Using OpenCV natively in C++ in a Flutter app with Dart FFI. Tested with Flutter 2.5.2.

Read the full articles:

• Mobile platorms: https://medium.com/flutter-community/integrating-c-library-in-a-flutter-app-using-dart-ffi-38a15e16bc14
• Desktop platorms: https://medium.com/flutter-community/integrating-c-library-in-a-desktop-flutter-app-using-dart-ffi-32560cb1169b

Run init.sh script from a scripts folder or do the following steps manually:

1. Download OpenCV for Android and iOS: https://opencv.org/releases/
2. Copy or create symlinks:
   • opencv2.framework to native_opencv/ios
   • OpenCV-android-sdk/sdk/native/jni/include to native_opencv
   • Contents of OpenCV-android-sdk/sdk/native/libs/** to native_opencv/android/src/main/jniLibs/**

Run init_windows.ps1 PowerShell script from a scripts folder or do the following steps manually:

1. Download OpenCV for Windows: https://opencv.org/releases/
2. Unpack it. Set env. variable OpenCV_DIR to unpacked ...\opencv\build folder
3. Create a hard link from native_opencv\ios\Classes\native_opencv.cpp to native_opencv_windows\windows\native_opencv.cpp
4. Make sure native_opencv_windows\windows\CMakeLists.txt contains correct .dll names (OpenCV_DEBUG_DLL_NAME,OpenCV_RELEASE_DLL_NAME)

Before doing anything else, you need to download OpenCV source code and build a framework by running opencv/platforms/apple/build_xcframework.py script.

Run init_macos.sh script from a scripts folder or do the following steps manually:

1. Create a hard link from native_opencv/ios/Classes/native_opencv.cpp to native_opencv_macos/macos/Classes/native_opencv.cpp
2. Copy opencv2.xcframework to native_opencv/macos

Before doing anything else, you need to download OpenCV source code and build libraries by following the official tutorial.

After making a build, set up an environmental variable OpenCV_DIR to the build's folder.

Run init_linux.sh script from a scripts folder or do the following steps manually:

1. Create a hard link from native_opencv/ios/Classes/native_opencv.cpp to native_opencv_linux/linux/native_opencv.cpp

  /snap/flutter/current/usr/include/x86_64-linux-gnu/gnu/stubs.h:7:11: fatal error: 'gnu/stubs-32.h' file not found
  # include <gnu/stubs-32.h>
            ^~~~~~~~~~~~~~~~
  1 error generated.
  ninja: build stopped: subcommand failed.

or

/snap/flutter/current/usr/bin/ld: warning: /usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/crt1.o: unsupported GNU_PROPERTY_TYPE (5) type: 0xc0008002
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Exception: Build process failed

Install flutter manually instead of from snap, see: https://flutter.dev/docs/get-started/install/linux#install-flutter-manually