I have a static library written in C++ that I have compiled for iOS and can use in a iOS test application but I cannot use opencv for videoio.

When I try to use either

cv::VideoCapture videoReader(filePathThatExistInMySandboxDocumentsFolder.mp4);

or

cv::VideoWriter videoWriter(filePathToNotExistInMySandboxDocumentsFolder.mp4,
 cv::VideoWriter::fourcc('H', '2', '6', '4'), 30, cv::Size(1920, 1080), true); 

I get the following exception

[ WARN:[email protected]] global cap.cpp:196 open VIDEOIO(AVFOUNDATION): raised unknown C++ exception!

Looking at cap.cpp line 196 I can see that the backend is found and that the exception originated from line 150 createCapture but after this I start to get lost.

1. I have built OpenCV myself with this option cmake -DCMAKE_TOOLCHAIN_FILE=../platforms/ios/cmake/Toolchains/Toolchain-iPhoneOS_Xcode.cmake -DIOS_ARCH=arm64 -DCMAKE_INSTALL_PREFIX=$OCV_INSTALL -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF -DBUILD_opencv_objc=OFF -DBUILD_opencv_apps=OFF -DBUILD_TESTS=OFF -DBUILD_PERF_TESTS=OFF -DBUILD_EXAMPLES=OFF -DWITH_OPENEXR=OFF -DWITH_CAP_IOS=OFF .. WITH_CAP_IOS ON or OFF gives the same exception My static library is linked with these opencv libraries

libopencv_core.dylib
libopencv_video.dylib
libopencv_videoio.dylib
libopencv_imgproc.dylib
libopencv_imgcodecs.dylib
libopencv_features2d.dylib

2. I have given the application permission to use the camera in case there is a dependency to the camera.
3. Writing images to disk works
4. The same code works without issues on MacOS also using AVFoundation as the backend
5. In Xcode AVFoundation.framework is added to Build Phases -> Link Binary with Libraries
6. The same code runs without problems on MacOS with opencv built for that platform

and I have run out of ideas of what to check next to be able to read and write videos on iOS using OpenCV?

Updates since original post: Some helpful things that might be useful to others are to build OpenCV with -DCMAKE_BUILD_TYPE=Debug and -DOPENCV_VIDEOIO_DEBUG=ON the later outputs useful information about videoio. I can write MJPEG files by forcing backend to CAP_OPENCV_MJPEG instead of CAP_AVFOUNDATION

cv::VideoWriter videoWriter(filePathToNotExistInMySandboxDocumentsFolder.mp4, cv::CAP_OPENCV_MJPEG,
 cv::VideoWriter::fourcc('H', '2', '6', '4'), 30, cv::Size(1920, 1080), true); 

My conclusion is that, despite having added AVFoundation to my Xcode project, OpenCV linked in my C++ lib cannot find AVFoundation. So the question seems to be how to make AVFoundation visible to the C++ library?