cvQueryFrame(capture) always returns null

Question

Does anyone know why I keep getting null frames? I tried skipping the first five and still null.

int _tmain(int argc, char** argv)
{
CvCapture *capture  = cvCaptureFromFile(argv[1]);

int fps = (int)cvGetCaptureProperty(capture, CV_CAP_PROP_FPS);
IplImage* frame;
cvNamedWindow("video", CV_WINDOW_AUTOSIZE);
while(1)
{
    frame = cvQueryFrame(capture);
    if(!frame)
        break;
    cvShowImage("video", frame);
    char c = cvWaitKey(1000/fps);
    if(c == 33)
        break;
}
cvReleaseCapture( &capture);
cvDestroyWindow( "video" );

return 0;
}

Answer 1

Video file must UNCOMPRESSED avi! So actually I was getting null frames because cvCapture returned a null because my input video file was not uncompressed.

Answer 2

I use your code for test, then it run well with 'xvid' format video. I think OpenCV 'capture' function maybe process some popular and old format of videos. Video with format "H264" may be not work.

Answer 3

When cvCaptureFromFile() fails it returns NULL , and I suspect it is failing:

CvCapture *capture  = cvCaptureFromFile(argv[1]);
if (!capture)
{
    // print error, quit application
}

It usually fails for one of these reasons: either it can't find the file, or OpenCV doesn't know how to open it. For instance, .mkv files are not supported by OpenCV.