报错如图

Check contents of vgg_generated_48.i ...
CMake Error at E:/opencv3.0/opencv/sourcesContrib/opencv_contrib-master/modules/xfeatures2d/cmake/download_vgg.cmake:13 (ocv_download):
Unknown CMake command "ocv_download".
Call Stack (most recent call first):
E:/opencv3.0/opencv/sourcesContrib/opencv_contrib-master/modules/xfeatures2d/cmake/download_vgg.cmake:20 (vgg_download)
E:/opencv3.0/opencv/sourcesContrib/opencv_contrib-master/modules/xfeatures2d/CMakeLists.txt:4 (include)