International Journal of Electronics and Communications (AEÜ)

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1 Int. J. Electron. Commun. (AEÜ) 66 (2012) Contents lists available at SciVerse ScienceDirect International Journal of Electronics and Communications (AEÜ) jou rn al h omepage: A real-time dual watermarking algorithm of H.264/AVC video stream for Video-on-Demand service He Yingliang, Yang Gaobo, Zhu Ningbo College of Computer and Communication, Hunan University, Changsha, China a r t i c l e i n f o Article history: Received 23 June 2010 Accepted 19 August 2011 Keywords: Video watermarking H.264/AVC Video-on-Demand CDMA a b s t r a c t A real-time dual watermarking algorithm of H.264/AVC compressed video is proposed for Video-on- Demand (VOD) service. The copyright information and user information are modulated by CDMA spreading strategies as watermark. At the encoder side, copyright information is embedded into the first non-zero coefficient of Intra 4 4 coded blocks in the luminance components of I frames. An effective error compensation mechanism is simultaneously introduced into the embedding process, which strictly restricts the distortion caused by quantization and watermark embedding within a step size of quantization. At the server side, a XOR based mapping rule is utilized for watermark embedding because the number of non-zero coefficients after quantization is quite less in P frames. Every two bits of the private user information is mapped to three non-zero coefficients of P frames. Experimental results on test sequences demonstrate that the proposed approach has just slight influence on bit rate and PSNR with real-time performance for VOD services, and achieves the dual purposes of copyright protection and pirate tracking Elsevier GmbH. All rights reserved. 1. Introduction Digital watermarking has recently been used as a complementary protection mechanism enabling pirated contents to be tracked back to the authorized user from which they originated [1]. Compared with traditional encryption solution, video watermarking technique can protect the copyright by embedding both ownerspecific and user-specific information (watermark) into the digital media. Since the watermark can only be detected or removed by the originator, it can be used to identify the copyright owner and to track the illegal distributor. In fact, video watermarking techniques can be exploited for different purposes. For example, digital watermarks are typically used in copy control applications to prevent unauthorized copying. In broadcast monitoring application, the watermark can verify the video being broadcast. Digital fingerprints can be embedded into a video to trace back a malicious user. Watermarking based content authentication schemes can judge whether the media content has been altered and even locate the modified positions. In recent years, multi-purpose video watermarking techniques have Corresponding author at: College of Computer and Communication, Hunan University, Lushan Road, Changsha, Hunan , China. Tel.: address: gbyang hunu@hotmail.com (G. Yang). been proposed to integrate two or three above schemes to achieve multiple objectives [2]. Video-on-Demand (VOD) has been widely used to provide an alternative channel to conventional TV. IEEE , the recently proposed wireless broadband access standard (WiMax), has significant support for VOD services [3]. Meanwhile, copyright protection of digital media has become a major concern for content providers who support large scale VOD systems [4]. Copyright owners (e.g., movie studios) do not want to risk their material potentially being pirated after it is digitally transmitted, unless there is reliable content protection such as video watermarking. In the VOD service system, video is distributed from a centralized location like the local central office. A customer can signal the network to select a movie, and the network will start transmission within a reasonable time. Different from conventional video watermarking, copyright protection for VOD service should meet others specific requirements. First, real-time performance is very important. Since VOD servers usually use compression techniques to reduce the storage and bandwidth requirements, it is impractical to firstly decode the video bit-stream, and then embed a watermark, and finally re-encode it. An alternative approach is to embed the watermark directly in compressed domain. Second, for different users, the watermarked video should be different so as to trace back a malicious user. Third, the influence of the bit rate changes caused by watermarking during the transmission of video streams should be considered since video is usually transmitted to users via networks with varying bandwidth /$ see front matter 2011 Elsevier GmbH. All rights reserved. doi: /j.aeue

