Process color watermarking: the use of visual masking and dot gain correction

Subtractive-Color-Reproduction is the fundamental mechanism in 4-color process printing, which employs CMYK (cyan, magenta, yellow and black) dots for color reproduction. Watermarks embedded in RGB images are distorted when the image is transformed from what to process color for printing usage. Based on the feature of CMYK color space and human visual system, this paper presents a robust CMYK watermarking algorithm for printing images, which embeds an informative watermark and a structure template signal in DCT domain of the Yellow information channel and DFT domain of the black information channel respectively. The embedded template enables robustness against geometric distortions in the print-scanning process. Another spatial visual mask is proposed to reshape the embedded energy after it is inverted to the spatial domain, which significantly improves the color fidelity and the image quality. In addition, a dot gain compensation model is put forward for color correction. A series of proof tests have been carried out on an offset press, which show that the watermark preserves the fidelity of the image and can be extracted with a high quality; moreover, a remarkable fidelity of color and tones on printed copies is well preserved.