Spot color watermarking: the use of the IWT-SVD and tone correction model

Abstract

Watermarking has long been a necessary technology in the package printing industry for copyright protection and counterfeit prevention. However, A large number of spot colors are used in the industry, which poses a challenge for the watermarking application. In light of the integer wavelet transform (IWT) and singular value decomposition (SVD), we herein present a robust watermarking algorithm for spot color printing based on the color space transformation principle. First, a spot color channel was separated from the original image using the Neugebauer model. The image of the spot color channel was then transformed with the IWT, and the watermark was embedded in singular values of the 1-level IWT decomposed sub-bands. To enhance the security, Arnold scrambling and encryption were adopted, and a digital signature was embedded in the watermarked images. Hence, the extraction of the digital signature is required prior to the extraction of the watermark information. In the extraction stage, the watermark was extracted from the scanned grayscale image. The spot color scale was designed, and the tone correction model was established for the scanned grayscale image. A series of tests were conducted, and the results showed that the method is effective in resisting spot color distortion attacks in the print-scanning process, and the imperceptibility, capacity as well as security in the spot color are also favorable.