2x1=10

Don’t judge me, I know how this title sounds. The harsh real­i­ty is — as per writ­ing of this post — that I always have a hard time get­ting CMake to rec­og­nize the right Python path when I’m using an Ana­con­da envi­ron­ment. In the­o­ry it’s just

git clone https://github.com/opencv/opencv.git opencv
mkdir opencv/build && cd opencv/build
cmake -DCMAKE_INSTALL_PREFIX=/usr/local  ..
make -j8
(sudo) make install

and there you go. It will hap­pi­ly pick up your environment’s Python instal­la­tion and build that for you. Of course, it looks more along the lines of

cmake \
    -DCMAKE_INSTALL_PREFIX="/usr/local" \
    -DOPENCV_EXTRA_MODULES_PATH="../opencv_contrib/modules" \
    -DBUILD_DOCS=OFF \
    -DBUILD_TESTS=OFF \
    -DBUILD_EXAMPLES=OFF \
    -DBUILD_PERF_TESTS=OFF \
    -DBUILD_opencv_dnn=OFF \
    -DENABLE_FAST_MATH=ON \
    -DWITH_OPENMP=ON \
    -DWITH_TBB=ON \
    -DMKL_WITH_TBB=ON \
    -DMKL_WITH_OPENMP=ON \
    -DCMAKE_CXX_COMPILER="/usr/bin/g++-5" \
    -DCMAKE_C_COMPILER="/usr/bin/gcc-5" \
    -DCUDA_HOST_COMPILER="/usr/bin/gcc-5" \
    -DCUDA_FAST_MATH=ON \
    -DCUDA_ARCH_BIN="5.2" \
    -DWITH_CUBLAS=ON \
    ..

When­ev­er you then add

conda activate my-environment

to the mix though, all hell breaks loose and you end up with par­tial­ly con­fig­ured Python 2 and no Python 3 sup­port at all. The trick seems to be not to rely on OpenCV’s stan­dard Python con­fig­u­ra­tion val­ues

PYTHON3_LIBRARY
PYTHON3_EXECUTABLE
PYTHON3_INCLUDE_DIR
PYTHON3_INCLUDE_DIR2
PYTHON3_NUMPY_INCLUDE_DIRS

but rather to use the seem­ing­ly undoc­u­ment­ed val­ues

PYTHON3_LIBRARIES
PYTHON3_INCLUDE_PATH

as well, giv­ing it the nice appear­ance of

cmake \
    -DCMAKE_BUILD_TYPE=RELEASE \
    -DCMAKE_INSTALL_PREFIX="/your/anaconda3" \
    -DOPENCV_EXTRA_MODULES_PATH="../opencv_contrib/modules" \
    -DBUILD_DOCS=OFF \
    -DBUILD_TESTS=OFF \
    -DBUILD_EXAMPLES=OFF \
    -DBUILD_PERF_TESTS=OFF \
    -DBUILD_opencv_dnn=ON \
    -DTINYDNN_USE_NNPACK=OFF \
    -DTINYDNN_USE_TBB=ON \
    -DTINYDNN_USE_OMP=ON \
    -DENABLE_FAST_MATH=ON \
    -DWITH_OPENMP=ON \
    -DWITH_TBB=ON \
    -DMKL_WITH_TBB=ON \
    -DMKL_WITH_OPENMP=ON \
    -DCMAKE_CXX_COMPILER="/usr/bin/g++-5" \
    -DCMAKE_C_COMPILER="/usr/bin/gcc-5" \
    -DCUDA_HOST_COMPILER="/usr/bin/gcc-5" \
    -DCUDA_FAST_MATH=ON \
    -DCUDA_ARCH_BIN="5.2" \
    -DWITH_CUBLAS=ON \
    -DBUILD_opencv_python2=OFF \
    -DPYTHON_EXECUTABLE="/your/anaconda3/bin/python3" \
    -DPYTHON_LIBRARY="/your/anaconda3/lib/libpython3.6m.so" \
    -DPYTHON3_LIBRARY="/your/anaconda3/lib/libpython3.6m.so" \
    -DPYTHON3_EXECUTABLE="/your/anaconda3/bin/python3" \
    -DPYTHON3_INCLUDE_DIR="/your/anaconda3/include/python3.6m" \
    -DPYTHON3_INCLUDE_DIR2="/your/anaconda3/include" \
    -DPYTHON3_NUMPY_INCLUDE_DIRS="/your/anaconda3/lib/python3.6/site-packages/numpy/core/include" \
    -DPYTHON3_INCLUDE_PATH="/your/anaconda3/include/python3.6m" \
    -DPYTHON3_LIBRARIES="/your/anaconda3/lib/libpython3.6m.so" \
    -DHDF5_C_LIBRARY_z="/your/anaconda3/lib/libz.so" \
    ..

And then it’ll do what you want it to do. Because I’m tired of retry­ing every­time, on github.com/sunsided/opencv-cmake is my repo with sup­port for the OpenCV Extras Mod­ule and some doc­u­men­ta­tion for this.

So, what paths go in there? Well, this blog post has a very flashy way of find­ing out.

For CMAKE_INSTALL_PREFIX you use

python3 -c "import sys; print(sys.prefix)"

so it will install OpenCV direct­ly into your Ana­con­da instal­la­tion.
For PYTHON3_EXECUTABLE you call

which python3

and PYTHON3_INCLUDE_DIR is giv­en by

python3 -c "from distutils.sysconfig import get_python_inc; print(get_python_inc())"

Final­ly, a PYTHON3_PACKAGES_PATH can be found using

python3 -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())"

which you a can use to deter­mine the PYTHON3_NUMPY_INCLUDE_DIRS, although CMake strange­ly did that part right for me.