ICCVG 2002 Zakopane, 25-29 Sept. 2002 Roger ŚWIERCZYŃSKI Institute of Electronics and Telecommunication Poznań University of Technology roger@et.put.poznan.pl FRAGILE WATERMARKING USING SUBBAND CODING Abstract The paper deals with a new technique of fragile image watermarking. Proposed technique exploits inter-subband dependencies in order to minimize the influence of inserted watermark on the host image quality. Using the unique watermark based on the low-frequency subband, proposed scheme is resistant against attacks based on series of images analysis. Tampering detection ability of the proposed technique has been examined for a several types of image manipulation. keywords : fragile watermarking, subband coding, tamper proofing 1 INTRODUCTION Digital watermarking is an adaptation of the watermark security technique well known from the paper documents, bills and money. As in the paper world, this technique consist of placing some additional information to the original data in a way, that in normal conditions changes of data are imperceptible. Only in specific conditions by using the specialized software the embedded watermark can be visualized. Good example of using embedding scheme comes from Divx system. Each player of that system embedded a unique watermark that supported transaction tracking intended to deter piracy. [Cox02] Second area of interests is data authentication which has applicability in law, commerce and journalism. Some examples are [Lin99]: watermarking of databases resources to detect any tampering trials; ensuring news agencies that presented material is not fabricated or edited to falsify events (so called trustworthy camera ); courtroom evidence (for example shields number photo authentication). Last but not least, watermarking has been used not only for the security purposes, like authenticity verification or intellectual rights proof but also for such far from original concept solutions like error detection. [Chen01] and [Robi02] reported that by embedding 1/6 02-09-30
R. Świerczyński a watermark on the quantized DCT coefficients and examining its integrity on the decoder side, the error detection capability of video decoders is significantly increased compared to widely used syntax-based error detection schemes. Depending on the type of application, the watermark has to be robust or fragile. When watermark is resist to manipulations of the data (for example lossy compression) it is called robust and well suited for the copyright protection applications. [Liu02, Srik01] On the other hand - when authenticity of given data is a prime question the watermark should act in an opposite way. Any manipulation of original data should destroy watermarked signal. Such watermark is called fragile and is expected to have ability to show not only that the tampering process has occurred, but also where. There exist also so called semi-fragile watermarking techniques, where some manipulations are allowed (for example JPEG compression to a predefined quality factor [Delp00, Lin00]) but other data manipulations are detected as tampering. A good overview of all areas of digital watermarking is presented in [Barn00, Peti99,Voya00]. 2 PROPOSED TECHNIQUE 2.1 Subband coding using inter-subband dependencies Subband coding (SBC) is a well established technique used in a variety of applications. The basic idea of SBC is to split up the two-dimensional frequency band of an image into subsampled channels. It is well known that most of the input image energy is concentrated in the low-frequency band ( subband). The high-frequency bands exhibit low energy and mostly small values associated with pixels in corresponding sub-images. Therefore it is a very good place to embed a watermark its influence on image quality is relatively small. Moreover since almost all nowadays applications use mentioned above properties for compression by removing the higher frequency information the higher subbands are ideally suited for inserting a fragile watermark. It is also well known that there exist a some kind of dependency between subbands. Usually the high energy pixels in higher subbands are present in areas where in subband rapid changes occur. In some previous works [Dom94, Świe97] we have presented that affecting these pixels may introduce a visible artifacts in decoded image. Thus we use a simple method of detecting such active pixels which can be omitted in watermarking process. We define a pixel at position (x,y) to be active if: max ( I ( x, y) I ( x, y 1), I ( x, y) I ( x 1, y), I ( x, y) I ( x 1, y 1) ) T > where I (, ) is the intensity of a pixel in a low-frequency subimage at given position and T is a predefined threshold. Figure 1 shows a quality of reconstructed, watermarked image for a given threshold for boats and clown static images and a frame from flower garden sequence. See also figure 2 showing percentage of changed i.e. watermarked points in HH subimage versus the threshold for the same images. From the two figures one can see that threshold values in range 60-80 give over 90% percent of watermarked pixels and quality over 48 db. Figure 3 shows the active point of the test image flower garden and T=75. One can see that the edges are well protected against changing.
