cv::namedWindow() with Qt support crashes when used in another Qt app (Patch #919)

When using the function cv::namedWindow() with compiler-flag WITH_QT in a project that uses Qt, namedWindow() will crash if there is already another QApplication instance. The failed assertion is:

#!C
Q_ASSERT_X(!self, "QCoreApplication", "there should be only one application object");

This is caused in file [source:trunk/opencv/modules/highgui/src/window_QT.cpp:#L441 highgui/window_QT.cpp, line 441]. In this line, a new QApplication object is created, even if one already exists. This happens in projects that create their own QApplication and event loop, and then want to use cv::namedWindow (for later cv::imshow or whatever reason).

Affected version: OpenCV 2.2 and current trunk from SVN
Tested on: Windows 7 Prof. 32bit, Visual Studio 2008, OpenCV 2.2, options WITH_QT and WITH_QT_OPENGL

Code to reproduce:

#!C
#include <QApplication>
#include "opencv2/core/core.hpp" 
#include "opencv2/highgui/highgui.hpp" 

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    cv::namedWindow("test"); // CRASH!
    // it doesn't matter if namedWindow is called now
    // or later inside the event loop...
    return app.exec();
}

The same problem is described in http://tech.groups.yahoo.com/group/OpenCV/message/76941 . Although the resolutions and workarounds described there work, they don't solve the original problem: At the moment, you cannot use HighGUI with Qt backend in your own Qt applications.

I included a patch to fix this. It simply doesn't create a new QApp instance, if one already exists. It works in my environment. It will however cause problems when cvStartLoop is used for multi-threading (see comments), but that is not recommended right now anyway (see comments there, too).

Could somebody apply the patch, or can I commit the changes myself?

Regards, Linus Atorf