Using opencv in linux

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Hello, I am just getting started with opencv, and I know that this has probably been asked plenty of times, but so far I haven’t been able to successfully compile any source file that uses opencv libraries. I followed this tutorial. Specifically, I’ve run the commands from the Build with opencv_contrib section of the aforementioned tutorial.

After this, I tried running the example given here just as the tutorial indicates, but got the following error:

CMake Error at CMakeLists.txt:3 (find_package):
By not providing “FindOpenCV.cmake” in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by “OpenCV”, but
CMake did not find one.
Could not find a package configuration file provided by “OpenCV” with any
of the following names:
OpenCVConfig.cmake
opencv-config.cmake
Add the installation prefix of “OpenCV” to CMAKE_PREFIX_PATH or set
“OpenCV_DIR” to a directory containing one of the above files. If “OpenCV”
provides a separate development package or SDK, be sure it has been
installed.

Obviously, I am not quite sure what I am doing or what should I do in order to fix it.
Finally, and out of curiosity, if I wanted to run the example without using cmake, how could this be done? As far as I know, I should be using the -I and -L flags so that the compiler is able to find the headers and the libraries, but I’m not 100% sure how this is done either.
Thank you in advance for your help.

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Well, the error message says it all. It doesn’t find the OpenCVConfig.cmake file, so you have to set the path manually. Add:

set(OpenCV_DIR "/usr/local/lib/cmake/opencv4")

(or whatever your path is) to the CMakeLists.txt file.

To build your app manually, it’s a bit more complicated, as there are a dozen lib files to link your application to. If you have OpenCV built with pkg-config enabled, it’s easier:

gcc mycode.cpp -o mycode `pkg-config opencv4 --cflags` `pkg-config opencv4 --libs`