How do i use only certain modules in OpenCV?

edit

i don't want to download the entirety of OpenCV.

you have to. but you can discard the src tree and build folders after installing your locally build libs.

I just want to use Gaussian blur, Blackhat, Canny, findContours, moments.

so you need the core and imgproc modules at least (how would you obtain images ?). you can restrict the build to those:

cmake -DBUILD_LIST="core,imgproc"

and later, only link those libs to your program (do NOT use cmake or pkg-config, because they will drag in all there is)

g++ my.cpp -I/usr/local/include/opencv4  -lopencv_imgproc -lopencv_core -o myprog