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Multimedia Security

DCT-Domain Watermarking Chiou-Ting Hsu and Ja-Ling Wu, "Hidden digital watermarks in images," IEEE Trans. On Image Processing, vol. 8, No. 1, January 1999. Multimedia Security. Requirements Invisibility Robustness No ambiguity. Embedding. Comparison. Extraction.

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Multimedia Security

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  1. DCT-Domain WatermarkingChiou-Ting Hsu and Ja-Ling Wu, "Hidden digital watermarks in images," IEEE Trans. On Image Processing, vol. 8, No. 1, January 1999 Multimedia Security

  2. Requirements Invisibility Robustness No ambiguity Embedding Comparison Extraction A generic image watermarking system • Distortions • Image processing • Compression • …... Watermark Watermark

  3. Advantages of this scheme • Advantages: • Semantically meaningful watermark pattern • Good perceptual invisibility • Acceptable robustness • Various user-selected options • Reasonable complexity/execution time

  4. System overview • Watermarks are randomly permuted to spread their spatial relationship, and then embedded in the DCT domain of the host image, with consideration of invisibility/robustness

  5. Block Diagrams of the Original Algorithm Original Image Watermark Image FDCT Pseudo-random Permutation Block-based Mapping Embedding (Polarity Reversing) IDCT Watermarked Image

  6. Block Diagrams of the Original Algorithm Original Image Suspected Image FDCT FDCT Extract the Permutated Data (XOR) Reverse Block-based Permutation Reverse Pseudo-random Permutation Extracted Watermark

  7. Block DCT/IDCT • Advantages • Fast • Suitable for robustness against JPEG compression • Disadvantages • Block effect • Effect of picture cropping

  8. Semantic Meaningful Watermarks • Watermarks can be verified with naked eyes by understanding the semantics of the extracted watermark patterns The seal of CML (in Chinese characters)

  9. Pseudo-Random Permutation • A n-bit Linear Feedback Shift Register (LFSR) is used to generate the maximal length (2n-1) sequence Original Watermark Permuted Watermark The 14-bit Shift Register that permutes 1-16384

  10. Block-based Mapping • Watermark blocks with more signal pixels are embedded into image blocks with higher variances • to achieve better perceptual invisibility.

  11. Choices of embedding positions within each block: Low-frequency Bad invisibility High-frequency Bad robustness => Middle-frequency Fix positions in each block Embedding(1/2) Embedding 4 watermark pixels

  12. Embedding(2/2) • Polarity: the inequalityrelations between the scaled DC value and the selected AC coefficients • Effects of the JPEGquantization table are also considered

  13. Polarity Reversing • Polarity: the inequality relationship between DC & corresponding AC values within each DCT block. watermark bits: 0 1 0 1 0 0 1 polarity: 1 1 0 0 1 0 1 XOR 1 0 0 1 1 0 0 reversed polarity: Embedding Extraction

  14. Extraction • Exclusive-or (XOR) operations are performed on the two polarity patterns to obtain the permuted binary data.

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