Maintaining image quality when watermarking grayscale comic images for electronic books

Abstract

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.

A method for detecting watermarks in print using smart phone

“…The watermark strength can be determined by using a previously reported method [12]. The original image is analyzed by using a human visual model before watermark embedding, and visually important areas, such as edge shapes, where changes are more perceptible than changes in other areas are extracted.…”

“…Therefore, we set the watermark strength so that the image quality of the watermarked image would be nearly the same level as that of the unwatermarked image with 1/10 JPEG compression. The watermark strength was calculated in the same way for both the conventional and proposed embedding methods [12]. The PSNR values for a sample image watermarked using the conventional and proposed method were respectively 41.3 dB and 41.4 dB.…”