2 306 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) Fig. 1. System overview of the proposed approach. To the best of our knowledge, although the idea of compresseddomain video watermarking [5] is not new, video watermarking schemes specifically designed for VOD services, are few publicly reported. Wang [6] et al. proposed a real-time video watermarking scheme based on a parallel MPEG-2 engine. It embeds subscriber-specific watermark signature to the 8 8 DCT blocks of compressed video stream. Its main contribution lies in the buffer sharing mechanism with a dynamic allocation algorithm to significantly reduce the memory demand within this parallel watermarking software. The watermark algorithm is robust against various temporal attacks, and can provide satisfactory performance for large scale video distribution networks. However, it cannot realize blind detection and the demand for buffer size is great. The latest video coding standard H.264/AVC is expected to be more widely used in the future instead of MPEG-2 because of its higher compression efficiency. However, new features such as variable block size motion estimation/compensation, directional spatial prediction in intra-coded frame and context-adaptive entropy coding have been adopted in H.264/AVC. These features bring new challenges because of less redundancy existed in video stream compressed with H.264/AVC. Zhang et al. [7] proposed a robust video watermarking scheme for H.264/AVC, which can survive transcoding process and strong common signal processing attacks. Maneli et al. [8] proposed a framework for robust watermarking of H.264-encoded video with controllable detection performance. The Watson visual model is exploited for a 4 4 DCT block to obtain a larger payload and a greater robustness while minimizing visual distortion. Spyridon et al. [9] proposed a new method for data hiding in H.264/AVC streams, which exploits the IPCM encoded macroblocks during the intra prediction stage in order to hide the desired data. However, all these approaches are not proposed for VOD services and cannot achieve pirate tracking. Therefore, it is worthy of investigation to devise a new video watermarking scheme for H.264/AVC based VOD services. Dual video watermarking, which embeds two different watermarks to resist different types of attacks and complement each other, is suitable for VOD services. In this paper, a real-time dual video watermarking algorithm is proposed for H.264/AVC bitstream in VOD streaming application. Copyright information as well as private user information is modulated as watermark by code division multiple access (CDMA) technique because of its excellent performance in spread spectrum. At the encoder side, copyright information is embedded into the first non-zero coefficient of the Intra 4 4 coded blocks in the luminance component of I frames. To decrease the visual distortion caused by quantization and watermarking, an effective error compensation mechanism is introduced in the embedding process. At the side of sever, a XOR based mapping rule is exploited for watermark embedding in P frames. Every two bits of the private user information is mapped to three non-zero coefficients of P frames. The proposed approach can accomplish the dual purposes of copyright protection and pirate tracking for VOD services. The rest paper is organized as follows: Section 2 briefly describes the block diagram of VOD system with real-time watermarking embedding. In Section 3, the proposed dual watermarking is discussed in detail. Experimental results are illustrated in Section 4. Section 5 concludes this paper. 2. System overview Our watermark embedding algorithm is intended to use at the local video distributor who directly delivers video to the end users (subscribers). In such systems, a common video content must be watermarked to distinguish between different versions being delivered to many subscribers [5]. The system framework of the proposed approach is illustrated in Fig. 1. In WiMax (or other access networks), video is transported from a content storage facility to the head end, which further distributes the content to subscribers. In this architecture, video stream is watermarked during the encoder process with copyright information in digitalize producer studio, and then is stored in local storage stations. At the head end, the video stream is embedded with watermark signature for a specific subscriber. Since the input to head end from local storage station is compressed video, the user-specific watermark information is embedded directly in compressed domain, without fully decoding the input compressed video and re-encoding it for watermark signature embedding. Finally, the video with dual watermarks is sent out to different subscribers via WiMax downlink. Since different video streams that contain the same copyright information and unique subscribe information as watermark are provided for different users, the source of illegal copies as legal evidence can be identified by the watermark extraction and verification process when needed. For the H.264/AVC video coding standard, its high compression efficiency presents a major challenge for any video watermarking approach specifically designed for it. For example, unlike previous coding standards MPEG-2 and MPEG-4, the residual macroblocks in H.264/AVC intra coded frames are obtained by spatial prediction within a frame. Therefore, the intra coded frames consist mainly of residual data. After quantization, the majority of these residual

