In order to better protect the copyright of the color image

this paper explained theoretically why the green color image component should be chosen to embed into a watermark

and thus proposed a new blind watermark algorithm for a color image. First

the green component of the original image was divided block by block with size 8×8 pixels

and transformed with discrete cosine transformation (DCT). Second

the binary watermark image was spread spectrum modulated by two uncorrelated random sequences. At last

in order to maximize the enhanced watermark strength without affecting transparency

human visual masking factor was imported. And the modulated watermark with different intensities was embedded in mid-frequency region coefficients of DCT domain based on visual masking factor magnitude. The watermark was extracted by comparing the relativity between two random sequences with watermarked image

and the process did not need the original image

which means a blind watermark algorithm. Experimental results demonstrated that this method was invisible and robust against some usual attacks such as JPEG compression

cropping

adding noise

and embedding watermark in a green component is more robust against lossy compression than embedding watermark in a red or blue component.