How to save cv::mat in opencv c++ function to image file in IOS for debugging

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I am building an image processing project in IOS using c++ opencv and objective c in Xcode. It captures every frames and does some image processing work. When debugging the image processing c++ function for every frames from the phone's camera (running the project in the iphone), i need to save cv::mat to image files in IOS. Then I can check these image files to debug image processing algorithms.

In Android, we can use cv::imwrite to write cv::mat to image files to the defined path of the android phone. However, it doesn't work in IOS. It maybe some protection mechanism of IOS system.

There should be another method to achieve it. Or display the cv::mat when running IOS-based opencv project in the phone.

Thanks very much!