提示原因：surf、sift等算法涉及到专利问题，编译时看控制台输出窗口，发现提示This algorithm is patented and is excluded in this configuration; Set OPENCV_ENABLE_NONFREE CMake option and rebuild the library in function ‘cv::xfeatures2d::SIFT::create’。 解决方法：重新cmake一次，在cmake时勾选OPENCV_ENABLE_NONFREE可以解决。
