null pointer in opencv (QT) when running

Question

Hello i have a Macbook pro with osx 10.7. I just installed Qt (nokia), opencv, cmake. I have never worked with any of those . So i started my first tutorial. I created a new QT Gui application. I changed my main.cpp to :

#include <QtGui/QApplication>
#include <opencv/cv.h>
#include <opencv/highgui.h> 
#include "mainwindow.h"

int main(int argc, char *argv[])
{
   QApplication a(argc, argv);
   MainWindow w;
   w.show();

   IplImage* img = 0;
   img = cvLoadImage("cat.jpg");
   cvNamedWindow("Example1", CV_WINDOW_AUTOSIZE);
   cvShowImage("Example1", img);
   cvWaitKey(0);
   cvReleaseImage(&img);
   cvDestroyWindow("Example1");

   return a.exec();

}

I also changed my .pro file:

QT       += core gui

TARGET = untitled1
TEMPLATE = app


INCLUDEPATH = /usr/local/include

SOURCES += main.cpp\
        mainwindow.cpp

HEADERS  += mainwindow.h

LIBS += -lm -lopencv_core -lopencv_highgui -lopencv_video -lopencv_imgproc

FORMS    += mainwindow.ui

for the rest :

the cat.jpg is in the qt project folder (next to main.cpp)

Now when i build this and run it. i get this error:

Starting /Users/olivierjanssens/untitled1-build-desktop-Desktop_Qt_4_7_4_for_GCC__Qt_SDK__Debug/untitled1.app/Contents/MacOS/untitled1...
OpenCV Error: Null pointer (NULL array pointer is passed) in cvGetMat, file /Users/olivierjanssens/source/OpenCV-2.3.1/modules/core/src/array.cpp, line 2382
terminate called throwing an exceptionThe program has unexpectedly finished.
/Users/olivierjanssens/untitled1-build-desktop-Desktop_Qt_4_7_4_for_GCC__Qt_SDK__Debug/untitled1.app/Contents/MacOS/untitled1 exited with code 0

The window opens but then it crashes and no image is shown.

What am i doing wrong ?

Answer 1

cat.jpg is probably not in the right directory.

If you don't supply a path it searches the current directory, which is probably /Users/olivierjanssens/untitled1-build-desktop-Desktop_Qt_4_7_4_for_GCC__Qt_SDK__Debug or /Users/olivierjanssens/untitled1-build-desktop-Desktop_Qt_4_7_4_for_GCC__Qt_SDK__Debug/untitled1.app/Contents/MacOS. The directory with main.cpp in it isn't searched.

Try supplying a full path to cvLoadImage() (something like "/Users/olivierjanssens/untitled1/cat.jpg")