MASTER THESIS Video Watermarking - Computer Graphics Group ...

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embedding. Anyway, the pictures are used for measuring distortion caused by watermark embedding. Distortion in one picture is measured by peak signal-to-noise ratio (PSNR) which is the most commonly measure of quality of reconstruction in image compression: where MAXI is the maximum pixel color sample value (usually 255) of any picture, P is the original picture, Pi is a sample value of the original picture and P * i is a sample value of the watermarked picture. At the end of the entire embedding process, average PSNR over all watermarked pictures is estimated and printed. Distortion expressed by PSNR relates to perceptibility of the watermark. The practical results are presented in Chapter 5. 4.2.3 Detection In the detection mode, the plugin accepts a H.264 stream as well, invokes the detection process and outputs detection results as textual data. The pipeline is illustrated in Figure 12. Figure 12: Watermark pipeline in detection mode Again, only intra coded slices are taken into the detection process. Inputs for the process are: a potentially watermarked intra coded slice, content ID, copy ID. Only when whole picture is processed, output of this process is the probability that the cover content copy contains the identifier with value of copy ID. 26 (7)
At the beginning of the entire process, the same pure watermark as in the embedding process is generated. The pure watermark is further used for correlation with the detected watermark to retrieve hidden information. In each macroblock, watermark block element values carrying one hidden information bit are obtained using particular watermarking method (the scheme is depicted in Figure 13). In case of frequency domain, the transform coefficients are directly accessible. In case of spatial domain, the coefficients have to be dequantized and inverse transformed in order to obtain the residual. It is further added to predicted samples and clipped giving picture samples suitable for detection. In this case, the residual is not enough because it depends on selected intra prediction mode which could change after an attack. Figure 13: Watermark element detection scheme Obtained watermark block element values are compared with the corresponding element values of the pure watermark. When the two corresponding values match, 1 has been encoded, while when they differ, -1 has been encoded. Remember the modulation of pure watermark by bits of hidden information in the embedding process: when -1 was to be encoded, values of the watermark block elements are inverted, i.e. each 1 becomes -1 and vice versa (thus differ), and when 1 was to be encoded, values of the block elements remain unchanged (thus match). But in practice, especially after an attack, the corresponding values in one block need not 100% match or differ. Therefore, some correlation mechanism has to be proceeded. The correlation sum for i-th watermark block is computed: where W P ij is j-th element value of the pure watermark block and W * ij is j-th element value of the detected watermark block. When the block encodes 1 (corresponding values match – they are either (1, 27 (8)
Page 1 and 2: Charles University in Prague Facult
Page 3 and 4: Table of Contents Abstract 5 1 Intr
Page 5 and 6: Název práce: Watermarking videa A
Page 7 and 8: information about copyright owner a
Page 9 and 10: Common geometric distortions - the
Page 11 and 12: watermark, i.e. values of the water
Page 13 and 14: Chapter 3 The H.264 Standard The H.
Page 15 and 16: There are three types of slices: I,
Page 17 and 18: frequency transform. The choice is
Page 19 and 20: Chapter 4 Implementation Details Th
Page 21 and 22: auxiliary coded pictures), Supplem
Page 23 and 24: an intra coded slice, content ID -
Page 25: samples is subtracted from the resi
Page 29 and 30: file in order to provide detailed r
Page 31 and 32: as a part of the whole process. The
Page 33 and 34: coefficients are hardly set to non-
Page 35 and 36: Table 1 outlines characteristics of
Page 37 and 38: The third and the fourth row sets c
Page 39 and 40: Using block method, we cannot consi
Page 41 and 42: probabilities of watermark detectio
Page 43 and 44: 5.4.3 Cropping In the cropping test
Page 45 and 46: 5.4.6 Blurring Blurring can be cons
Page 47 and 48: 5.4.8 Multiple Watermark Embedding
Page 49 and 50: Despite the expectations, all the m
Page 51 and 52: ecompression and noising, and to so
Page 53 and 54: Bibliography [1] Ingemar J. Cox, Jo
Page 55 and 56: Appendix A Enclosed CD & Installati
Page 57 and 58: Appendix B The plugin usage The wat
Page 59: length is the length of the list of

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