3 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) BER of extracted watermark P frame I frame PSNR of video frames P frame I frame BER (%) N PSNR (db) N Fig. 2. The impact of length of the spread spectrum sequences N for Stefan sequence attacked by re-encoding (Left: the bit error rate of the extracted watermark; right: the average PSNR value between the original video frames and the watermarked ones). coefficients are near to zero, which makes it extremely difficult to embed watermark information without degrading the video quality and increasing the bit rate. In the above system, to meet with the copyright protection requirements of VOD service, the dual watermarking algorithm is specifically devised by taking the characteristics of H.264/AVC into consideration. For the copyright watermark, it is directly embedded into compressed video stream before the satellite uplink, which greatly reduces the computational complexity. Moreover, the coded block pattern (CBP) parameter is utilized to avoid watermark being embedded into those blocks with all zero coefficients, so as to guarantee only slight change of bitrate. For the user-specific watermarking, unique user information for each subscriber is directly embedded in compressed domain to achieve real-time performance, and XOR based mapping rule is exploited to lower the burden of bandwidth. 3. Proposed watermark schemes 3.1. Watermark pre-processing by CDMA The appeal of spread spectrum in watermarking is understandable. Digital video, by virtue of its time-space property, fits the direct sequence spread spectrum more readily. CDMA is an application of direct sequence spread spectrum. The application of CDMA to watermarking seems obvious. The goal is to use near-orthogonal watermarks. The advantages of CDMA lies in that it combines multiple signals that overlap in both time and frequency yet remain separable. The separability is achieved by first projecting individual messages onto near-orthogonal PN sequences prior to carrier modulation. Owing to the near-orthogonal sequences of CDMA spread spectrum strategy, the robustness and imperceptibility of the watermarking can be strengthened simultaneously [10]. To improve the robustness of dual-watermarking system, both copyright information and user-specific information are modulated by CDMA technique as watermark before embedding. Assuming the dual-watermark modulated by BPSK is arranged as the matrix form B = [b ij ] 1 i N1,1 j M, and the generated Gold sequences set can be denoted as G = {g k (n)} 1 k M,1 n N, where g k (n) {+1, 1} and M,N represent the number and length of the spread spectrum sequences, respectively. We obtain the matrix of information encoded by CDMA C = [C ij ] 1 i N1,1 j N as follows C = B GZ M i.e. C ij = b ik g k (j) k=1 Since C is an integral matrix, it must be translated into the binary representation before watermark embedding. The actual watermark after binary representation is supposed to be W. At the receiver, Ŵ is the estimated watermark extracted from W, which can be restructured as Ĉ = [Ĉ ij ] 1 i N1,1 j N. In the rest of this section, we will discuss the details of the decoding procedure of CDMA and its affection to robustness separately. (1) Without errors occurring in the extraction process With this assumption, we can get Ĉ = [Ĉ ij ] 1 i N1,1 j N = [C ij ] 1 i N1,1 j N = C, and can utilize the correlation detection to reconstruct the original watermark. N O ˆB = Ĉ G T = C G T = B G G T = B... (2) O N M M where ˆB = [ˆb ij ] 1 i N1,1 j M. Formula (2) is directly obtained from the orthogonal property of the Gold sequences. Obviously, the original watermark can be extracted as follows: if ˆb ij > 0, i.e. b ij > 0, then ŵ ij = w ij = 0; otherwise ŵ ij = w ij = 1. So, the original watermark can be completely recovered. (2) With some errors in the extraction process Without loss of generality, it is often regarded that white Gaussian noise exists in the extraction process for ŵ ij = w ij = 1, that is Ĉ ij = C ij + n, where n is a Gaussian random variable with zero mean and variance 2, i.e. n N(0, 2 ). So the bit error ratio (BER) of watermark after correlation detection is: P CDMA e = P(ˆb ij < 0 b ij = +1) P(b ij = +1) + P(ˆb ij > 0 b ij (1)

