Opencv: Null pointer (NULL array pointer is passed) in cvGetMat - c++

Question

I'm getting this error when trying to capture video from webcam using opencv. This is the code I'm using:
#include "opencv2/opencv.hpp"
using namespace cv;
using namespace std;
int main( int argc, const char** argv ) {
CvCapture *capture = cvCaptureFromCAM(0);
IplImage *frame;
cvNamedWindow("test");
while ( 1)
{
frame = cvQueryFrame(capture) ;
cvShowImage("test", frame);
int key = cvWaitKey(1);
if ( key == 27 ) break; // ESC key was pressed
}
// Memory deallocation
cvReleaseCapture(&capture);
cvDestroyWindow("test");
return 0;
}
The error occur at cvShowImage("text", frame):
OpenCV Error: Null pointer (NULL array pointer is passed) in cvGetMat,
file
/opt/local/var/macports/build/_opt_mports_dports_graphics_opencv/opencv/work/opencv-2.4.6.1/modules/core/src/array.cpp,
line 2382 libc++abi.dylib: terminate called throwing an exception
(lldb)
What is it error about and how can I fix it?

Answer 1

First you should check that whether there is a device at index 0, like this
if (!capture)
{
// print error and exit
cout << "ERROR: Capture is null!\n";
return -1;
}
Secondly, :
while ( capture)
{
frame = cvQueryFrame(capture) ;
cvShowImage("test", frame);
int key = cvWaitKey(1);
if ( key == 27 ) break; // ESC key was pressed
}
Hope this helps