$ workon cv
$ cd ~/opencv
$ mkdir build
$ cd build
$ cmake -D CMAKE_BUILD_TYPE=RELEASE
    -D CMAKE_INSTALL_PREFIX=/usr/local
    -D INSTALL_PYTHON_EXAMPLES=ON
    -D INSTALL_C_EXAMPLES=OFF
    -D OPENCV_ENABLE_NONFREE=ON
    -D OPENCV_EXTRA_MODULES_PATH=~/opencv_contrib/modules
    -D PYTHON_EXECUTABLE=~/.virtualenvs/cv/bin/python
    -D BUILD_EXAMPLES=ON ..