4 308 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) = 1) P(b ij = 1) (3) For the sake of simplicity, supposing P(b ij = +1) = P(b ij = 1) = 1/2, then Eq. (3) can be simplified as: P CDMA e = 1 2 [P(ˆb ij < 0 b ij = +1) + P(ˆb ij > 0 b ij = 1)] (4) Considering the relationship between ˆB = [ˆb ij ] 1 i N1,1 j M and Ĉ = [Ĉ ij ] 1 i N1,1 j N, we can draw the conclusion that ˆb ij N( N b ij, 2 ). When b ij = +1, ˆb ij N( N, 2 ), otherwise, ˆb ij N( N, 2 ). Then, Eq. (4) can be rewritten as: [ Pe CDMA = e (x 2 N) 2 2 dx e (x+ 2 N) 2 2 dx ] = Q ( N ) where Q (x) = (1/ + 2) e (t2 /2) dt, Q(x) is monotonically x decreasing because Q (x) = (e (x2 /2) / 2) < 0. On the other hand, the BER of watermark information without pre-processing by CDMA is P e = Q (1/ 2). Apparently, Pe CDMA is smaller than P e, which means that the introduction of CDMA can strengthen the robustness of watermarking algorithm. Moreover, the larger the N, the smaller the Pe CDMA. Nevertheless, the video quality will become worse and PSNR will be lower along with the increasing of N. Therefore, it is important to choose an appropriate N. Fig. 2 illustrates the influence of N to the robustness and imperceptibility of the watermarking. In the following, N is selected to 16 by experiment, which is compatible with the result of Fig Watermark embedding process of copyright information The Integer Transform (IT), a derivative of the 4 4 DCT, has been adopted in H.264/AVC standard for coding image blocks in residual data. The low-frequency coefficients of transformed residue are of high energy and their relative sizes are stable. Therefore, the low-frequency coefficients are utilized for watermark embedding. The coded block pattern (CBP) parameter in the H.264/AVC coder specifies which 8 8 block has non-zero coefficients. If the CBP parameter of a block is zero, it represents that all its DCT coefficients are zero. Therefore, CBP can be utilized to avoid watermark embedded into those blocks with all zero coefficients, and thus the change of bitrate by watermark embedding can be guaranteed to be minor. Copyright information is embedded into the coefficients of I frame after quantization. Let I L be the luminance component of I frame after quantization and len be the bits in the watermark to be embedded, the embedding process is as follows. (5) Fig. 3. Raster-scan order. If appropriate T is determined, these I4 blocks can be represented with a set V, V = {I4 1, I4 2, I4 3 I4 num } Step 2: Compute the quantization errors H.264/AVC adopts some new coding features to decrease the visual distortion, which is mainly caused by the rounding operation of quantization. Let F be the transform coefficients and FQ be the coefficients after quantization, the forward quantization operation is represented as: ( F ) FQ = round QP where QP is quantization step size and round(*) is the rounding operation. The inverse quantization is represented as follows: F = FQ QP (7) where F is the recovered coefficient by inverse quantization. Apparently, the error E caused by quantization can be defined as: E = F F (8) Watermark embedding may increase this kind of error because it usually modifies the DCT coefficients, which will further degrade the visual quality. Therefore, an error compensation mechanism is essential to decrease the errors caused by quantization and watermark embedding. During watermark embedding, an error function is defined for the first non-zero coefficient in the ith I4 block as follows: E(I4 i (1)) = abs(f(i4 i (1))) abs(f (I4 i (1))) (1 i num) (9) It is generally believed that the smaller is abs(e), the less it degrades the visual quality. (6) Step 1: finding the most appropriate T A statistics is made to those I4 blocks if they satisfy the following two requirements: First, its CBP does not equal zero. Second, the first non-zero coefficient is larger than T. The scanning order of DCT coefficients is illustrated in Fig. 3. Supposing num is the number of these I4 block. Obviously, the size of T can be used to adjust the embedding capacity, which should be adaptively determined. For different video sequences, it is determined in an iterative way on the basis of a pre-defined initial value by experience. The iterative process is defined as follows: (1) if num len then T = T 1; (2) Re-count num; go to (1) until num > len Step 3: watermark embedding and error compensation Most of the elements I4 i (1) in V are the DC coefficients. It is generally believed that they are more preferable to be embedded with watermark than AC components. However, if watermark is only embedded into DC components, it cannot resist collusion attack. In this paper, the DC coefficients as well as the low frequency AC coefficients are utilized for watermark embedding. Moreover, their absolute values are relatively large, which guarantees the relative changes of these coefficients by watermark embedding will be small. If the watermark bit to be embedded equals 1, I4 i (1) is modulated to be even according to the error compensation mechanism. Similarly, if the watermark bit be 1, I4 i (1) is modulated to be odd. Let sign(*) be the sign function and I4 i (1) w be the

