Using Dlib is not so difficult, as is a "header-only" library and then you don't need to compile it. However, as it took me some time to figure out the correct steps to integrate it with OpenCV for Windows and Linux, I write this short post as a quick-rough guide to get it started.
Download Dlib and extract the folder somewhere. In windows, I used C:\, in Linux /home/username on Linux. Open Qt Creator and select "Qt Console Application". Once the files are generated, go to the .pro file of your project, and add
INCLUDEPATH += C:\dlib-18.18
LIBS+= -lgdi32 -lcomctl32 -luser32 -lwinmm -lws2_32
INCLUDEPATH += /home/username/dlib-18.18
LIBS += -pthread
CONFIG += link_pkgconfig
PKGCONFIG += x11
[extra libraries are required only for the gcc compiler, the others should add them automatically]
Then, right click on the "Sources" folder of your project, and select "Add Existing File", and add dlib-18.18/dlib/all/source.cpp
That's it! Now let's have some random dlib code and see if it works:
In main.cpp, add #include <dlib/image_processing/frontal_face_detector.h> on top. Comment everything inside the main and add, instead
dlib::frontal_face_detector detector = dlib::get_frontal_face_detector();
this doesn't mean much, but it's just to try and see if it works. Now run qmake, and build everything. It should build without any error. You can run it, and use all the Dlib capability for your software.