Multiple spatial watermarking technique in color images

This paper presents a novel multiple digital watermarking technique for the copyright protection of digital color images. In order to improve the robustness against cropping attacks, a binary watermark image is divided into four parts. Each is encrypted by a secret key and embedded into different regions of the blue component of the color image in the spatial domain. Watermark extraction is based on the comparisons between the original intensity pixel values and the corresponding watermarked intensity pixel values in blocks of size 8 x 8. The watermark-extracted bits are determined using the probabilities of detecting bit '1' or bit '0'. The watermark can be extracted in several parts depending on the size of the host image, but only four of these are selected by a correlation coefficient detector and used to reconstruct the extracted watermark. Experimental results show that the proposed scheme successfully makes the watermark perceptually invisible and robust for a wide range of attacks, including JPEG-loss compression, median filtering, low pass filtering, rotation, rotation-scaling, rotation-crop, image cropping, image scaling, and self-similarity attacks.