目的 针对基于尺度不变特征变换(SIFT)的抗几何攻击图像水印算法在图像匹配时容易产生误匹配，影响提取水印的鲁棒性，提出一种改进的SIFT图像水印算法。方法 先对载体图像进行两级非下采样Contourlet变换(NSCT)，利用中间值量化算法把水印信息嵌入低频系数的方向子带中。图像匹配时利用广义霍夫变换删除误匹配特征点对，提高匹配精度。根据筛选后的特征点匹配对估算几何失真参数，对含水印的载体图像进行几何校正并提取水印，从而实现水印的抗几何攻击。结果 实验数据表明对原始特征点匹配对进行筛选后,至少提高了6.8%的匹配精度，提取水印的NC值平均达到0.9以上。结论 文中提出的改进算法不仅对几何攻击和常规信号攻击均具有较好的鲁棒性，还能保证水印的不可见性。

The work aims to propose an improved SIFT image watermarking algorithm with respect to the mismatching caused by the image watermarking algorithm robust to geometric attacks based on scale invariant feature transform (SIFT) when the images are matching. Firstly, the carrier image was subject to two-stage non-subsampled Contourlet transform (NSCT) and the watermarking information was embedded into the directional subbands with low-frequency coefficient through mean-value quantitative algorithm. When the images are matching, the generalized Hough transform was used to remove the mismatched feature point pairs to improve the matching accuracy. The geometric distortion parameters were estimated according to the screened feature point pairs. The carrier images with watermarks were subject to geometric correction and watermark extraction, so as to achieve the watermarking robust to geometric attacks. The experimental data indicated that, after the matching pairs of the original feature points were screened, the matching precision was improved at least by 6.8% and the average NC values of extracted watermark were all above 0.9. The results show that the improved algorithm proposed herein can not only resist the geometric attacks and normal signal attacks, but also guarantee the invisibility of watermarks.