampled at a certain rate in accordance with radial magnitudes to be enlarged and reduced for arrangements. When a watermark is arranged in a circular shape, an upper side is configured in a watermark stream and then the stream is copied for a lower side, to be arranged in the same rotation direction for use. An initially formed shape is a semi-circle rather than a concentric circle, which is because an initial component of the Fourier Transform is an origin symmetry, so a watermark is embedded by a semi-circle and then the semi-circle is copied in the origin symmetry, to thereby bring out an effect embedding concentric circles as in FIG. 6 b.    
         [0070]    The watermark so formed is inputted to the adder  140  to be added to an image signal outputted from the first SF transformer  120 .  
         [0071]    The adder  40  first divides the image into blocks of a predetermined size, that is, a watermark size in order to add a watermark configured from the watermark generator  130  and the image signal outputted from the first SF transformer  120 . The divided image signal and the watermark signal outputted from the watermark generator  130  are added. At this time, the intensity of a watermark embedded according to the characteristics of the image signal is determined beforehand for the addition.  
         [0072]    The determination of the watermark intensity can be accomplished in various forms. For example, it is determined by the number of colors used for each channel in a block, histogram shape, energy ratio of high and low frequencies, and so on.  
         [0073]    For example, when dividing an image block by block, the number of colors used for each block and a color value are obtained. In case that many color numbers are used and the color value is high, a real image corresponds to one that has severe color changes or colors of a brilliant form. Accordingly, a visual effect is not experienced so much even though the intensity of a watermark to be embedded in a corresponding block is high. However, in case that color changes are small, even a watermark embedded with a low intensity can give a feeling that much noise is included in an original image. Therefore, the number of colors used in a block and a color value are considered to determine the intensity of a watermark to be embedded to be strong when the value is high and to be weak when the value is low.  
         [0074]    Further, if the DCT transform is applied to an image to be expressed in a block, it is characterized in that a part corresponding to a low frequency region is clustered in the upper left of the block, a part corresponding to an intermediate frequency region in the center portion, and a part corresponding to a high frequency region in the lower right. That is, the DCT result enables the characteristics of an inputted image to be grasped depending on a ratio of a low frequency energy and a high frequency energy.  
         [0075]    Moreover, if an inputted image is analyzed channel by channel, each of R, G, and B channels has 8 bits (2 8 =256) and values 0˜255 are allocated to each color in case of a 24-bit color, and it is available that a histogram is prepared based on the values with respect to image regions and the changes or occupied colors in an image are grasped based on the shape and changes of the histogram. That is, if the number of used colors is small, the distribution of the histogram becomes narrow, and to the contrary, if the number of used colors is large, the distribution of the histogram becomes wide. The large number of used colors means that an image has severe changes, and, to the contrary, the small number of used colors means that an image is dull without particular changes. Accordingly, with this, it can be determined whether an image energy is concentrated in a high frequency region or in a low frequency region.  
         [0076]    In a method for adding a watermark, it is also possible to independently embed a watermark directly into each channel, that is, in case of a Gray image, into the Gray channel and, in case of an RGB image, into respective R, G, and B channels, without passing through the image converter  110 . If a watermark-embedded signal is outputted, the signal is inputted to the FS transformer  150  and then converted from a frequency domain to a spatial domain.  
         [0077]    The operations carried out in the FS transformer  150  is described with reference to FIG. 7.  
         [0078]    The FS transformer  150  basically carries out in the reverse order the process done by the first SF transformer  120 . That is, an FFT shift is carried out with respect to a watermark-embedded signal (Step S 300 ). This plays a role of shifting the signal into an original format by applying again the FFT shift done for the signal prior to embedding a watermark. After shifting, the signal component (magnitude component) and a phase component separated and extracted before from the first SF transformer  120  are added and then an inverse 2D FFT is applied to transform a frequency domain signal into a spatial domain signal (Step S 310 ).  
         [0079]    After the transform, an image signal is resized to prevent an overflow which can occur by an addition with a watermark carried out in the above step (Step S 320 ). For example, an R channel signal of 8 bits has values ranging from 0 to 255, which can have values less than 0 or larger than 255 by the addition with a watermark in the adder  140 . A watermark size basically has a value of −1 or 1, or, in case of resizing, has values of integer multiples of the above value, and, even though the size is not big, may have a value out of a range from 0 to 255 by the addition with an image signal. At this time, abrupt color changes are developed. Accordingly, in case that an addition result becomes less than 0 or larger than 255, an overflow occurs and corresponding values are adjusted to 0 or 255, respectively, that are boundary values the signal can have.  
         [0080]    As stated above, a watermark-embedded signal is transformed into a spatial domain by the FS transformer  150  and then recorded on a storage medium and the like by the image recorder  160 , such recording operations of which are described with reference to FIG. 8.  
         [0081]    The image recordation part  160  determines whether the watermark-embedded signal is a 24-bit image or not (Step S 400 ). If the watermark-embedded signal is a 24-bit image, the previous IQ components left after having extracted the Y component from the YIQ components are added to the Y component (Step S 410 ). Following the addition, a signal of the YIQ format is again converted into the RGB signal by using Formula 4 as follows (Step S 420 ).  
         [0082]    [Formula 4] 
         [         R           G           B         ]     =       [         1.0       0.956       0.621           1.0         -   0.272           -   0.647             1.0         -   1.106         1.703         ]          [         Y           I           Q         ]                             
 
