Ubuntu16.04安装eigen3

编译错误：

error: ‘face::EigenFaceRecognizer’ has not been declared
             model_ = face::EigenFaceRecognizer::create(config.model_num_components,
                            ^~~~~~~~~~~~~~~~~~~
/home/star/rikirobot/catkin_ws/src/opencv_apps/src/nodelet/face_recognition_nodelet.cpp:476:28: error: ‘face::FisherFaceRecognizer’ has not been declared
             model_ = face::FisherFaceRecognizer::create(config.model_num_components,
                            ^~~~~~~~~~~~~~~~~~~~
/home/star/rikirobot/catkin_ws/src/opencv_apps/src/nodelet/face_recognition_nodelet.cpp:484:48: error: ‘create’ is not a member of ‘cv::face::LBPHFaceRecognizer’
             model_ = face::LBPHFaceRecognizer::create(config.lbph_radius,
                                                ^~~~~~
opencv_apps/CMakeFiles/opencv_apps.dir/build.make:494: recipe for target 'opencv_apps/CMakeFiles/opencv_apps.dir/src/nodelet/face_recognition_nodelet.cpp.o' failed
make[2]: *** [opencv_apps/CMakeFiles/opencv_apps.dir/src/nodelet/face_recognition_nodelet.cpp.o] Error 1
CMakeFiles/Makefile2:11736: recipe for target 'opencv_apps/CMakeFiles/opencv_apps.dir/all' failed
make[1]: *** [opencv_apps/CMakeFiles/opencv_apps.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2
Invoking "make -j1" failed

出现错误的原因是找不到Eigen3里面的函数，只需要安装一下eigen3这个库即可。
解决方法：
1、首先，下载eigen3的文件包，地址：http://eigen.tuxfamily.org/index.php?title=Main_Page 在该网站中，可以下载任意版本对应的文件，我下载的是3.2.10版本的
2、解压缩，将安装包的名称重命名为eigen3。
安装Eigen3.2.10

cd eigen3

sudo mkdir build

sudo cmake ..

sudo make

sudo make install 

然后，运行以下命令：

sudo cp -r /usr/local/include/eigen3/Eigen /usr/local/include