Question

I’ve been trying to use openCV 2.2’s new Qt functions (See http://opencv.willowgarage.com/documentation/cpp/qt_new_functions.html#index-1384) for a project but I keep getting the error:

1> main.cpp 1>main.obj : error
LNK2019: unresolved external symbol
“void __cdecl
cv::displayStatusBar(class
std::basic_string,class
std::allocator > const &,class
std::basic_string,class
std::allocator > const &,int)”
(?displayStatusBar@cv@@YAXABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0H@Z)
referenced in function _main
1>C:\Documents and Settings\Orange\my
documents\visual studio
2010\Projects\FYP\FYP.exe : fatal
error LNK1120: 1 unresolved externals

I’m using Visual Studio 2010, windows xp, qt 4.7.1 and openCV 2.2.
I have already added the core lib files for both openCV and qt in my project settings under properties>Linker>Input. I have tried compiling and running openCV code without experiencing any problem except when I tried to use the new Qt functions such as displayStatusBar(…) and createButton(…). I’ve been searching for a solution for a couple of days now and I still could not find one possibly because openCV2.2 was released only recently. Has anyone tried and encountered this problem and have a solution?

Btw the reason I want to use the openCV provided functions for creating simple windows and buttons instead of qt is because I wish to avoid having to convert between the Mat data type in openCV and QImage in Qt.

Thanks in advance to any replies.

Attached code:

#include <cv.h>
#include <highgui.h>

using namespace cv;

int main()
{
    namedWindow("Test", CV_WINDOW_AUTOSIZE);            
    // ...
    // Code links without problem when the following line is commented out.
    displayStatusBar("Test", "Hello World", 5000);  

    return 0;
}

Answer 1

I found out the problem so I think I should post an answer here for the benefit of others who may have experienced the same problem.

Basically the binaries provided for OpenCv 2.2 by default do NOT include the Qt new functions. To enable it, one needs to download the OpenCv source and use CMake to generate makefiles and source that include the Qt new functions and compile the source from scratch.

Here are roughly the steps to do:

1. First, install QT using the SDK from here:
   http://qt.nokia.com/downloads/ . NOTE: You will need to add the QT libs and header files to your system path and to your linker path so that your compiler can locate them. It is NOT simply download and install. For windows, see: http://developer.qt.nokia.com/doc/qt-4.8/install-win.html to find out how to add the environment variables. You will need to set them up for CMake to detect QT and generate the proper files. For linux users, see: http://blog.sudobits.com/2010/06/30/how-to-install-qt-on-ubuntu-10-04/
2. Download and install CMake from here:
   http://cmake.org/cmake/resources/software.html .
3. Download OpenCV’s source code for your OS from here:
   http://opencv.willowgarage.com/wiki/
4. Now run the GUI of CMake and use it to select the folder where you
   have extracted the OpenCV source. Select the type of OS you are
   compiling for i.e. Unix Makefiles, Visual Studio solution, XCode
   etc.
5. Now a list of options should appear. There will be an option for QT
   as well as QT with openGL. Select the first and the one with openGL
   if you require it. Next, click the “configure” button followed by “generate” button in
   CMake. The necessary project files would then be generated.
6. Depending on the
   type of project you created (Unix Makefiles, Visual Studio project
   etc.), you can now go to the folder where they are generated by
   CMake and compile the project. For e.g. if you chose CMake to
   generate Visual Studio solution, there would be a Visual Studio
   solution file in the folder. Open it and select Build All. If you
   chose Unix Makefiles just call “make” at the command line followed by “sudo make install”.
7. Now all you need is the libraries in the /lib folder and the header
   files in the /include folder. Copy this 2 folders to whereever you
   wish to place them. For e.g. on Ubuntu, after you have typed “sudo make
   install” they would have been automatically copied to /usr/local/lib
   and /usr/local/include. For windows user, they would be in the
   folder you specified in CMake.
8. To link your project (be it visual studio or using make in unix),
   you have to tell your linker where to find the libs and include
   files. You specify them as you would when you downloaded the OpenCV
   binaries.

Thats it. You should now have a working OpenCV with QT.

Instructions on how to compile from scratch is given on OpenCv’s webpage here: http://opencv.willowgarage.com/wiki/InstallGuide.