If something goes wrong, you will want to first test that OpenCV is working properly.

You can do that by running the example files

cp -r /usr/share/doc/opencv-doc/examples ./
cd examples/c
./build-all.sh

There are 2 versions of OpenCV.

OpenCV 2 is the second version released in 2009.

This is why you have both folders /usr/include/opencv and /usr/include/opencv2.

For new projects, use OpenCV 2 only.

The only header needed is opencv2/opencv.hpp which includes all the others.

Note that:

pkg-config opencv --cflags

Only includes opencv, so it seems that to use opencv2, you have to

#include <opencv2/opencv.hpp>