Detection of Double Compression with the Same Bit rate in MPEG-2 Videos ... 1 School of Information Science and Technology, Sun Yat-Sen University, ...
Detection of Double Compression with the Same Bit rate in MPEG-2 Videos Zhensheng Huang1, Fangjun Huang1, Jiwu Huang2 1
School of Information Science and Technology, Sun Yat-Sen University, Guangdong 510006, China 2 College of Information Engineering, Shenzhen University, Shenzhen 518060, China ABSTRACT
an evidence of double compression, and a new method was proposed. In [4], Sun et al. further improved the detection accuracy rates of Chen and Shi's method with considering first digit distributions of the intra-coded frames of MPEG-2 video. All of the above method can only detect the double MPEG-2 compression when the primary compression and the second compression have different bit rates. To the best of our knowledge, no method has been reported for detecting double MPEG-2 compression with the same bit rate until now. In this paper, we present a new method for detecting double MPEG-2 compression with the same bit rate which is inspired by our previous work on double JPEG compression [5]. Our new method is based on the observation that the number of different coefficients between the I frames of the singly and doubly compressed MPEG-2 videos is much larger than the number of different coefficients between the I frames of the corresponding doubly and triply compressed MPEG-2 videos. Based on this observation, a novel recompressing strategy is presented and it can detect double MPEG-2 compression effectively. The rest of this paper is organized as follows. Section 2 gives a brief review of the quantization and de-quantization process in MPEG-2 coding system. In Section 3, our new detection algorithm is presented. The experimental results and discussions are given in Section 4 and the conclusion is drawn in Section 5.
The detection of double compression with the same bit rate in MPEG-2 is of great significance in the field of video forensics. However, when the singly compressed and the doubly compressed videos have the same bit rate, no detecting method has been reported yet. In this paper, we propose a new method that can detect double MPEG-2 compression with the same bit rate. Our method is based on the observation that the number of different coefficients between I frames of the singly and doubly compressed MPEG-2 videos is much larger than the number of different coefficients between the I frames of the corresponding doubly and triply compressed MPEG-2 videos. Via a new recompressing strategy, the aforementioned characteristics of MPEG-2 video can be utilized to discriminate the singly compressed and doubly compressed MPEG-2 videos. Various experiments on different bit rates demonstrate the efficiency of our new method. Index Terms— Digital forensic, double compression, MPEG-2, DCT coefficients, bit rate. 1. INTRODUCTION Nowadays, the usage of surveillance systems and video cameras is growing rapidly. However, modifying a digital video without any obvious trace is easy to be accomplished via using today's video editing software such as Adobe Premiere Pro 2.0 [1]. Detection of double MPEG-2 compression is of great significance because of the following two reasons. Firstly, MPEG-2 is still one of the most popular video formats on the internet nowadays. Secondly, the doubly compressed MPEG-2 videos often result from video tampering. Previously, some methods for detecting double MPEG-2 compression were proposed. Via examining the periodic artifacts in discrete cosine transform (DCT) histograms of I frames, Wang and Farid [2] presented a detection method for double MPEG-2 compression firstly. In [3], Chen and Shi pointed out that the disturbance in the probability distribution of the first digits of non-zeros quantized alternating current (AC) coefficients could be considered as
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2. MPEG-2 QUANTIZATION AND DE-QUANTIZATION Although the de-quantization process and method in the MPEG-2 coding systems are deterministic, which must strictly comply with the MPEG-2 standard, the encoder is free to perform quantization process in any way. In the following section, the quantization and de-quantization process of intra mode belonging to Test Model 5 of MPEG-2 [6] will be introduced. In the module of quantization process of TM5, each block is transformed with 2-dimensional DCT transform first, and then the DCT coefficients are quantized respectively according to the mode of current macroblock. In the intra mode, the quantizer of DC coefficients can only select a fixed step based on precision parameter that will be transmitted in
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the picture coding extension. The step size is modified by two mechanisms, i. e., the weighting matrix wI (i, j ) and the
Suppose one I frame of MPEG-2 video is denoted by In (n=1, 2), where n represents the number of times that the video has been compressed with the same bit rate. For example, I1 means that this frame belonging to a singly compressed MPEG-2 video, while I2 means that this frame belonging to a doubly compressed MPEG-2 video. We denote Dn as the number of different DCT coefficients between In and In+1 (n=1, 2). According to [5], when recompressing a JPEG image over and over again with the same quantization matrix, the number of different JPEG coefficients between the sequential two versions will monotonically decrease. In the process of MPEG-2 compression, the I frame is compressed with the same philosophy as that in JPEG compression. Thus the number of different coefficients D1 between the I frames of singly and doubly compressed MPEG-2 videos will be larger than the number of different coefficients D2 between the I frames of doubly and triply compressed MPEG-2 videos. This intrinsic property of the I frames in MPEG-2 video can be used to discriminate the singly compressed video and doubly compressed video with the same bit rate. Next, 20 MPEG-2 videos whose bit rate range from 7M bit/s to 9 M bit/s are used to exemplify the relationship between D1 and D2. Experimental results are illustrated in Fig. 1, where the horizontal axis represents the file number of videos, and the vertical axis represents the average number of different coefficients between the I frames of singly compressed and doubly compressed MPEG-2 videos, or doubly compressed and triply compressed MPEG-2 videos. It is observed from Fig. 1 that, for MPEP-2 videos with different bit rate, D1 will be bigger than D2 in general.
