OpenCV build steps:
download opencv and contrib

1, CMake 3.16.1, python 3.7
2, Create build folder
3, Click Configure
4, Search test and java, remove
5, Search "free", select OPENCV_ENABLE_NOFREE
6, Search extra_modules_path, select to opencv_contrib path to module folder
7, search world, select to build dll one for all, otherwise don't select
8, Click configure again and again. // https://blog.csdn.net/free_luojing/article/details/123261061. CMakeDownloadLog.txt, https://ghproxy.com/
9, click Generate
10, open vs project such as Opencv.sln
11, Build->batch build-> select build all(debug and release) and install(debug and release)
12, usefull file in install folder