iOS上的OpenCV - VideoCapture属性始终返回1

Question

我一直在尝试构建一个简单的OpenCV（iOS）应用程序，该应用程序从包中加载视频并查询其帧数，持续时间等，然后它将尝试从中获取单个帧。不幸的是，当我使用VideoCapture类时，所有属性都返回值1.然后我尝试导航到frame #100，但这也不起作用。我这样做是错误的吗？

cv::VideoCapture capture([[movieURL path] UTF8String]);
if (!capture.isOpened()) {
    Log(@"Could not open the video:%@", [movieURL path]);
}

double rate = capture.get(CV_CAP_PROP_FPS);

cv::Mat frame; //current video frame

//Delay between each frame in ms
//corresponds to video frame rate
int delay = 1000/rate;

long numOfFrames = static_cast<long>(capture.get(CV_CAP_PROP_FRAME_COUNT));

Log(@"Video Frame rate:%f, delay:%d, numOfFrames:%ld", rate, delay, numOfFrames);

double position = 100.0;
capture.set(CV_CAP_PROP_POS_FRAMES, position);

long frameNum = static_cast<long>(capture.get(CV_CAP_PROP_POS_MSEC));

Log(@"Moved to Frame 100. Getting frame number:%ld", frameNum);