Robust gray-level watermark embedding and retrieving technique

The present invention relates to information security, and proposes a novel digital gray-level watermark system by applying the discrete cosine transform and digital quantization technique to embed the gray scale watermark into the digital image effectively, so as to achieve the purpose of protecting the intellectual property of image. The gray scale watermark is processed by discrete cosine transform in the present invention, so that the important information of watermark is concentrated on few frequency coefficients, which not only can strengthen the resistance of the watermark to the intentional/casual damage to the image, but also can enhance the image quality of the watermark brought through image attack after embedding the watermark. A quantization table is also designed to reduce the amount of data of gray scale watermark, so that the parameter value passing through digital quantization after discrete cosine transform can be close to the frequency value at the low frequency position in the digital image. Finally, the digital watermark is embedded in the way of replacing one-by-one. The digital watermark embedded into the digital image provides a high resistance to the image attack in the present invention, and can resist the attack of JPEG distortion compression and damaging signal processing.