Spot color printing image digital watermarking method and system

Abstract

Based on the conversion principle of color space in spot color printing, the invention combines IWT and SVD to propose a digital watermarking method and system for spot color printing images. The method and system decompose the carrier image into spot color channels, perform IWT conversion on the spot color channels, and then directly embed the watermark information into the singular values ​​of each subband of the first-level IWT; in order to improve the security of the watermark, the watermark image is Arnold configured Random encryption processing, and embed a digital signature that can resist the printing-scanning process in the U and V of the embedded watermark image; in order to improve the robustness of digital signature and watermark, a tone correction model for scanning spot color images is proposed , correct the scanned image, then extract the digital signature for verification, and extract the watermark information if the verification is passed. The invention has excellent performance in watermark transparency, robustness and capacity, can resist offset printing-scanning process, has low algorithm complexity, high execution efficiency, and the extraction time is less than 1 second.

Description

technical field [0001] The invention belongs to the technical field of digital watermarking of printed images, and in particular relates to a method and system for digital watermarking of spot-color printed images. Background technique [0002] There are two ways of color reproduction of printed images: the first is subtractive color reproduction, which uses cyan (C), magenta (M), yellow (Y) and black (K) inks to overprint to achieve color image reproduction; the second The first is spot color printing reproduction, which refers to the method of using other inks other than CMYK four-color ink to reproduce the color of the original. Such as figure 1 As shown, when the spot color PANTONE168 C uses four-color printing, it can be decomposed into four channels of C, M, Y, and K, while it is only decomposed into one spot color channel when using spot color printing. Therefore, there are fewer channels for spot color printing images to embed watermark information, and the require...

AI Technical Summary

Problems solved by technology

[0004] In 2003, Alattar, O. and Reed, A. et al. proposed the spot color digital watermarking algorithm for the first time. First, calculate the spot color brightness mapping factor and yellow saturation to modulate the watermark signal, and then embed it into the spot color channel. At the same time, pass the intensity The factor dynamically adjusts each pixel, and the algorithm is effective for most spot colors, but not for some spot colors, such as: silvery yellow, silvery gold, silver and gold, etc.; 2015, Reed A. and Filler T. et al. proposed a spot color watermarking algorithm in packaging printing. This algorithm uses CMY three-color overprinting for a certain proportion of spot colors, and embeds watermark information in the CMY three-color channel, such as PANTONE 221 spot color, which can be decomposed into 75% The spot color channel and 25% of the CMY channel are superimposed, and the watermark is embedded in the 25% of the CMY channel, but the use of CMY to simulate the spot color will produce a certain color difference, and three additional printing plates of C, M and Y are added, which increases the printing cost. cost

However, the SVD watermarking algorithm also has defects. Based on the SVD watermarking algorithm, only the singular value eigenvector S of the carrier image is used when embedding the watermark information. Because the singular value S of the image only represents the brightness information of the image, and the two U and V The orthogonal matrix reflects the texture and geometric structure characteristics of the image. If U and V are used to synthesize the S vector of the image without watermark when extracting the watermark, an image similar to the original watermark image structure can also be reconstructed. resulting in detection security issues

For this problem, the solution based on the digital signature mechanism is relatively mature, but the robustness against printing needs to be improved

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