I tried following the official tutorial on how to build and install openCV, but it fails: https://docs.opencv.org/master/d7/d9f/tutorial_linux_install.html
Many other tutorials install:
sudo apt-get install libopencv-dev
Is libopencv-dev needed (the official tutorial does not mention it)? Out of desperation I am downloading the 350MB and installing it (my harddrive is only 20GB).