How do I set up Qt windows for OpenCV in Ubuntu ?

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Hi, I've been following an OpenCV tutorial from a book. Prior to it, I have OpenCV already installed on my machine. I'm working with OpenCV & C++ on a terminal and compile my program with CMake. Here's my Ubuntu & OpenCV version :

Ubuntu 18.04.4 LTS
OpenCV version : 4.2.0

In the book whenever they use imshow(), it shows a window with a toolbar and status bar on it, mine is not. And later on the next few chapters, they still use this toolbar for many purposes. After searching for a while, I found out that it is using Qt GUI. So, I went on to install Qt using these following codes :

sudo apt-get install qtcreator
sudo apt-get install qt5-default

And when I checked my Qt version, this is what I got :

$ qmake --version                                                                  
QMake version 3.1
Using Qt version 5.9.5 in /usr/lib/x86_64-linux-gnu

So, now I have Qt installed in my machine, but how do I integrate this with my OpenCV so I can use it on my programs other than putting highgui lib ? I encountered this error :

The library is compiled without QT support in function

I've been looking around online but still don't know how. Do I need to entirely uninstall OpenCV and compile it again with QT using CMake ?

UPDATE

Apparently there was no other way beside completely uninstall OpenCV and then compile & reinstalling it with QT. This is what I do :

1. Uninstall OpenCV (already reinstalled 3 times)

   sudo apt-get purge '*opencv*'
   sudo find / -name "*opencv*" -exec rm -rf {} \;
   

2. Cloning OpenCV and OpenCV contrib from Github, and during building with CMake, put WITH_QT = ON. here's the full set up that I used :

   cmake -D CMAKE_BUILD_TYPE=RELEASE \
   -D CMAKE_INSTALL_PREFIX=/usr/local \
   -D INSTALL_C_EXAMPLES=ON \
   -D INSTALL_PYTHON_EXAMPLES=ON \
   -D OPENCV_GENERATE_PKGCONFIG=ON \ 
   -D WITH_TBB=ON \ 
   -D WITH_V4L=ON \ 
   -D WITH_QT=ON \ 
   -D WITH_OPENGL=ON \
   -D OPENCV_EXTRA_MODULES_PATH=~/opencv_build/opencv_contrib/modules \
   -D BUILD_EXAMPLES=ON ..
   

But why does I still get error on any project with QT on it (I have QT properties on my createButton function) even after I entirely reinstall, build and make OpenCV using WITH_QT=ON ? Does CMake failed to locate QT ? Here's the full exeption :

terminate called after throwing an instance of 'cv::Exception' what():  
OpenCV(4.2.0-dev) /home/raisa/opencv_build/opencv/modules/highgui/src/window.cpp:597:
error: (-213:The function/feature is not implemented)
The library is compiled without QT support in function 'createButton'

[2]    10467 abort (core dumped)  ./exerc13

Here's the function createButton that causing error

//Create buttons
createButton("Blur",  blurCallback,  NULL, QT_CHECKBOX, 0);
createButton("Grey",  greyCallback,  NULL, QT_RADIOBOX, 0);
createButton("RGB",   bgrCallback,   NULL, QT_RADIOBOX, 1);
createButton("Sobel", sobelCallback, NULL, QT_PUSH_BUTTON, 0);

Literally just because I have QT properties there and it's causing error "The library is compiled without QT support". Any advice ?

UPDATE 2

I read about explicit path definition for Qt integration instead of just putting WITH_QT=ON. So I decided to go with cmake-gui and ... (more)