quantization factor scale q_scale. The former is used to modify the step size within a block and the latter is used in order that the step size can be modified at the cost of only a few bits. The AC coefficients ac(i, j ) must be quantized by individual quantization factors according to (1)
32 ac(i, j ) ac(i, j ) wI (i, j ) where
(1)
is defined as integer division with truncation of
the result toward zero. The final quantized level QAC (i, j ) is given by: a ac (i, j ) sign(ac(i, j )) b q _ scale (2) QAC (i, j ) 2 q _ scale where
1 x 0 sign( x) 0 x 0 1 x 0
(3)
The de-quantization process is essentially a multiplication by the quantizer step size. With utilizing wI (i, j ) and q_scale, the decoder can calculate the quantizer step size for each block, and produce the reconstructed AC coefficients ac(i, j ) according to (4)
2 QAC (i, j ) wI (i, j ) q _ scale ac(i, j ) (4) 32 Finally, the resulting coefficients ac(i, j ) are saturated, and then a mismatch control operation is performed to output the final reconstructed DCT coefficients. 3. PROPOSED DETECTION SCHEME In this section, some statistical difference between the singly compressed and doubly compressed MPEG-2 videos with the same bit rate will be demonstrated firstly. Secondly, we will show statistical difference between the singly compressed video and the doubly compressed video , or doubly compressed video and the triply compressed video with different bit rates. At last, a new method which can be used to detect double MPEG-2 compression with the same bit rate will be presented.
Figure 1 The values of D1 and D 2 for videos with bit rate ranging from 7M bits/s to 9M bits/s
3.2 Statistical Difference Between Singly Compressed MPEG-2 Video and Doubly Compressed MPEG-2 Video, or Doubly Compressed Video and Triply Compressed Video in I frames with different bit rates.
3.1
Statistical Difference Between Singly Compressed MPEG-2 Video and Doubly Compressed MPEG-2 Video in I frame with the same bit rate.
As before, I1, I2 and I3 represent the I frames belonging to the singly, doubly and triply compressed MPEG-2 videos
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with the same bit rate respectively, and D1 represents the number of different coefficients between I1 and I2, and D2 represents the number of different coefficients between I2 and I3. If the I frame belonging to a singly compressed video, when it has been recompressed with a different bit rate, the obtained I frames are represented by I2'; If the I frames
1) Decompress V into the spatial domain, and then recompress it using the same bit rate. We can obtain the MPEG-2 video V', the number of different DCT coefficients between I frames of V and V' is denoted by D'. 2) Decompress V into the spatial domain, and then recompress it using another bit rate, which is near the original bit rate. We can obtain another new MPEG-2 video
belonging to a doubly compressed video, when it has been recompressed with a different bit rate, the obtained I frames are represented by I3'. The number of different coefficients between I1 and I2', and I2 and I3' is represented by D1' and D2',
V'', the number of different DCT coefficients in I frame between V' and V'' is denoted by D''. 3) Our decision rule is as follows.
respectively. Some experiments for demonstrating the relationships between D1 and D1', D2 and D2' are shown in Fig. 2, where the horizontal axis represents the file number of videos and the vertical axis represents the corresponding difference values, i.e., D1, D2, D1' and D2'. The original 40 MPEG-2 videos are with the bit rate of 9M bit/s. The first 20 videos are singly compressed, and the rest 20 videos are doubly compressed with the same bit rate. To obtain the value of D1' and D2', we have re-compressed the aforementioned 40 videos with 8.90M bit/s. It is observed from Fig. 2 that D1 is bigger than D1', and D2' is bigger than D2 in general. This relationship is very important and it can be utilized in detecting the double MPEG-2 compression. We will demonstrate how to use it in the next section.
if D' ' D' ,V is doubly compressed if D' ' D' ,V is singly compressed It is noted that in step 1, we do not know whether the test MPEG-2 video is singly or doubly compressed. However, if the test video is a singly compressed video, a relatively big value D' will be obtained as we discussed in Section A. Otherwise, if the test video has been doubly compressed, we will get a relatively small value D'. Then after conducting the second step, the obtained value of D'', which is mainly determined by the recompressed bit rate, will have more chances to be less than D' when the test video is singly compressed than that when the test video is doubly compressed. For the same reason, the obtain value of D'' will have more chances to be no less than D' when the test video is doubly compressed than that the test video is singly compressed. As seen, the key for the success of our detection method is the bit rate we select to recompress the video in step 2. The bit rate being selected improperly will lead to a poor detection result. For example, if the new bit rate is too far from the primary bit rate of the test video, the obtained value of D'' may be bigger than the value D' in general no matter whether the test video is singly or doubly compressed. Thus even if the test video is a singly compressed, it will be determined as a doubly compressed video. As to our method, the main challenge is to find the proper recompress ing bit rate, which can make the obtained D'' less than D' if the test video is a singly compressed video and no less than D' if the test video is a doubly compressed video. In our experiments, we select the recompress bit rate near the primary bit rate of the detected video. For example, if the bit rate of the detected videos is 9M bit/s, we can select it as 8.91M bit/s, 8.92M bit/s and so on.