5 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) Fig. 4. The visual impact before and after watermarking. watermarked I4 i (1), the process of watermark embedding is illustrated as follows: embedding. H.264/AVC allows tree-structured partitioning motion estimation with 1/4 pixel accuracy for luminance components to if W1 = 1 abs(i4 i (1)) + 1, if I4 i (1) mod2 = 1&E(I4 i (1)) 0 I4 i (1) w abs(i4 = sign(i4 i (1)) i (1)) + 1, if I4 i (1) mod2 = 1&abs(I4 i (1)) = T + 1&E(I4 i (1)) < 0 abs(i4 i (1)) 1, if I4 i (1) mod2 = 1&abs(I4 i (1)) > T + 1&E(I4 i (1)) < 0 abs(i4 i (1)), else if W1 = 1 abs(i4 i (1)) + 1, if I4 i (1) mod2 = 0&E(I4 i (1)) 0 I4 i (1) w abs(i4 = sign(i4 i (1)) i (1)) + 1, if I4 i (1) mod2 = 0&abs(I4 i (1)) = T + 1&E(I4 i (1)) < 0 abs(i4 i (1)) 1, if I4 i (1) mod2 = 0&abs(I4 i (1)) > T + 1&E(I4 i (1)) < 0 abs(i4 i (1)), else If abs(f(i4 i (1))) is larger than abs(f (I4 i (1))), it reflects that the decimal part which is less than 0.5 is omitted. In this instance, if it needs to be modified because of watermark embedding, subtracting one from abs(i4 i (1)) will further enlarge the visual distortion. Actually, the copyright information embedding algorithm adds 1 to abs(i4 i (1)) to compensate this kind of errors and makes abs(f(i4 i (1))) less than abs(f (I4 i (1))). Though there are still errors in E(I4 i (1)), the absolute value of error is very small. The change of original data is less than a step size of quantization, and it is about one half of step size under ideal circumstances. On the contrary, when abs(f(i4 i (1))) is less than abs(f (I4 i (1))), abs(i4 i (1)) should decrease one as abs(i4 i (1)) > T + 1 and augment one as abs(i4 i (1)) > T + 1, which guarantees the first non-zero coefficient is larger than T. Therefore, the quantization error can be well compensated during the process of watermark embedding Watermark embedding process for user-specific information To protect the copyright and trace the origin of piracy, different user-specific watermark information should be embedded for different subscriber. At the server of VOD, the first P frame of every GOP in the H.264/AVC stream is selected for watermark achieve higher compression efficiency, which makes the non-zero coefficients of residue rather less. To decrease the change of bitrate as less as possible, watermark is usually embedded into the nonzero coefficients. However, the lack of the non-zero coefficients will influence the watermark capacity ultimately. To solve this problem, matrix coding is exploited to map two bits of watermark to every three non-zero coefficients and only one coefficient is necessary to be modified trivially. Supposing every authorized user is assigned with a legal ID, such as the number of his/her personal identification card. Obviously, this kind of ID can be distinguished from each other, and further be used as watermark to trace the illegal distribution. The watermark embedding process details as follows: Step 1: selection of candidate block for watermark embedding Supposing P L be the luminance component of the first P frame in every GOP, the number of I4 blocks after integer transform will be W P H P /16 if P L is of size W P H P. Let P4 i (1 i W P H P /16) be an I4 block, the candidate blocks for watermark embedding should meet the following two requirements simultaneously: (1) its CBP does not equal 0; (2) there are at least three non-zero DCT coefficients.

6 310 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) x bit/frame psnr (db) without watermark 1.37 watermarking with compensation watermarking without compensation 1.36 watermarking algorithm in [5] frame (a) bitrate change of I frame without watermark watermarking with compensation watermarking without compensation watermarking algorithm in [5] frame (b) PSNR change of I frame 2 x bit/frame without watermark 0.8 with watermark frame (c) bitrate change of P frame psnr (db) without watermark with watermark frame (d) PSNR change of P frame Fig. 5. The bitrate and PSNR before and after watermarking Step 2: map rule for watermark embedding Every two bits of watermark is embedded into a candidate I4 block. Let P4 i (1), P4 i (2) and P4 i (3) be the first three non-zero coefficients as the scan order illustrated in Fig. 2, and a, b and c be the remainder of them when they are divided by 2, respectively. The mapping rule is defined as follows: if W2(i) = 00 (i = 1, 2,, length(w2)/2) 3.4. Watermark extraction process The watermark extraction process corresponds to the dual of the embedding process. The watermarked H.264/AVC stream is partially decoded to obtain the DCT coefficients of I and P frames after quantization. For each I frame, the value of T is first determined as it does in watermark embedding. If the