APPARATUS AND METHOD FOR EMBEDDING AND EXTRACTING DIGITAL WATERMARKS BASED ON WAVELETS
FIELD OF THE INVENTION
[0001] The present invention relates to digital watermarking; and, more particularly, to an apparatus and method for embedding and extracting digital watermarks based on wavelets which is robust to external attacks while being capable of minimizing a degradation in picture quality caused by embedding of the watermarks.
BACKGROUND OF THE INVENTION
[0002] Change from the analog age to the digital age has been rapidly progressing, as apparent from generalization of digital media, great and a wide growth of electronic publishing industries, digitalization of diverse multimedia contents, and rapid development of digital communication networks such as the Internet, all of which have been recently made. That is, transfer and exchange of diverse data associated with e-books, Internet TV, images, videos, MP3, etc. are currently enabled. Using such multimedia services, therefore, users can rapidly and easily obtain desired information.
[0003] However, the change to the digital age involves various adverse effects. For example, the development of digital techniques has allowed a large number of copies to be produced. Furthermore, the development of communication networks has allowed the distribution of copies without any limitation. For this reason, creative works of individuals may be unreasonably used by stealth. Practically, such adverse effects have been highlighted as significant problems to be surely eliminated to providers, who provide data services such as MP3 files or moving picture data over the Internet.
[0004] Meanwhile, copies of analog data, for example, books, analog tapes, films, or painted pictures have a degraded quality. In order to prevent such a degradation in quality, it is necessary to produce copies identical to the original data. However, this is technically impossible. Although owners of copyrights have a preference for digital data because of the above mentioned drawback of analog data, the digital data also has a drawback in that it is impossible to distinguish the original data from its copies due to the digital property thereof. For this reason, it is strongly required to provide solutions for protection of copyrights of digital data against unauthorized duplication, distribution and modification of the digital data, and authentication associated with those copyrights.
[0005] To this end, techniques for preventing unauthorized copying of digital data have been developed. For example, information protection schemes such as cryptography and firewall have been proposed. However, such methods are incompatible with the features of the Web because most of them basically prevent access to data. Furthermore, there is no reliable measure to prevent unauthorized copying and modification of digital data made by users allowed to have access to the digital data.
[0006] Accordingly, research has been performed to provide various copying prevention techniques for effectively preventing copying of digital data, thereby protecting the
copyrights of the digital data. For example, research has been actively performed in association with a digital watermarking method, which is known to be effective for prevention of copying of digital data.
[0007] Watermarking is a technique developed to prevent copying of digital contents. In accordance with this technique, the owner of a copyright can embed, in a multimedia content created by him, a specific stream of digital data representing information about the ownership of the multimedia content while being visually and audibly imperceptible. Such a specific digital data stream is called a "watermark". Digital watermarking methods are mainly classified into a method of embedding a watermark in a spatial domain, and a method of embedding a watermark in a frequency domain. Watermarking in spatial domains can be easily performed while requiring a relatively small amount of calculation. However, it is difficult to apply this watermarking method to images compressed by a technique such as JPEG (Joint Photographic Experts Group). In addition, this watermarking method has a problem in that the embedded watermark is quite sensitive to noise. For this reason, watermarking in frequency domains has been known as being more effective than the watermarking in spatial domains. Therefore, the watermarking in frequency domains has been mainly used.
[0008] For the watermarking method based on frequency domains, I. J. Cox has proposed a method in which the entire domain of an image is processed by DCT (Discrete Cosine Transform) without being divided into blocks so that random noise proportional to DCT coefficients are embedded, as watermark signals, in the domains, except for the low frequency domain. In addition, various watermarking methods based on DCT domains have been proposed. For example, a watermarking method based on block DCT has been proposed in which insertion of a watermark is determined based on a JND (Just Noticeable Difference) value using human visual characteristics. In accordance with this method, a product by the JND value is embedded as a watermark signal. Recently, a method has been proposed in which a visually-imperceptible watermark is embedded in a DC component of a DCT domain.
[0009] Meanwhile, in pace with the recently increased demand for highly efficient compression of image and video data, research for compression of image data is actively conducted in association with image data compression using a wavelet transform, as compared to image data compression using a DCT, which involves a blocking phenomenon in the encoding of super-low-speed moving pictures. In particular, the watermarking methods based on DCT domains is ineffective in JPEG 2000, that is, a new image compression standard recently established for Internet environments because compression of images is performed based on a wavelet transform in the JPEG 2000, different from the existing JPEG standards based on DCT. Based on this background, research for watermarking methods based on wavelets is actively conducted.
