Mat leaking memory using Canny

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I'm doing live video processing on iOS using OpenCV, without using CvVideoCamera. My app is crashing due to Memory Pressure.

The native iOS camera calls this function every time a frame is captured:

- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
       fromConnection:(AVCaptureConnection *)connection
{
    //convert the frame to a UIImage:
    UIImage *image = [self imageFromSampleBuffer:sampleBuffer];

    //convert the UIImage to a Mat:
    Mat srcMat = [self cvMatFromUIImage:image];

    //Process the Mat:
    Mat dst, cdst;
    Canny(srcMat, dst, 50, 200, 3);
    cvtColor(dst, cdst, COLOR_GRAY2BGR);
}

The app crashes about 15 seconds later due to memory pressure.

After some investigation, I found that the call to Canny() is responsible, but I can't figure out why...

It looks like the Mat objects are staying in memory, but aren't Mat objects released automatically?