openCV cvCreateVideoWriter always returns null

Question

I am trying to save a video file with openCV under Mandriva Linux 2009.1. Unfortunately cvCreateVideoWriter always returns null although I could see and modify both and mpeg fiels, in other words cvCaptureFromCam, cvCaptureFromAvi, cvRetrieveFrame is working.

I tried all the possible codecs from this tutorial and this codec page. I also tried 0 for uncompressed avi and -1 for a selection box in the fourcc parameter, but no choice has popped up and nothing happened. I also modified isColor to 0.

I also have write permissions in the working directory.

Here is the relevant code snippet:

    CvVideoWriter *writer = 0;
    int isColor = 1;
    int fps = 25;
    int frameW = 640;
    int frameH = 480;


    if( argc == 1 || (argc == 2 && strlen(argv[1]) == 1 && isdigit(argv[1][0])))
        capture = cvCaptureFromCAM( argc == 2 ? argv[1][0] - '0' : 0 );
    else if( argc == 2 ) {
        printf("argc == 2 :%s:\n", argv[1] );
        capture = cvCaptureFromFile( argv[1] );
    }

    cvInitSystem(argc,argv);

 writer = cvCreateVideoWriter("out.avi",CV_FOURCC('I','Y','U','V'),fps,cvSize(frameW,frameH),isColor);

What could be the problem?

Answer 1

This will depend on how your OpenCV library was configured. In particular I believe HAVE_FFMPEG must be defined in order for the 'IYUV' FOURCC value to be handled. Was your library built with HAVE_FFMPEG defined?

The default writer object (which is created when fourcc is zero, or the fourcc value is not handled by one of the available handlers) creates an image file per frame. In this case the filename needs to be something like "file%04d.foo" or "file0001.foo", otherwise writer creation fails. You might try passing something like "test0001.png" for the filename parameter as an experiment to verify that the default image writer is created successfully (this won't help directly, but might be a way to confirm that your fourcc value is not being recognized by an available writer.)

According to documentation, the option to open a selection dialog when the fourcc equal -1 is Windows-specific.