[C++] #include opencv2.xxx.h no such file or directory

• because /usr/include does not have opencv2 folder or opencv2/xxx.h header file

• If you installed opencv4, you might have installed the opencv installation path on /usr/local/ during installation.

• /usr/local/include/opencv4/opencv2 folder can be found to have the xxx.h header file!

따라서

#include <opencv4/opencv2/xxxx.h>

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c++ include 기본 경로를 보는 명령어는
echo | gcc -v -x c -E -
echo | gcc -v -x c++ -E -

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make했을때 경로에러가 난다면 Makefile 에

컴파일시 include 경로를 추가해주면 된다.