# Build OpenCV Applications with Clang ## In this learning path - [Introduction](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/) - [OpenCV and Compilers for Windows on Arm](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/1-opencv-compilers/) - [Setup](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/1.5-opencv-setup/) - [Build OpenCV Applications with Clang](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/2-2-build-clang/) - [Build OpenCV Applications with MSVC](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/2-1-build-msvc/) - [Next Steps](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/_next-steps/) Follow the steps below to build OpenCV and a test application using the library with the Clang compiler. ## How do I build OpenCV with Clang? ### Clone the OpenCV repository Open up a Windows PowerShell and checkout the source tree: ```bash git clone https://github.com/opencv/opencv cd opencv git checkout tags/4.10.0 ``` > You might be able to use a later version, but these steps have been tested with the version 4.10.0. ### Pre-build configuration You can use CMake from the command line. First, run the following command to run the pre-build configuration. ```bash mkdir build_clang cd build_clang cmake ` -S .. ` -B . ` -G "Ninja" ` -DCMAKE_C_COMPILER=clang ` -DCMAKE_CXX_COMPILER=clang++ ` -DCMAKE_BUILD_TYPE=Release ` -DBUILD_opencv_world=ON ` -DWITH_ITT=OFF ` -DWITH_OPENCL=OFF ` -DWITH_OPENCLAMDBLAS=OFF ` -DWITH_OPENCLAMDFFT=OFF ` -DWITH_OPENCL_D3D11_NV=OFF ` -DWITH_DIRECTML=OFF ` -DWITH_DIRECTX=OFF ` -DWITH_ADE=OFF ` -DWITH_CAROTENE=OFF ``` The given options specify the following: - The source code is located one level above the current directory. - The build will be performed in the current directory. - The Clang compiler will be used as the compiler. - The compiled library is generated as a single file that includes all of OpenCV’s functionality. - Unnecessary options have been disabled, assuming processing on Arm CPUs. If the configuration is successful, a message similar to the following should be displayed at the end of the execution: ``` -- General configuration for OpenCV 4.10.0 ===================================== -- Version control: 4.10.0 -- -- Platform: -- Timestamp: 2024-11-08T09:23:57Z -- Host: Windows 10.0.22631 ARM64 -- CMake: 3.28.1 -- CMake generator: Ninja -- CMake build tool: C:/Users/username/work/venv/Scripts/ninja.exe -- Configuration: Release -- -- CPU/HW features: -- Baseline: NEON -- requested: NEON FP16 -- Dispatched code generation: NEON_DOTPROD NEON_FP16 NEON_BF16 -- requested: NEON_FP16 NEON_BF16 NEON_DOTPROD -- NEON_DOTPROD (1 files): + NEON_DOTPROD -- NEON_FP16 (2 files): + NEON_FP16 -- NEON_BF16 (0 files): + NEON_BF16 -- -- C/C++: -- Built as dynamic libs?: YES -- C++ standard: 11 -- C++ Compiler: C:/Program Files/LLVM/bin/clang++.exe (ver 18.1.8) [...] -- C Compiler: C:/Program Files/LLVM/bin/clang.exe [...] -- Install to: C:/Users/username/work/opencv/build_clang/install -- ----------------------------------------------------------------- -- -- Configuring done (244.5s) -- Generating done (1.4s) -- Build files have been written to: C:/Users/username/work/opencv/build_clang ``` ### Build and install Run the following commands to build and install OpenCV: ```bash ninja ninja install ``` > The build takes approximately 25 mins on a Lenovo X13s. When the build and the install steps are complete, confirm the shared library has been created by inspecting the results in the `install/bin` directory: ```bash ls ./install/bin ``` ``` Mode LastWriteTime Length Name ---- ------------- ------ ---- -a---- 08/11/2024 09:51 40448 opencv_annotation.exe -a---- 08/11/2024 09:51 126464 opencv_interactive-calibration.exe -a---- 08/11/2024 09:51 40448 opencv_model_diagnostics.exe -a---- 08/11/2024 09:51 38400 opencv_version.exe -a---- 08/11/2024 09:51 35840 opencv_version_win32.exe -a---- 08/11/2024 09:23 26391552 opencv_videoio_ffmpeg4100_64.dll -a---- 08/11/2024 09:51 51712 opencv_visualisation.exe -a---- 08/11/2024 09:50 20207104 opencv_world4100.dll ``` Also inspect the `install/lib` directory: ```bash ls ./install/lib ``` ``` Directory: C:\Users\username\work\opencv\build_clang\install\lib Mode LastWriteTime Length Name ---- ------------- ------ ---- -a---- 08/11/2024 09:23 434 OpenCVConfig-version.cmake -a---- 08/11/2024 09:23 15254 OpenCVConfig.cmake -a---- 08/11/2024 09:23 936 OpenCVModules-release.cmake -a---- 08/11/2024 09:23 3749 OpenCVModules.cmake -a---- 08/11/2024 09:50 2862548 opencv_world4100.lib ``` The library used in your application is `opencv_world.lib/dll`. Once the library files are correctly generated, run the following command to ensure there are no errors. ```bash ./install/bin/opencv_version.exe ``` ``` 4.10.0 ``` ## Build OpenCV Applications Once the OpenCV library has been successfully created, you can create a simple application and try using it. ### Prepare an application program First, use a text editor to save the following C++ program as `test_opencv.cpp` in the `build_clang` directory. ```cpp #include #include int main() { cv::Mat image = cv::Mat::zeros(100, 100, CV_8UC3); if (image.empty()) { std::cout << "Failed to create an image!" << std::endl; return -1; } cv::circle(image, cv::Point(50, 50), 30, cv::Scalar(255, 0, 0), -1); cv::imwrite("test_image.png", image); cv::waitKey(0); return 0; } ``` This program is a simple example that uses OpenCV to create a 100x100 black image, draw a blue circle on it, and save it as a file. ### Compile the program Compile the code using the command below: ```bash clang++ .\test_opencv.cpp -o test_opencv.exe -I.\install\include -L.\install\lib -lopencv_world4100 ``` The given options specify the following: - **`-o`**: Specifies the name of the generated executable file. - **`-I`**: Indicates the directory where the OpenCV header files to be included are located. - **`-L`**: Specifies the directory where the libraries for linking are located. - **`-l`**: Specifies the name of the library to link. When linking `opencv_world4100.lib`, omit the `.lib` extension and specify it as `-lopencv_world4100`. ### Run the program To run the executable, you need to ensure that the directory containing the dynamic libraries (DLLs) is added to the `PATH` environment variable, or place the DLLs in the same location as the executable. > The command below adds the DLL directory to the beginning of the `PATH` environment variable. Since this is a temporary setting, the `PATH` will revert to its original state when the PowerShell session is closed. To set it permanently, you need to use the Windows system settings or the `[Environment]::SetEnvironmentVariable()` method. ```bash $env:PATH = "./install/bin;" + $env:PATH ``` Run the test program: ```bash .\test_opencv.exe ``` When you execute the command, it will finish quickly, and `test_image.png` is generated. If you do not have the DLL directory in your search path, the program appears to run, but no `test_image.png` is generated. Open the image file, it should look like the example shown below. ![test_image.png](https://learn.arm.com/learning-paths/laptops-and-desktops/win-opencv/test_image.png) Congratulations! You are now ready to create your own OpenCV applications using Clang.