Emaus: Compiling OpenCV...

Download OpenCV and the contributed functionality (extra modules), and extract them (will be placed in opencv-4.1.0 and opencv_contrib-4.1.0:

wget -O opencv.zip https://github.com/opencv/opencv/releases/download/4.1.0/opencv-4.1.0-docs.zip
unzip opencv.zip

wget -O opencv_contrib.zip https://github.com/opencv/opencv_contrib/archive/4.1.0.zip
unzip opencv_contrib.zip

cd ~/opencv-4.1.0/

Create a directory build where the build artifacts will be placed, then configure the build with cmake and build:

mkdir build && cd build

# This takes just a minute
cmake -D CMAKE_BUILD_TYPE=RELEASE \
    -D ENABLE_PRECOMPILED_HEADERS=OFF \
    -D CMAKE_INSTALL_PREFIX=/usr/local \
    -D INSTALL_PYTHON_EXAMPLES=ON \
    -D OPENCV_EXTRA_MODULES_PATH=~/opencv_contrib-4.1.0/modules \
    -D BUILD_EXAMPLES=ON ..

# This takes many hours (7h on mine)
make
# It is possible to run with several cores if you have good enough cooling

The compilation got stuck at the “final” stage, where cv2.cpp.o was being built. The green LED which symbolizes mmc0/SD card activity was lit green constantly and the terminal hanged (most likely due to constant swapping). After some time the compilation failed with an error similar to the following:

[ 85%] Building CXX object modules/python3/CMakeFiles/opencv_python3.dir/__/src2/cv2.cpp.o
c++: internal compiler error: Killed (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <file: usr="" share="" doc="" gcc-6="" readme.bugs=""> for instructions.
modules/python3/CMakeFiles/opencv_python3.dir/build.make:62: recipe for target 'modules/python3/CMakeFiles/opencv_python3.dir/__/src2/cv2.cpp.o' failed
make[2]: *** [modules/python3/CMakeFiles/opencv_python3.dir/__/src2/cv2.cpp.o] Error 4
CMakeFiles/Makefile2:24733: recipe for target 'modules/python3/CMakeFiles/opencv_python3.dir/all' failed
make[1]: *** [modules/python3/CMakeFiles/opencv_python3.dir/all] Error 2
Makefile:160: recipe for target 'all' failed
make: *** [all] Error 2

I managed to solve it by increasing the swap file size (taken from this tutorial) from 100MB (default size) to twice the RAM-size (about 2GB). This is done by temporarily making a change in the file /etc/dphys-swapfile:

# [...]
#CONF_SWAPSIZE=100

# [...]
CONF_SWAPFACTOR=2

The service that manages the swap file then has to be restarted:

sudo /etc/init.d/dphys-swapfile stop
sudo /etc/init.d/dphys-swapfile start

Typing the free -m command should then show something similar to:

             total       used       free     shared    buffers     cached
Mem:           859        621        238         43        117        416
-/+ buffers/cache:         87        772
Swap:         1718          0       1718

sudo make install
sudo ldconfig
sudo apt-get update
# Adding the newly compiled OpenCV library to a virtual environment
ln -s /usr/local/lib/python3.7/site-packages/cv2/python-3.7/cv2.cpython-37m-arm-linux-gnueabihf.so ~/venv/lib/python3.7/site-packages/cv2.so

Compiling OpenCV 4.1 on Raspberry Pi Jessie for Python 3.7.3