Figure 2 The relationships between D1 and D 1', D 2 and D2' for videos with 9M bit/s
4. EXPERIMENTAL RESULTS
3.3
In our experiments, 200 widely known YUV sequences in Common Intermediate Format (CIF) [7] are selected as source sequences for testing, each YUV sequences contains 100 frames with the resolution of 352 288 . TM5 codec is used to encode and decode videos. All of the YUV
Detection of Double MPEG-2 Compression with the Same Bit Rate Given a MPEG-2 video V under examination, our detection method is proposed as follows :
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sequences are first encoded to MPEG-2 videos with different bit rate ranging from 6M bit/s to 8M bit /s. The experimental results are shown in Table 1. The probability that the singly compressed has been correctly predicted is represented by true positive rate (TPR) and the probability that the doubly compressed has been correctly predicted is represented by true negative rate (TNR). The final detection accuracy rate (AR) is computed as (TNR+TPR)/2. As seen, via selecting a proper recompressing bit rate, the double MPEG-2 compression with the same bit rate can be detected effectively by using our methods.
literature. In this paper, we have presented a method to detect it. Through a new re-compressing strategy, the difference between a singly compressed video and a doubly compressed video can be discriminated effectively. ACKNOWLEDGMENT This work was supported by the National Science & Technology Pillar Program (2012BAK16B06), the National Natural Science Foundation of China (61173147, U1135001,61332012), the Fundamental Research Funds of Central Universities (121gpy31), and the Project Sponsored by the Scientific Research Foundation for the Retu rned Overseas Chinese Scholars, State Education Ministry ([2012]1707).
Table 1 The experiments on videos with different bit rate Recompressing Bit rate TPR TNR AR bit rate 5.89 61% 87.5% 74.25% 5.90 76.5% 86% 81.25% 5.91 77% 81.5% 79.25% 5.92 77.5% 80.5% 79% 6M bit /s 5.93 79% 78.5% 78.75% 5.94 81.5% 77% 79.25% 5.95 82.5% 73.5% 78% 6.89 71.5% 87% 79.25% 6.90 76% 85.5% 80.75% 6.91 76.5% 86% 81.25% 6.92 82.5% 81% 81.75% 6.93 84% 79% 81.5% 7M bit/s 6.94 89% 72.5% 80.75% 6.95 92.5% 65.5% 79% 7.89 68% 88% 78% 7.90 79% 79% 79% 7.91 85% 77% 81% 7.92 85.5% 73.5% 74.5% 7.93 87% 70.5% 79.5% 8M bit/s 7.94 91% 67% 79% 7.95 95% 58.5% 76.75% 8.89 78% 79.5% 78.75% 8.90 82.5% 79.5% 81% 8.91 87% 69.5% 78.25% 8.92 90.5% 62.5% 76.5% 8.93 93% 58% 75.5% 9M bit/s 8.94 95.5% 54% 74.75% 8.95 97% 51.5% 74.25%
REFERENCES [1]
Adobe Premiere Pro 2.0: A popular M PEG-2 encoder.
http://www.adobe.com/products/premiere/ [2]
[3]
[4]
[5]
[6] [7]
5. CONCLUSION Because the difference between the singly compressed MPEG-2 video and doubly compressed video with the same bit rate is small, detection of double MPEG-2 compression with the same bit rate is a challenging problem. To the best of our knowledge, no method has been reported in the
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W. Wang and H. Farid, “Exposing digital forgeries in video by detecting M PEG compression,” in Proc. M ultimedia Security Workshop (M M and Sec), pp. 37–47, Sep. 2006. W. Chen and Y.Q. Shi, “Detection of double M PEG compression based on first digit statistics,” Lecture Notes Computer Science (IWDW 2008), vol. 5450,pp.16–30, 2009. T. Sun, W. Wang, and X. Jiang, “Exposing video forgeries by detecting M PEG double compression,” in Proc. IEEE Int. Conf. Acoust. Speech Signal Process. (ICASSP), pp. 1389– 1392, M ar. 2012. F. Huang , J. Huang and Y. Q, Shi. “Detecting Double JPEG Compression With the Same Quantization M atrix,” IEEE Trans on Information Forensics and Security, vol. 5. no. 4, pp. 848-856, 2010 Test M odel 5 for ISO/M PEG II, Apr. 1991. http://media.xiph.org/video/derf/