         [0083]    A signal converted as above is stored in a storage medium in a watermarked image (Step S 430 ).  
         [0084]    However, if the watermarked signal is not a 24-bit image in the step S 400 , the step  430  directly proceeds for storage since the watermarked signal is a image signal inputted from the preceding image conversion part  110  without a separate conversion step so that the above conversion is unnecessary. Further, even in case that a watermark is embedded in every channel process by the processing of the RGB format instead of converting an inputted image into the YIQ format, the step S 430  directly proceeds for storage.  
         [0085]    The watermark embedding apparatus  100  as stated above, arranges a watermark in the two-dimensional space for embeddings and embeds a watermark in the frequency domain, bringing out an effect that a watermark does not change even when taking variations as to an image such as rotations, cuttings, or the like with respect to a watermark-embedded image.  
         [0086]    Further, the watermark detection apparatus  200  for detecting a watermark from the watermark-embedded image signal as above is described with reference to FIG. 1, and FIGS.  9  to  14 .  
         [0087]    A watermarked image can flow into a pirate or an illegal user via various ways, be pirated, and be modified. However, in case that a watermark is spatially arranged and embedded in the frequency domain by the watermark embedding apparatus  100  according to the present invention, the watermark embedded in an image has characteristics robust enough to maintain its shape even when the image undergoes variations due to image rotations, cuttings, or the like. Descriptions are made on a apparatus and method for detecting a watermark embedded by such a manner.  
         [0088]    If an image in which a watermark is embedded and recorded is inputted to the watermark detection apparatus  200 , the image is first converted into a signal of a certain form through the image conversion part  210 . The structure and operations of the image conversion part  210  in the watermark detection apparatus  200  is the same as those of the image conversion part  110  in the watermark embedding apparatus  100 . That is, if an inputted image is a 24-bit image, the inputted image is converted into the YIQ format from the RGB format, only the Y component is extracted and outputted to detect a watermark. If not a 24-bit image, the inputted image is outputted without the conversion. Further, if the inputted image is in 24 bits, the RGB signal form can be outputted as it is.  
         [0089]    An image signal outputted from the image conversion part  210  is inputted into the pre-processing part  220 . The pre-processor  220  is for emphasizing the characteristics of a watermark included in the image signal, and carries out a high-pass filtering, sharpen filtering, or high-boost filtering process. Such filters employed in the pre-processor  220  are illustrated for examples in FIG. 9 and FIG. 10.  
         [0090]    [0090]FIG. 9 is a view for showing examples of various spatial filters performing a role of boosting high-frequency components of an image signal, FIG. 9A, FIG. 9B, and FIG. 9C show mask forms for a high boost filter, a Laplacian filter, and Difference-of-Gaussian (DoG) filter, respectively.  
         [0091]    The high boost filter in FIG. 9A serves detecting a watermark, and plays a role of boosting a watermark signal. That is, it plays a role of reducing an image component energy and increasing a watermark signal energy. Further, the DoG filter of FIG. 9C is based on Formula 5 as follows.  
         [0092]    [Formula 5] 
         DoG        (     x   ,   y     )       =              -       (       x   2     +     y   2       )       2        σ   1   2               2                   πσ   1   2         -            -       (       x   2     +     y   2       )       2        σ   2   2               2                   πσ   2   2                                 
 
