Maintaining image quality when watermarking grayscale comic images for electronic books

Digital watermarking of comic images effectively protects the copyright of the content in electronic book applications. The characteristics of comic images were analyzed to determine the effect of digital watermarking on the line shapes. Modification of the shape edges by conventional watermarking methods can degrade image quality because the human eye is sensitive to the edges. The analysis revealed that slight changes in the black pixels in a comic image are barely perceived by the human eye when the image is displayed on a tablet PC. A method was then developed for embedding watermarks that minimizes modification of the white pixels compared to modification of the black ones. Experimental evaluation showed that this method improves the quality of watermarked grayscale comic images. Watermark robustness was almost the same as that with the conventional method in an assumed use case.