结合图像空间域和小波域特性的自适应盲水印算法

A adaptive digital watermarking algorithm combining spatial and DWT domain

本文针对传统的基于人类视觉系统(HSV)特性的空间域与变换域结合算法的不足,提出了一种有效的自适应盲水印新算法。通过利用图像的局部纹理特性产生伪随机码与水印序列组成扩频水印来增强水印的抗干扰能力。利用密钥产生一串伪随机数控制水印嵌入位置来增强水印的保密性。水印嵌入过程中使用了改进的量化水印算法。通过计算相邻特征平均值来自适应调整待量化小波系数的量化间隔,大大提升了水印系统的逼真度。根据奇偶判决法来实现盲检测。经实验表明水印是不可察觉的,算法对于遭受JPEG、均值滤波、维纳滤波等手段攻击后提取出的水印仍与原始水印具有0.85以上的相似度,具有很好的鲁棒性。

A novel effective adaptive digital watermarking algorithm was proposed without using the traditional method of combining spatial and DWT domain based on the features of human visual system （HVS）. The loaded image was divided into several blocks and calculated their variances. A pseudo random sequence was produced after comparing these variances with the variance of the whole loaded image. Then a spread spectrum watermark sequence was created after combining original watermark and pseudo random sequence, and the robustness was accordingly enhanced by utilizing the spread spectrum codes. The loaded image was decomposed by DWT （Discrete Wavelet Transform） into 2-level sub-band images. The watermark sequence was embedded into the high-frequency coefficients of the loaded image. An innovative quantization method was proposed. To improve the security of the watermark system, the cryptography technology was used in the embedding procedure. A long periodic pseudo random m-sequence was created to choose the location of watermark to be embedded in. Each bit watermark that used to embed in load image needs to mark choice every time. The traditional quantization methods usually use the constant quantized interval and the scheme proposed in this paper is called odd-even adjudegment rule. The scheme could adaptively adjust the quantized interval by calculating the adjacert symbols＇ mean value of the wavelet coefficients which is selected to embed the watermark. The top left, top and left coefficients were selected as the adjacent symbols of embedded wavelet coefficient＇s neighboring symbols. The fidelity of the watermarked image and the invisibility of the watermark have been enhanced by this method. The main idea of odd-even adjudgement rule is to quantize the coefficients into the most adjacent odd quantization mark or even quantization mark according to the watermark＇s value. The blind extraction could be realized by using this rule. Experimental results show that the watermark proposed in this paper is invisible （the PSNR is higher than 41） and is robust in the case of against some normal attacks such as JPEG compression, additive noise and filtering, etc. （the similarity between the original watermark and extracted watermark is almost above 0.85）.