         [0093]    In addition to the filters in FIG. 9, a filter as shown in FIG. 10 may be used for reducing an image component energy and intensifying a watermark component energy.  
         [0094]    The pre-processor  220  as stated above is for intensifying a watermark component from an image signal, for which any one of the filters shown in FIG. 9 and FIG. 10 may be used for processing.  
         [0095]    [0095]FIG. 11 is an exemplary view for showing results processed by filters of FIG. 9. FIG. 11A is a view for showing an example of a watermarked image before filtering, and FIGS. 11B to  11 D respectively show the processed results by a high boost filter, Laplacian filter, and DoG filter.  
         [0096]    A signal passing through the pre-processor  220  is inputted to the second SF transformer  230 . The operation flows in the second SF transformer  230  is described with reference to FIG. 12. The second SF transformer  230  is for extracting an embedded watermark, which basically has the same transform process as one of the first SF transformer  120  in the watermark embedding apparatus  100  (Step S 200 ). That is, the 2DFFT transform separates a magnitude component and a phase component, the magnitude component is extracted, and then the magnitude component is FFT-shifted.  
         [0097]    After the FFT shift, a watermark-embedded regions are extracted in one dimension (Step S 234 ). Since a watermark-embedded position does not change even when image transforms such as rotations, enlargements/reductions, cuttings, and so on are applied, the above processing can be carried out. The watermark-embedded regions may vary in sizes thereof. For example, in case of arranging watermarks in a concentric shape, one can be identical to a real watermark size, but inner and outer watermarks arranged about the watermark are extended in lengths thereof and formed through manipulations such as sampling and the like. Accordingly, the watermarks so changed in sizes are resized to original sizes (Step S 238 ).  
         [0098]    In the meantime, the watermark generator  240  of the watermark detection apparatus  200  is the same in a basic structure as the watermark generator  130  of the watermark embedding apparatus  100 , but has not the watermark configurer. That is, the watermark generator  240  generates watermarks cast in one dimension as to respective pseudo-noise codes generated by a user key and an inherent key.  
         [0099]    The correlation calculator  250 , as stated above, calculates a correlation Corr between a watermark component of an image signal processed by the second SF transformer  230  and a watermark signal generated from the watermark generator  240  by using Formula 6 as below.  
           Corr=IFFT ( FFT ( W   EXT )× conj ( FFT ( W   m )))  [Formula 6] 
         [0100]    Here, W EXT  denotes a watermarked embedded in an image signal extracted by the second SF transformer  230 , and W m  respective watermarks generated by using a user key and an inherent key by the watermark generator  240 . IFFT denotes a one-dimensional inverse fast Fourier transform, FFT a one-dimensional fast Fourier Transform, and conj a complex conjugate.  
         [0101]    The correlation calculations using the above Formula 6 are carried out by multiplying data obtained through the two-dimensional fast Fourier transform with respect to a watermarked image W EXT  with data obtained through the two-dimensional fast Fourier transform with respect to a watermark W m  generated by a user key or an inherent key from the watermark generation part  230 , and then the inverse fast Fourier transform is applied to the multiplication to be converted into a spatial domain. As above, the transform into a frequency domain and the calculations based on the multiplication reduce the number of calculations compared to taking convolution with an image watermarked in the spatial domain and a watermark, enabling faster data processing.  
         [0102]    [0102]FIG. 13 is a view for showing a floated correlation calculated based on Formula 6 as to a presumptive case that a watermark is embedded. A correlation obtained by Formula 6 is not a certain value, but plural values in a one-dimensional sequence form, so such plural values are compared to enable a maximum peak value and its position to be obtained through a process as follows.  
         [0103]    The watermark detector  260  checks, like a watermark is generated through an inherent key and a user key in the watermark generator  230  if peaks occur as shown in FIG. 14, whether these two key values exist and the peaks occur at the same position (Step S 500 ). A sharpness degree is calculated based on Formula 7 if the two peak positions are the same (Step S 510 ). The calculation of the sharpness degree, through a fourth moment (Kurtosis) K, checks whether the value of K is more than a certain threshold value (Step S 520 ), and it is determined that a watermark is detected when the two conditions are all satisfied.  
               K        (       x   1     ,   …              ,     x   N       )       =       {       1   N            ∑     j   =   1     N                       [         X   j     -     X   _       σ     ]     4         }     -   3             [     Formula                 7     ]                               
 
         [0104]    Here,  
         x   1     ,   …              ,     x   N                           
 
         [0105]    denotes a result value of a correlation between two watermarks W m  and W EXT , {overscore (x)} an average of  
         x   1     ,   …              ,     x   N     ,                         
 
         [0106]    and σ a standard deviation.  
         [0107]    The determination as to whether a value of K is more than a certain threshold value in the above procedure is to determine whether a watermark is embedded through a comparison between a peak value and a set threshold value since a peak appears high at an calculated value in case that the watermark is embedded. In here, the threshold value is shown as a value allocated in a certain manner by experiments. However, when the condition is not satisfied in the Step S 520 , it is determined that a watermark is not detected.  
       INDUSTRIAL APPLICABILITY  
       [0108]    As described above, the present invention relates to a method and apparatus for embedding and detecting a watermark, which uses a watermark formed in a radial or circular format in the two-dimensional form upon embeddings and changes its configuration for the embedding into an image signal in the frequency domain, enhancing the robustness of a watermark against signal variations. Further, upon the detection of the watermark, a watermark embedded in the radial or circular format in the two-dimensional form as above can be effectively detected, enhancing the accuracy and promptness for watermark detections.  
         [0109]    In particular, the present invention greatly reduces the complexity of an entire system compared to a method using the existing log-polar mapping and removes data losses which can occur in the step of the log-polar mapping and the inverse log-polar mapping, to thereby facilitate the watermark detections as well as remarkably reduce image losses when embedding a watermark.  
         [0110]    Although the preferred embodiment of the present invention has been described in particular, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiment, but various changes and modifications can be made within the spirit and scope of the present invention as defined by the appended claims.