基于菲涅耳全息图和离散余弦变换的“盲数字水印”技术

基于光学菲涅耳全息图和相位密码板,结合离散余弦变换,设计了一种新的正实值编码的“盲数字水印”计算方法.相位密码板是多个点光源的菲涅耳衍射光场的相角之和,原始水印图像在其菲涅耳衍射域中与通过相位密码板的参考光作相干叠加,形成菲涅耳全息图;之后将其嵌入到原始宿主图像的离散余弦变换中,同时将此叠加水印信息的原像素值用其邻近的原像素均值来替换;通过作逆离散余弦变换,获得了已嵌入水印信息的正实数值的目标图像;通过对载有水印信息的目标图像作逆运算,从中提取了原始水印图像.数值计算结果表明：该水印计算法对JPEG有损压缩、剪切、噪音污染和重采样等攻击,具有很强的鲁棒性.由于本算法属“盲数字水印”技术,以及水印信息的灵活嵌入和多重密钥（衍射距离、多个点光源位置等）的随意选择,从而使该算法具有很高的安全性和实用价值.

Blind Digital Watermarking Technique Based on Fresnel Hologram and Discrete Cosine Transform

The blind digital watermarking algorithm with positive real values is proposed based on optical Fresnel hologram,phase encryption mask (PEM) and DCT (Discrete-Cosine-Transform).The PEM is the sum of the phase angles of the E-fields which are the Fresnel diffraction transform of serial point lights with the different coordinates.The Fresnel hologram of original watermark image is obtained by a reference light through the PEM,and watermarking information are embedded into DCT of original host image.Meanwhile the original pixel value,which has been embedded watermark information,is replaced by the average value of its adjacent pixels,and the watermarked target image is regained after inverse DCT.The original watermark image can also be retrieved from the target image after the above inverse processing.Numerical experiments show that the algorithm is robust to the attacks of the JPEG lossy compressing,cropping,superposing noise and resampling from the screen copy.Because the watermark can be retrieved from the target image without the original host image,the watermark can be embedded in the host image flexibly,and the multi-keys (light wavelength,diffraction distance,a serial point light positions) can be designed in the large degrees of freedom,the algorithm has high security and practical value.