A DCT-domain watermarking algorithm with 8 bits gray image's watermark is proposed. First

the method of block classification based on HVS is improved

which enhances the information hiding ability of cover image. In the watermark embedding process

we transform the image's watermark into DCT domain

and embed the low-frequency coefficients to the blocks that have bigger elongation factor. At the same time

we adjust the DCT coefficients of image's watermark to make them in the same level. This increases the SNR of extracted watermark. The experimental results show that the mentioned algorithm is more robust to many attacks

especially to JPEG compression

rotation and cropping.