Please read this if you have Opencv installed on your system, but fail to build the Opencv binding demo.
The Opencv binding capabilities for this library are demonstrated with a short graphical demo, included here.
Building the app using the provided Makefile ($ make demo) relies on the pkg-config utility.
Recently, with Opencv4 (and probably also with latest 3.x releases), the Opencv team seems to have dropped support of pkg-config (see opencv/opencv#13154).
The demo program has been tested using Opencv 4.10, build from source. The trick is to activate the pkg-config option when building Opencv with Cmake. I used:
cmake .. -DOPENCV_GENERATE_PKGCONFIG=ON -DWITH_FFMPEG=ON
make
sudo make install
However, while this indeed generates the opencv.pc file (at least with Opencv3.4), it does not install it on your system.
Thus the makefile for target demo fails.
You need to copy it from the opencv build folder to the pkg-config usual location.
From the OpenCv 'build' folder, something like this should be sufficient:
$ sudo cp unix-install/opencv.pc /usr/share/pkgconfig/
You can also try to copy it in /usr/local/lib/pkgconfig/, seems to be another usual location.
FWIW, in case it does not get generated with your Opencv release, here is the content of that file for release 4.10.
Name it opencv.pc and copy it to /usr/share/pkgconfig/.
Be sure to run $ sudo ldconfig after (re)installing Opencv!
# Package Information for pkg-config
prefix=/usr/local
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include/opencv4
Name: OpenCV
Description: Open Source Computer Vision Library
Version: 4.10.0
Libs: -L${exec_prefix}/lib -lopencv_gapi -lopencv_highgui -lopencv_ml -lopencv_objdetect -lopencv_photo -lopencv_stitching -lopencv_video -lopencv_calib3d -lopencv_features2d -lopencv_dnn -lopencv_flann -lopencv_videoio -lopencv_imgcodecs -lopencv_imgproc -lopencv_core
Libs.private: -ldl -lm -lpthread -lrt
Cflags: -I${includedir}