[0010] Various watermarking methods based on the wavelet transform have been proposed. For example, there is a watermarking method in which watermark signals having different lengths are embedded in all high-frequency domains, except for the lowest-frequency domain, respectively. Also, a watermarking method has been proposed in
which a watermark signal is embedded in a coefficient having a larger value. In most of the proposed methods, a watermark is embedded in frequency components, except for DC components, that is, the lowest-frequency components, after a frequency transform including a wavelet transform, taking into consideration the human's visual characteristics more sensitive to a variation in low-frequency components than to a variation in high-frequency components. However, these methods still have problems in that the watermark is considerably damaged when the highfrequency components are eliminated in accordance with a compression process such as JPEG compression.
[0011] On the other hand, a technique for embedding watermarks in DC components has been disclosed in "Embedding Image Watermarks in DC Components", "IEEE Trans. Circuits and Systems for Video Technology" Volume 10, No. 6, pp 974-979, Sep. 2000. In this technique "Embedding Image Watermarks in DC Components", embedding of a watermark in a DC component has been proposed as a method for enhancing the robustness of the watermark to attacks based on, for example, JPEG. This technique is not based on wavelets, but based on DCT That is, an image is subjected to a DCT for respective blocks thereof, and the resultant blocks are sorted into two groups in accordance with texture intensities thereof so that different scaling factors are adaptively applied to respective block groups, in order to make watermarks have different intensities. However, this technique cannot provide solutions to a degradation in picture quality due to embedding of watermarks in DC components.
SUMMARY OF THE INVENTION [0012] Therefore, an object of the invention is to provide an apparatus and method for embedding and extracting digital watermarks based on wavelets, in which the watermarks are embedded in DC component domains of wavelettransformed domains in the order of pixels having a higher high-frequency dependency, so that they are robust to external attacks such as compression while minimizing a degradation in picture quality caused by the embedding thereof.
[0013] In accordance with one aspect, the present invention provides a digital watermark embedding apparatus based on wavelets including: a high-frequency component removing unit for removing high-frequency components from a target image corresponding to a target domain of a wavelet-transformed original image, in which watermarks are to be embedded, thereby generating a mirror image corresponding to the target domain free of high-frequency components; an index information generating unit for comparing data values of pixels in the target image with data values of pixels in the mirror image, respectively, thereby detecting position information of the pixels having higher high-frequency dependencies in the target image, the index information generating unit serving to arrange the detected pixel position information in the order of pixels having higher high-frequency dependencies, thereby generating an index information about the arranged pixel position information; a watermark generating unit for generating a data stream of the watermarks to be embedded in the target image; and a watermark embedding unit for embedding the watermarks of the watermark data stream generated from the watermark generating unit in pixel data of the target image at positions determined based on the index information from the index information generating unit, respectively.
[0014] In accordance with another aspect, the present invention provides a digital watermark extracting apparatus based on wavelets including: a high-frequency component removing unit for removing high-frequency components from a target image corresponding to a target domain of a wavelet-transformed original image, in which original watermarks are to be embedded, thereby generating a mirror image corresponding to the target domain free of highfrequency components; an index information generating unit for comparing data values of pixels in the target image with data values of pixels in the mirror image, respectively, thereby detecting position information of the pixels having higher high-frequency dependencies in the target image, the index information generating unit serving to arrange the detected pixel position information in the order of pixels having higher high-frequency dependencies, thereby generating an index information about the arranged pixel position information; a watermark generating unit for generating a data stream of the original watermarks to be embedded in the target image; a watermark extracting unit for receiving the index information from the index information generating unit, receiving a watermark-embedded image corresponding to a watermark-embedded domain of the wavelet-transformed original image, and extracting a data stream of watermarks from in the watermark-embedded image, based on the index information; and a watermark comparing unit for checking a similarity between the original watermark data stream from the watermark generating unit and the extracted watermark data stream from the watermark extracting unit, thereby determining whether or not the original watermarks are embedded in the wavelet-transformed original image.
[0015] In accordance with another aspect, the present invention provides a digital watermark embedding method based on wavelets in a digital watermark embedding apparatus including a high frequency removing unit, an index information generating unit, a watermark generating unit, and a watermark embedding unit, including the steps of: (a) executing a multi-level wavelet transform at a level corresponding to the size of a data stream of watermarks to be embedded, for an original image, in which the watermarks are to be embedded, and setting a target domain of the wavelet-transformed image, in which the watermarks are to be embedded; (b) removing high-frequency components from a target image corresponding to the set target domain, thereby generating a mirror image corresponding to the target image, but free of high-frequency components; (c) comparing data values of pixels in the target image with data values of pixels in the mirror image, respectively, thereby detecting position information of the pixels having higher high-frequency dependencies in the target image, and arranging the detected pixel position information in the order of pixels having higher high-frequency dependencies, thereby generating an index information about the arranged pixel position information; and (d) embedding the watermarks of the watermark data stream in pixel data of the target image at positions determined based on the index information, respectively.
