I see no reason why a visual effect needs Distance. IMO square should look just fine. Untested. Oh. Also reduced flake count at Randi's request.
#include <QApplication>
#include "mainform.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MyWindow *mainWin = new MyWindow;
mainWin->show();
return app.exec();
}