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A collusion attack is an attack in which a large number of users collude to remove the watermark. In the case of block-wise watermarking, the attack can be easily carried out by cutting out the embedded pattern for each block from a single image and analysing the pattern without the collusion of many users. In order to avoid such attacks and to obtain higher””security, ”°Camouflage patterns”±””are introduced, which show the same spectrum but have different dot patterns. The camouflage pattern consisted of an inversion pattern and a phase-shift pattern.
For the inversion pattern, eight patterns are generated for one ID pattern, and the i-th pattern 𝑝𝑖,𝜎𝑖(x,y) are as follows.
The vertical and horizontal directions are relative to the major and minor axes of the ellipse, respectively. The negative pattern is a pattern in which the white and black dots are reversed. Because the numbers of white and black dots are the same, the number of dots does not change even if they are reversed. Figure3(a) shows the inverted pattern for R=32. The pattern 𝜎𝑖was generated by a random number generator and randomized for each block.
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IEEE/ICACT20220093 Slide.11
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