Frequency Domain Watermarking Resists Print-Scan Distortion
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Solution Overview
Problem
Conventional digital image watermarking techniques are susceptible to distortion during the print and scan process, particularly due to the differences between the CMYK color model used by printers and the RGB color model used by scanners, leading to sampling and quantization errors that impair the detection of hidden information.
Innovation Solution
A digital image watermarking system that embeds secret information using a Discrete Hadamard Transform (DHT) and Discrete Wavelet Transform (DWT) to create a robust watermark in the frequency domain, specifically modifying coefficients in the LL sub-band to withstand print-and-scan distortions and other attacks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional watermarking techniques are used in the spatial domain, then the watermark can be embedded easily, but the detection reliability deteriorates due to print-and-scan distortions
Solution Approach 1:
The patent replaces spatial domain watermarking (mechanical approach) with frequency domain watermarking using Discrete Hadamard Transform and Discrete Wavelet Transform. This substitution allows the watermark to be embedded in transformed coefficients rather than direct pixel values, making it resistant to print-and-scan distortions while maintaining ease of implementation through standard transform operations.
Solution Approach 2:
The patent changes the domain parameter from spatial domain to frequency domain by applying DHT and DWT transforms. This parameter change transforms the watermark embedding process from operating on pixel intensities to operating on transform coefficients, fundamentally altering how the watermark interacts with image processing operations and providing robustness against distortions.
2Device complexity
If the watermark is embedded in the spatial domain, then the embedding process is simple, but the watermark integrity deteriorates under JPEG compression and filtering attacks
Solution Approach 1:
The patent substitutes direct spatial domain embedding with frequency domain embedding using DHT and DWT. This allows the watermark to be distributed across frequency components rather than concentrated in spatial locations, providing stability against compression and filtering that operate differently in the frequency domain.
Solution Approach 2:
The patent applies preliminary transform operations (DHT followed by DWT) to the cover image and watermark before embedding. This preliminary action in the frequency domain prepares the data structure to be inherently more resistant to subsequent processing operations like JPEG compression and median filtering that occur after embedding.
3Ease of operation
If conventional color model conversion is used during print-and-scan, then the process is straightforward, but sampling and quantization errors increase
Solution Approach 1:
The patent changes the operational domain from spatial to frequency domain, which fundamentally alters how color model conversions affect the watermark. In the frequency domain, the watermark coefficients are less sensitive to the sampling and quantization errors introduced by CMYK-RGB conversions during print-and-scan processes.
Solution Approach 2:
The patent substitutes direct pixel-level operations with frequency domain operations. This substitution means that even when color model conversions introduce errors at the pixel level, the watermark embedded in frequency coefficients remains detectable because the transform operations provide a buffer against these errors.
Data Source
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AI summary
A method of producing a printed image comprising digital information embedded in a digital cover image. The method involves obtaining a frequency domain representation of the digital cover image, partitioning the frequency domain representation into blocks and selecting a sub-set of the frequency components of each block. A respective bit of the digital information is embedded in a respective one of the sub-sets by changing the polarity of the value of at least one of the respective frequency coefficients in the sub-set to create a modified block. The modified blocks are used to create a spatial domain representation of the digital cover image in which the digital information is embedded. The modified spatial domain representation of the digital cover image is then printed. The method is robust against distortion errors that can be introduced during print and scan operations and so the embedded digital information can readily be detected when the printed image is subsequently scanned.