Fragile watermarking using subband coding 100 Watermarked [%] 90 80 70 60 50 Flower Boats Clown 40 30 52 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 175 Threshold PSNR [db] Fig. 1 Percentage of watermarked points versus threshold T 51,5 51 50,5 50 49,5 Flower Boats Clown 49 48,5 48 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 175 Threshold Fig. 2 Quality of watermarked image versus the threshold T Fig. 3 Active pixels for flower garden and T=75 A method based on activity considerations is also presented in [Yama01], but the authors have embedded watermark into subband, obtaining robust watermarking scheme. Another way of choosing pixels for embedding (in this case randomly) in the higher subbands is presented in [Kund99].
R. Świerczyński 2.2 Watermarking The proposed method consist of the following steps. First the host image is divided into four subbands, using perfect reconstruction Haar filter bank. These filters are usually not used in subband coding area, since they produce a strong aliasing effect in case of compression. Since in this solution compression is not considered, the high frequency information is preserved, and as a result aliasing is not present. Next the activity of the pixels in the subband is examined, as described in previous section. The non-active pixels in higher subband are watermarked. In experiments we have used a simple LSB changing method, i.e. each LSB of the non-active pixel in higher subband takes an LSB value of the subband pixel located on the same position. Since LSB plane of the subband is different for each image, this proposed scheme is resistant against attacks based on series of images analysis. By omitting some pixels in embedding process we do not provide tampering detection ability for whole image. But looking at picture 3, one can see that the non-protected pixels do not gather in any larger objects, since any practical manipulation will still be detected. An example is shown on figure 4. The original frame from test sequence has been manipulated by: erasing one of the person; copying the second person to different position; adding falsified date of creation. Points detected as changed are shown in figure 5 (left). These points are nonlinear filtered to obtain more compact areas and superimposed into host image (fig. 5 right). All manipulations has been detected. Fig. 4 Original (left) and tampered (right) frame from flower garden sequence
Fragile watermarking using subband coding Fig. 5 Tampered points (left) and tampering detection result (right) 3 CONCLUSIONS This paper presents a new method of fragile watermarking of images. Proposed method is based on subband technique. It concerns the subbands properties in such way, that errors introduced by watermarking process are minimized. It is also show by example, that proposed method has a good ability of detecting which areas of original image has been tampered. By using unique watermark this technique is also resistant into series of images analysis attack. REFERENCES [Barn00] M. Barni, F. Bartolini, A. Piva, Digital watermarking of visual data: state of the art and new trends, Proceedings of the Eusipco, Tampere 2000, pp. 1657-1664 [Chen01] M. Chen, Y. He, R. L. Lagendijk, Error detection by fragile watermarking, Proceedings of the Picture Coding Symposium 2001, Seoul, Korea, pp. 287-290; [Cox02] I. Cox, M. Miller, The first 50 years of electronic watermarking, Eurasip Journal on Applied Signal Processing, vol 2002, no. 2, Feb. 2002, pp. 126-132; [Delp00] E. Lin, Ch. Podlichuk, E. Delp, Detection of image alternations using semi-fragile watermarks, Proceedings of the SPIE Int. Conf. on Security and Watermarking of Multimedia Content II, vol. 3971, San Jose, Jan 2000; [Dom94] M. Domański, R. Świerczyński, Subband coding of images using hierarchical quantization, Proceedings of the Eusipco 1994, Signal Processing VII: Theories and Applications, 1994, pp. 1218 1221; [Kund99] D. Kundur, D. Hatzinakos, Digital watermarking for telltale tamper proofing and authentication, Proceedings of the IEEE, Special Issue on Protection of Multimedia Content, vol. 87, no. 7, July 1999, pp. 1167-1180;
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