ios VideoWriter

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Hello,

I'm experimenting with writing videos in iOS 7.1, by simply copying one video file into another. Here's the code:

cv::VideoCapture iVideo(iFile); 
if ( !iVideo.isOpened() ){
    NSLog(@"Cannot open video capture file");
    return;
};

// -- a bunch of iVideo.get() calls to determine CV_CAP_PROP_***.  
// They all return "1".  Why??

cv::Size size(720, 480);  // Manually set, since we can't get() it.
int fourcc = CV_FOURCC('m', 'p', '4', 'v');  // Manually set since we can't get() it.

cv::VideoWriter videoWriter;
videoWriter.open(oFile, fourcc, 30, size, true);

if (!videoWriter.isOpened() ){
    NSLog(@"Cannot open video writing file");
    return;
}

cv::Mat frame;
iVideo >> frame;
int frameCount = 0;
while ( !frame.empty() && frameCount < 50){
    frameCount++;
    videoWriter << frame;  // <----- throws EXC_BAD_ACCESS here.
    iVideo >> frame;
}

The oddest thing is the code works when I'm targeting OS X. All CV_CAP_PROP_* are returned properly. But the same code doesn't work when targeting iOS.

Also, I've tried saving the frames using imwrite(). That works too.

I hope someone can help. Thanks!