OpenCV videoCapture 为 iOS 应用程序文档目录中的文件提供了错误的属性

Question

I am working on an iOS app that records video then does some processing on that video to create another video file. I'm getting really odd behavior from videoCapture though. It acts like the file has been opened but get() calls seem to return 1 for all the properties I care about (and I know they are not 1). My code looks like this:

VideoCapture cap(fileIn); // open the video file for reading

if ( !cap.isOpened() )  // if not success, exit program
  {
    cout << "Cannot open the video file" << endl;
    return -1;
  }
  else{
    cout << "File Open " << fileIn << endl;
  }

  double dFps = cap.get(CV_CAP_PROP_FPS); //get the frames per seconds of the video
  cout << "Frames per second = " << dFps << endl;

  double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH); //get the width of frames of the video
  double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT); //get the height of frames of the video
  cout << "Frame Size = " << dWidth << "x" << dHeight << endl;

  Size frameSize(static_cast<int>(dWidth), static_cast<int>(dHeight));

  double dFrameCount = cap.get(CV_CAP_PROP_FRAME_COUNT);
  cout << "Frame count = " << dFrameCount << endl;

  VideoWriter oVideoWriter (fileOut, CV_FOURCC('m','p', '4', 'v'), dFps, frameSize, true); //initialize the VideoWriter object

  if ( !oVideoWriter.isOpened() ) //if not initialize the VideoWriter successfully, exit the program
  {
    cout << "ERROR: Failed to write the video" << endl;
    return -1;
  }
  else{
    cout << "Output file open." << endl;
  }

and the console output is:

File Open /var/mobile/Containers/Data/Application/7CEE8B87-BD50-4172-8F62-A74AD7F3C45A/Documents/15318B74-07D9-43EE-932F-224EDCA9CDA7.mov
Frames per second = 1
Frame Size = 1x1
Frame count = 1
Output file open.

I tried changing the file path to an invalid file in case it wasn't finding the file at all but that gave me "Cannot open the video file". Any ideas would be appreciated. I'm thinking it could possibly be a permissions issue because I saw similar behavior when I tried to access a video in the camera roll with this code but this file is in my app's documents directory so I should have full access to it.

Answer 1

It appears that this has been an issue for at least the last 2 years ( OpenCV on iOS - VideoCapture properties always return 1 ) and that the iOS version of videocapture simply doesn't work for opening files.

I found an alternate way to do this (opening the video with apple apis then converting each frame to an opencv Mat) but in general it seems that the iOS port of opencv is far from complete and nearly completely undocumented.

Answer 2

First, get the URL of video file from UIImagePickerController object

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any]) {
    picker.dismiss(animated: true, completion: nil)
    
    let url = info[UIImagePickerController.InfoKey.mediaURL] as? NSURL
    let asset   = AVURLAsset.init(url: url! as URL)
    
    
    let mainstoryBoard = UIStoryboard(name:"Main", bundle: nil)
    let viewcontroller = mainstoryBoard.instantiateViewController(withIdentifier:"ViewController") as! ViewController
    viewcontroller.url = url
    viewcontroller.asset = asset
    self.navigationController?.pushViewController(viewcontroller, animated: true) 
}

After that, use this url to send to cvwrapper.mm file as shown in openVideoFile function

- (void)openVideoFile:(NSURL *)fileName {
    
    NSString *filename = fileName.path;
    std::string filePath = [filename UTF8String];
    
    cv::VideoCapture capture(filePath);
    
    if (!capture.isOpened()) {
        printf("@Error Opening video file");
        // failed, print error message
    }
    else {
        printf("File Opened");
    }
}