Question:- How can i check in opencv which of the extra modules i have built in linux

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as a human:

• just look into /usr/local/lib
• have a look at the "to be build" section in the cmake output
• cerr << cv::getBuildInformation(); (to retrieve that later)

from a program:

• #include "opencv2/opencv_modules.hpp" , then check, if e.g. HAVE_OPENCV_XFEATURES2D or HAVE_OPENCV_DNN are defined