扩频水印鲁棒性分析

数字水印的鲁棒性是水印技术实用化的一个重要指标。与通过StirMark测试和各种仿真测试不同，本文定义互信息作为代价函数，建立扩频水印系统鲁棒性描述和度量的一般模型，并对加性扩频水印的鲁棒性进行详细分析，推导出评估鲁棒性的互信息度量计算模型，仿真分析了盲检测和非盲检测条件下互信息函数对鲁棒性的评估结果。实验以统计误比特率的方法计算图像DCT域中低频系数为载体的扩频水印误码率，当水印噪声比变化时，互信息函数和误码率之间的匹配关系验证了互信息度量模型的有效性。互信息函数可以作为代价函数评估水印的鲁棒性，并预测误码率的变化趋势。 

Analysis for Robustness of Spread spectrum watermarking

Robustness is one of the most important requirements when digital watermarking is applied. Different from StirMark test and various simulation tests， a mutual information function is defined as a criterion measuring the robustness of spread spectrum watermarking algorithm. As an example of the additive spread spectrum watermarking scheme， the calculation formula of mutual information function is derived to evaluate the robustness of algorithm. Numerical computation of mutual information is performed with change of watermark noise rate (WNR) under blind detection and non blind detection. Error probability of watermarking scheme can be calculated to validate the measurement of mutual information function. In the experiment， middle and low frequency AC coefficients are selected as the host， and the statistic bit error rate is derived under the additive spread spectrum watermarking scheme and additive noise. Experiment results show evaluation conclusion of mutual information method is matched with that of empiric bit error rate (BER) against the WNR. Mutual information function can be selected as a cost function to evaluate the robustness of watermarking algorithm， and predict the BER.