[0016] In accordance with another aspect, the present invention provides a digital watermark extracting embedding method based on wavelets in a digital watermark extracting apparatus including a high frequency removing unit, an index information generating unit, a watermark generating unit, a watermark extracting unit, and a watermark comparing unit, including the steps of: (a') generating position information about pixels, in which a data stream of original watermarks is to be embedded, based on a target image corresponding to a target domain of a wavelettransformed original image, in which the target watermark data stream is to be embedded; (b') receiving data of pixels in a watermark-embedded domain, in which the original watermark data stream has been embedded; (c') extracting a watermark data stream from the received pixel data, based on the pixel position information; and (d') checking a similarity between the original watermark data stream and the extracted watermark data stream, thereby determining whether or not the original watermarks are embedded in the wavelet-transformed original image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the drawings, in which:
[0018] FIGS, la and lb are concept diagrams respectively illustrating a procedure for setting a target domain, in which watermarks are to be embedded, in accordance with an embodiment of the present invention;
[0019] FIG. 2 is a concept diagram illustrating a procedure for removing high-frequency components from the target domain;
[0020] FIG. 3 is a block diagram illustrating an apparatus for embedding digital watermarks based on wavelets in accordance with an embodiment of the present invention;
[0021] FIG. 4 is a concept diagram illustrating a procedure for embedding watermarks based on wavelets using the watermark embedding apparatus in accordance with an embodiment of the present invention;
[0022] FIG. 5 is a block diagram illustrating an apparatus for extracting digital watermarks based on wavelets in accordance with an embodiment of the present invention; and
[0023] FIG. 6 is a concept diagram illustrating a procedure for extracting watermarks based on wavelets using the digital watermark extracting apparatus in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS
[0024] Now, preferred embodiments of the present invention will be described with reference to the annexed drawings.
[0025] FIGS, la and lb are concept diagrams illustrating a wavelet transform procedure for embedding watermarks in an image in accordance with an embodiment of the present invention. Where it is desired to embed watermarks in a particular image shown in FIG. la for protection of the copyright for the image, it is necessary to decompose the original image into wavelets in order to determine domains, in which a watermark is to be embedded. That is, an n-level wavelet transform should be performed for the original image, as shown in FIG. lb. The level of wavelet transform determines the size of a DC domain, in which a watermark is to be embedded. Accordingly, the wavelet transform level
must be appropriately determined so that it prevents a degradation in picture quality caused by the embedding of the watermark. For example, when a DC domain has the same size as its original image, it can allow a maximum number of watermarks to be embedded therein. Generally, where an n-level wavelet transform for an image having an MxN size is performed, a domain LLn may be determined as a target domain, in which a watermark is to be embedded, as expressed by the following Expression 1:
M N [Expression 1]
size(L£„) = — x —
[0026] The size of the target domain may be determined, taking into consideration the length and embedding strength of a watermark data stream to be embedded, and the level of a degradation in picture quality caused by the embedding of the watermark data stream.
[0027] Where the domain LLn is determined as a target domain, in which a watermark is to be embedded, a procedure for removing high-frequency components from the target domain LLn should be executed. In this procedure, the high-frequency dependency of each pixel in the target domain LLn is checked in order to conduct the embedding of a watermark data stream in the pixels of the target domain LLn in the order of pixels having a higher high-frequency dependency. In accordance with this procedure, it is possible to prevent a degradation in picture quality caused by the embedding of watermarks. In the illustrated embodiment of the present invention, the original image is wavelet-transformed to estimate and detail domains. The watermark data stream is embedded in the estimate domain, which consists of DC components, for a desired robustness of the watermark data. Where the watermark data stream is randomly embedded in the pixels of wavelet-transformed DC component domain, a severe degradation in picture quality may occur. In order to minimize a degradation in picture quality caused by the embedding of watermark data, therefore, the watermark data is preferentially embedded in those, exhibiting a higher high-frequency dependency, of the pixels of the DC component domain, in accordance with the present invention, taking into the consideration the fact that the visual characteristics of humans are more sensitive to a variation in low-frequency components than to a variation in high-frequency components.
[0028] FIG. 2 is a concept diagram illustrating the procedure for removing high-frequency components from the target domain LLn. This high-frequency component removing procedure will now be described, along with a procedure for producing information about high-frequency dependency indicia, with reference to FIG. 2.
[0029] Referring to FIG. 2, a 1-level wavelet transform is additionally executed for the domain LLn determined as a target domain, in which watermarks are to be embedded (Step Si). That is, the image of the target domain LLn is divided into a DC component domain LLn+1 having estimate components, and high-frequency component domains LHn+ I, HLn+1, and HHn+1 each having detail components, as shown by a block 200 in FIG. 2. Subsequently, the components of frequency bands other than that of the DC components are processed to have a value of "0", as shown
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