how to install opencv on ubuntu

https://linuxize.com/post/how-to-install-opencv-on-ubuntu-18-04/

sudo apt install build-essential cmake git pkg-config libgtk-3-dev
sudo apt install libavcodec-dev libavformat-dev libswscale-dev libv4l-dev libxvidcore-dev libx264-dev
sudo apt install libjpeg-dev libpng-dev libtiff-dev gfortran openexr libatlas-base-dev
sudo apt install python3-dev python3-numpy libtbb2 libtbb-dev libdc1394-22-dev

mkdir ~/opencv_build && cd ~/opencv_build
git clone https://github.com/opencv/opencv.git
git clone https://github.com/opencv/opencv_contrib.git

If you want to install an older version of OpenCV, cd to both opencv and opencv_contrib directories and run git checkout

cd ~/opencv_build/opencv
mkdir build && cd build 

cmake -D CMAKE_BUILD_TYPE=RELEASE \
    -D CMAKE_INSTALL_PREFIX=/usr/local \
    -D INSTALL_C_EXAMPLES=OFF \
    -D INSTALL_PYTHON_EXAMPLES=OFF \
    -D OPENCV_GENERATE_PKGCONFIG=ON \
    -D OPENCV_EXTRA_MODULES_PATH=~/opencv_build/opencv_contrib/modules \
    -D OPENCV_ENABLE_NONFREE=ON \
    -D BUILD_EXAMPLES=ON ..

make
sudo make install
sudo ldconfig

pkg-config --modversion opencv4