MASTER THESIS Video Watermarking - Computer Graphics Group ...

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detection. Conversion into 8×8 blocks is out of the question because of intra prediction. The intra prediction process for a 4×4 sub-block within one 8×8 block uses the other 4×4 sub-blocks within that 8×8 block. Therefore, when the 8×8 block is watermarked, the error caused by watermark embedding should be compensated in the 4×4 sub-blocks when using the other 4×4 sub-blocks for prediction. But the compensation may severely impair the already embedded watermark... There is only conversion into 4×4 blocks left. In this case, intra prediction does not give trouble. The conversion is performed by decoding (i.e. dequantizing and inverse transforming) a 8×8 block, partitioning into 4×4 blocks and encoding (i.e. forward transforming and quantizing) the blocks in order to obtain transform coefficients for watermark embedding. This works pretty well but the quantization causes visible blocking artefacts. With respect to the reasons above, no conversion is applied and only 4×4 blocks are taken for watermark embedding. 4×4 blocks have been chosen because 4×4 block transform is more usual than 8×8 transform. <strong>Video</strong> sequences given to both embedding and detection can be converted to required format beforehand at the cost of eventual quality degradation. The pure watermark blocks are generated with dimension of 4 to cover 16 4×4 macroblock blocks. Embedding into one block proceeds as follows. Only the half of transform coefficients that represent higher frequencies are taken. Although higher frequencies are more vulnerable to eventual attacks, human visual system is more sensitive to distortion in lower frequencies and modification of low frequency coefficients causes obtrusive blocking artefacts. When 1 (the weight factor a in fact) is to be embedded, the coefficient with the greatest absolute value is chosen. The coefficient modification keeps the sign of the coefficient but eventually increases its absolute value to required robustness level. If the coefficient is positive but lower than a, the coefficient is increased to a. If the coefficient is negative but greater than -a, the coefficient is decreased to -a. The coefficient is remained unchanged if it is greater than a in absolute value. If the coefficient is 0, a or -a is randomly assigned. The purpose why all this is done is to enforce a non-zero value in the half while producing to the lowest distortion. When -1 is to be embedded, all coefficients in the half are set to zero. This causes loss in detail. Detected watermark element values are obtained directly from transform coefficients. If all transform coefficient values in the half are zero, -1 is returned, otherwise (if there is at least one non-zero coefficient) 1 is returned. Recompression at lower bit-rate causes more loss in detail, thus zero 32
coefficients are hardly set to non-zero values. On the other hand, the coefficients set to the weight factor a in absolute value can be zeroed. Therefore, a should be set to such a high value to remain non-zero even if the video sequence undergoes an attack. Considering multiple embedding attack, this method is quite vulnerable because the watermark elements are directly embedded and thus multiple embedded watermarks overwrite each other. The watermark can be completely destroyed by zeroing the coefficients in the half in all macroblocks but it results in visible blocking artefacts. 4.3.3 Coefficient Watermark Coefficient watermarking method belongs to frequency domain techniques as well. The method consists in coding one watermark element into one transform coefficient of a macroblock residual block. Again, only 4×4 blocks are supported because of the same reasons as in block watermarking method and the pure watermark block has dimension of 4. The transform coefficient where a watermark element is to be embedded into is pseudo-randomly chosen from the half of coefficients which represent higher frequencies. The pseudo-random generator is initialized for each picture by both content ID and user ID in order to increase robustness against multiple watermark embedding and collusion attacks. The value of the coefficient is altered in the same way as in block watermarking method, i.e. when 1 is to be embedded, the value is eventually increased to the weight factor a in absolute value, and when -1 is to be embedded, the coefficient is set to zero. In the detection process, the transform coefficient is pseudo-randomly chosen in the same way as in the embedding process. If the coefficient is zero, -1 is returned, otherwise 1 is returned. This method should have similar robustness qualities to block watermark method but distortion caused by watermark embedding should be lower because only one coefficient is altered in a block. 33
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 and 26: samples is subtracted from the resi
Page 27 and 28: At the beginning of the entire proc
Page 29 and 30: file in order to provide detailed r
Page 31: as a part of the whole process. The
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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