为了增强水印安全性, 用三维物体作为水印信息嵌入载体, 提出了一种基于层析法的多重计算全息数字水印技术。该方法用层析技术得到三维物体的菲涅尔(Fresnel)全息图; 利用Arnold变换对全息图进行置乱加密, 置乱后的图像作为待嵌入水印; 选择离散余弦变换(DCT)作为嵌入与提取算法; 将Arnold变换次数与三维物体每层的衍射距离作为加密密钥。数字仿真与实验结果显示, 当密钥正确时, 从水印中提取的全息图的再现结果与直接再现全息图的结果一致, 可以再现出三维物体相应层次的信息; 密钥错误时, 从水印中提取的全息图的再现结果无法识别, 水印提取失败。实验结果证明了水印嵌入及提取算法的正确性, 表明提出的方法可以实现水印信息的立体化和多重化, 具有较高的安全性和易于实现等优点。

To improve the security of watermark, a three-dimensional (3D) object was employed as the watermark to embed in a host, and a new technique of multiple watermark embedding was proposed based on Computer-generated Hologram (CGH). The tomography was used to obtain the Fresnel hologram of the 3D object. Then, the Arnold transformation was used to encrypt the hologram by scrambling the image, and the scrambled image was taken as the watermark to be embedded in. Furthermore, the Discrete Cosine Transformation (DCT) was chosen as the embedding and extracting algorithm and the Arnold transformation times and the different diffraction distances of every layer were regarded as encryption keys. Numerical and optical experimental results show when the keys are all correct, the reconstructed result of hologram extracted from the watermark is accord with that direct reconstructed hologram, and can display different information at different layers of the 3D object; otherwise, no useful information could be distinguished when the keys are all incorrect. These results prove the effectiveness and security of proposed method, and show that the method can allow the watermark to be three-dimensional and multiplicity.