first non-zero coefficient of I4 block is larger than T, this block can be assured to have watermark ( W2 ) if W2(i) = 00 (i = 1, 2,, length 2 P4 i (1) w = P4 i (1) P4 i (2) w = P4 i (2) P4 i (3) w = P4 i (3) if a b = 0 b c = 0 P4 i (1) w = P4 i (1) P4 i (2) w = P4 i (2) P4 i (3) w = P4 i (3) ± 1 if a b = 0 b c = 1 P4 i (1) w = P4 i (1) ± 1 P4 i (2) w = P4 i (2) P4 i (3) w = P4 i (3) if a b = 1 b c = 0 P4 i (1) w = P4 i (1) P4 i (2) w = P4 i (2) ± 1 P4 i (3) w = P4 i (3) if a b = 1 b c = 1 (10) where is logical XOR operation, and length(w2) denotes the length of the watermark W2. It should accord to the sign of P4 i (l)(l = 1,2,3) to select ±, which ensures the change of the video to be minor and does meet the requirement of imperceptibility. It is done in the similar way for the rest three occasions; therein the watermark bits to be embedded are 01, 10 or 11. Apparently, there are no computation-intensive operations in the above watermark embedding process. Furthermore, only partial decoding of input video stream is needed. Therefore, the requirement for buffer size is low and it can meet the real-time performance. in it. Let I4 i (1) w be the first non-zero coefficients of I4 block, the copyright information detection process describes as follows: if I4 i (1) w > T { W1 = 0 if I4 i (1) w mod 2 = 0 W1 = 1 else (11) For the first P frame in every GOP, if there are at least three nonzero coefficients in P4 block, it indicates that there are two bits of user-specific information mapped into this block. Let P4 i (1) w,

7 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) Table 1 The comparison of bitrate and PSNR for various video sequences. 0 (I) 1 (P) 8 (I) 9 (P) 12 (I) 13 (P) Container Original frame PSNR BIT Watermarked frame PSNR BIT Stefan Original frame PSNR BIT Watermarked frame PSNR BIT Bus Original frame PSNR BIT Watermarked frame PSNR BIT P4 i (2) w and P4 i (3) w be the first three non-zero coefficients, a, b and c be the remainder of them divided by 2 respectively. The userspecific information detection process is as follows: Let a = P4 i (1) w mod 2, b = P4 i (2) w mod 2 and c = P4 i (3) w mod 2 W2 = 00 if a b = 0&b c = 0 W2 = 01 if a b = 0&b c = 1 (12) W2 = 10 if a b = 1&b c = 0 W2 = 11 if a b = 1&b c = 1 As discussed above, the extraction process does not require the participation of the original video stream and can be accomplished blindly. 4. Experimental results and discussion In order to verify the performance of the proposed approach, it has been tested on several video sequences with different motion complexity, such as Paris, Foreman, Stefan and Bus (in CIF format, 300 frames, GOP = IPPP, QP = 28). Foreman sequence is of moderate motion complexity and Paris is of nearly still background and small foreground motions, whereas Stefan and Bus sequences represent the acutely moving sequences. The original video streams are obtained by the H.264/AVC reference software version JM10.0, and the watermarking algorithm is integrated with the H.264/AVC coder Analysis of visual quality, bitrate and PSNR after watermark embedding To confirm the imperceptibility of the watermark embedding algorithm, we randomly extract some frames and compare the video frames before and after watermarking. Fig. 4 illustrates the experimental results for Paris sequence. Among them, (a) and (b) are the original I frame and P frame without watermark, and they denote the 21st and 50th frame of original sequence respectively. (c) and (d) are the corresponding watermarked frame of (a) and (b). There is no obvious difference of visual quality between the original frame and watermarked one, which mainly benefits from the mechanism of quantization error compensation for the copyright watermarking in I frame, and matrix coding based XOR embedding for the user-specific watermarking in P frame. The bitrate and PSNR of the former 60 watermarked frames of Paris sequence are given in Fig. 5. The same results of [6] are also used for comparison. From (a) and (c), it can be concluded that there are only very slight bitrate increase after watermark embedding. When quantization error compensation is used, it is just about 0.09% percent of bitrate increase for I frame, which is smaller than the work in [6]. For P frame, it is only about 0.15% percent of bitrate increase. The PSNR degradations for I and P frames by watermark embedding are about db and 0.07 db respectively, as illustrated in (b) and (d). Especially, the PSNR improves about 0.14 db compared with the work in [6], and the mechanism of quantization error compensation can improve the PSNR about 0.05 db on average. Table 1 describes the PSNR and bitrate for other three video sequences. For all of these sequences, the dual-watermark information is only embedded into six frames as listed in the first row of this table. Obviously, the average PSNR of these I frames degrades less than 0.1 db and the