Watermarking schemes using the addition property among DCT coefficients

In general, a signal embedded in the low-frequency components of an image is less likely to be affected by attack. However, there is a problem of substantial degradation of image quality. In particular, if each divided block is processed, the blocking effects generated between adjacent blocks is more easily perceived. In this paper, an embedding method without blocking effects is proposed, making effective use of the superposition of the low-frequency components in the two-dimensional DCT basis images. Due to the effect of superposition of the two-dimensional DCT bases, the watermark information is not detected directly from the block with embedding, but extraction is performed from the subblocks in the block. In the proposed method, in order to provide robustness against geometrical transformation, the synchronization is recovered prior to extracting the water-mark information. The robustness can be further enhanced by means of error correcting code.