1. Downloading pre-built OpenCV4

wget https://github.com/sol-prog/raspberry-pi-opencv/releases/download/opencv4rpi2.1/opencv-4.1.0-armhf.tar.bz2

2.  Extract the archive

tar xvf opencv-4.1.0-armhf.tar.bz2

3. Move the extracted archive to the /opt folder

sudo mv opencv-4.1.0 /opt

4. Remove the archive (optional)

rm opencv-4.1.0-armhf.tar.bz2

 5. Install video and image support packages

sudo apt install libjpeg-dev libtiff-dev libjasper-dev libpng-dev libwebp-dev libopenexr-dev
sudo apt install libavcodec-dev libavformat-dev libswscale-dev libv4l-dev libxvidcore-dev libx264-dev libdc1394-22-dev libgstreamer-plugins-base1.0-dev libgstreamer1.0-dev

6. Install packages needed for OpenCV's interface

sudo apt install libgtk-3-dev libqtgui4 libqtwebkit4 libqt4-test python3-pyqt5 

7. Install other OpenCV related packages

sudo apt install libatlas-base-dev liblapacke-dev gfortran

8. Add OpenCV to the system path

cd ~
echo 'export LD_LIBRARY_PATH=/opt/opencv-4.1.0/lib:$LD_LIBRARY_PATH' >> .bashrc
. .bashrc

9. Restart the terminal or log back into the RPi if using a SSH

10. Install git if necessary

sudo apt-get install git

11.  Clone a config file to use OpenCV for C++

git clone https://gist.github.com/sol-prog/ed383474872958081985de733eaf352d opencv_cpp_compile_settings
 cd opencv_cpp_compile_settings/
 sudo cp opencv.pc /usr/lib/arm-linux-gnueabihf/pkgconfig
 cd ~
 rm -rf opencv_cpp_compile_settings/

 12. Check OpenCV has installed and been correctly added to the system path

pkg-config --modversion opencv

1. Install CMake on the Raspberry Pi

sudo apt-get -y install cmake

2. Save the following under CMakeLists.txt (within the same directory as the project)

cmake_minimum_required(VERSION 2.8)
project( Project )
find_package( OpenCV REQUIRED )
include_directories( ${OpenCV_INCLUDE_DIRS} )
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
add_executable( Project main.cpp )
target_link_libraries( Project ${OpenCV_LIBS} )

3. Run cmake then make, and finally the output file

$ cmake .
....
$ make 
...
$ ./Project