increase of bitrate is less than 0.2% on average. In addition, for the P frames, the average changes of PSNR and bitrate are also so trivial that it can guarantee the visual quality after watermark embedding. Therefore, our dual-watermarking algorithm is of high transparency Robustness analysis To further evaluate the performance of the proposed approach, the similarity between the original and extracted watermarks and BER (Bit Error Rate) of watermark are computed as follows: M N i=1 j=1 Sim = I(i, j) Î(i, j) M (13) N M N i=1 j=1 I2 (i, j) i=1 j=1î2 (i, j) M N i=1 j=1 BER = I(i, j) Î(i, j) 100% (14) M N where I and Î are the original watermark and the extracted watermark, respectively, and M, N are the size of watermark information. The higher the Sim is, or the less the BER is, the more the robust of the proposed approach is. When Sim = 1 or BER = 0, it reflects that there is no difference between the original watermark and the extracted one. In real-time video watermarking applications, we should consider the re-encoding attack during transmission and other kinds

8 312 Y. He et al. / Int. J. Electron. Commun. (AEÜ) 66 (2012) Table 2 The similarity of original watermark and extracted watermark. (Sim BER(%)) Foreman Stefan Bus I frame P frame I frame P frame I frame P frame No attacks (1 0) (1 0) (1 0) (1 0) (1 0) (1 0) Random noise 2 = 2 ( ) (1 0) ( ) (1 0) ( ) (1 0) 2 = 4 ( ) ( ) ( ) ( ) ( ) (1 0) Filtering Median ( ) (1 0) ( ) ( ) ( ) (1 0) Average ( ) ( ) ( ) ( ) ( ) ( ) Re-encoding R = 4 Mbps ( ) ( ) ( ) ( ) ( ) ( ) R = 2 Mbps ( ) ( ) ( ) ( ) ( ) ( ) Collusion attack ( ) ( ) ( ) ( ) ( ) ( ) Package dropping ratio = 5% ( ) ( ) ( ) ( ) ( ) ( ) ratio = 10% ( ) ( ) ( ) ( ) ( ) ( ) of attacks such as frame dropping, frame re-ordering, etc. To verify the robustness of the proposed approach, performance tests are conducted for the most common signal processing and reencoding attacks in video streaming. All of the experimental results are summarized in Table 2. For attacks such as adding random noise and filtering, Sim is larger than 0.92, and BER is less than 8.9%, which is owing to the introduction of CDMA for pre-processing. For the re-encoding attack with different constant bit rates, Sim is larger than 0.78, which is also acceptable for both copyright protection and pirate tracking. Since watermark information is repeatedly embedded into I frame and the first P frame of every GOP, the watermark information in multiple GOP can be gathered to rebuild watermark. Moreover, it is quite flexible to select the watermark embedding position for both I and P frames. Since user-specific watermark information is embedded into different positions of P frame, it makes the proposed approach robust to collusion attack and frame dropping, which is also verified in Table Complexity analyses From the above discussions, it is easily gained that the dual watermark embedding processes can be accomplished with simple operations. Actually, for the copyright watermark embedding, it only needs the manipulation of module-2 and comparison, and merely module-2 for the embedding of user-specific watermark. Furthermore, the input video stream does not need to be completely decoded for watermark embedding, which can meet with the real-time requirement of VOD systems. Assuming the length of embedding watermark bit is L, in the worst-case complexity, analysis can be submitted to O-notation, which refers to asymptotic upper bound of process. Based on this idea, we derive the complexity of the proposed embedding approaches is O(L), which indicates the complexity is linear with the length of the watermark bits. 6. Conclusions and future work In this paper, a dual watermarking algorithm is proposed for VOD services. Since the proposed approach does not need the full decoding of input H.264/AVC video stream, it is of low computational complexity and can meet the requirement of real time performance for VOD applications. In recent years, peer-to-peer (P2P) networks offer a tempting solution to the scalability problems of VOD streaming. A P2P VOD service is more challenging to design than a P2P live streaming system because, in addition to providing low start up delays, the system should also allow users arriving at arbitrary times to watch the video. The lack of synchronization among users reduces the block sharing opportunities and increases the complexity of the block transmission algorithms. Future research will focus on the watermarking of VOD service on P2P networks. Acknowledgements This work is supported in part by National Natural Science Foundation of China under grant , Key Project of Hunan Provincial Natural Science Foundation under grant 11JJ2053, and Special Pro-phase Project on National Basic Research Program of China under grant 2010CB References [1] Doërr G, Dugelay JL. A guide tour of video watermarking. 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