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I am creating an Android library that uses OpenCV via NDK. It doesn't require any Java parts of OpenCV, native parts only, so I decided not to use OpenCV for Android to reduce the size of the OpenCV code the library needs.

Now I need to download and build the native (C++) part of OpenCV using CMake so that the library won't require its users to download and install OpenCV on their own.

Currently my module's build.gradle contains this:

externalNativeBuild {
    cmake {
        cppFlags '-std=c++11 -frtti -fexceptions'
        arguments '-DANDROID_ARM_NEON=TRUE'
    }
}

And my CMakeLists.txt is:

cmake_minimum_required(VERSION 3.18.1)

project("mylib")

set(OPENCV_INSTALL_DIR ${CMAKE_BINARY_DIR}/opencv-install)

include(ExternalProject)
ExternalProject_Add(opencv
        GIT_REPOSITORY https://github.com/opencv/opencv.git
        GIT_TAG 4.5.5
        UPDATE_DISCONNECTED TRUE
        CMAKE_ARGS
            <OpenCV arguments I need>
            -DCMAKE_INSTALL_PREFIX=${OPENCV_INSTALL_DIR}
        )
include_directories(${OPENCV_INSTALL_DIR}/include)
link_directories(${OPENCV_INSTALL_DIR}/lib)
set(OPENCV_LIBRARIES <OpenCV modules I use>)

add_library(mylib SHARED mylib.cpp)

add_dependencies(mylib opencv)

find_library(log-lib log)

target_link_libraries(mylib ${log-lib} ${OPENCV_LIBRARIES})

I am using Android Studio. When I try to build the library, the downloading process goes fine, but at the configure step I get the following error:

FAILED: opencv-prefix/src/opencv-stamp/opencv-configure 
cmd.exe /C "cd /D <my lib module path>\.cxx\Debug\612145b5\x86_64\opencv-prefix\src\opencv-build && <my Android SDK path>\cmake\3.18.1\bin\cmake.exe -DBUILD_TESTS=OFF -DBUILD_PERF_TESTS=OFF -DBUILD_EXAMPLES=OFF -DBUILD_opencv_apps=OFF -DBUILD_JAVA=OFF -DBUILD_FAT_JAVA_LIB=OFF -DBUILD_opencv_python2=OFF -DBUILD_opencv_python3=OFF -DWITH_GTK=OFF -DWITH_WIN32UI=OFF -DWITH_FFMPEG=OFF -DWITH_V4L=OFF -DCPU_BASELINE=NEON -DCMAKE_INSTALL_PREFIX=<my lib module path>/.cxx/Debug/612145b5/x86_64/opencv-install -GNinja <my lib module path>/.cxx/Debug/612145b5/x86_64/opencv-prefix/src/opencv && <my Android SDK path>\cmake\3.18.1\bin\cmake.exe -E touch <my lib module path>/.cxx/Debug/612145b5/x86_64/opencv-prefix/src/opencv-stamp/opencv-configure"
CMake Error: CMake was unable to find a build program corresponding to "Ninja".  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage

As I see, CMake cannot find the following components:

  1. Ninja
  2. C++ compiler
  3. C compiler

I can get rid of the first error, if I add ninja's path from NDK to the PATH variable, I don't think thats how it should be done though. Neither do I want to pass CMAKE_CXX_COMPILER and CMAKE_C_COMPILER to CMake manually, as it seems like it should be done automatically by Android Studio.

So, how can I fix this?

Eyjafl
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  • So, the reason was that `ExternalProject_Add` doesn't forward any arguments to OpenCV. When I added `-DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM} -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER} -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}` to `CMAKE_ARGS` these errors disappeared. Then I had to also add `-DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY` to pass compiler checks, but now I'm stuck with a lot of errors about unknown types and headers. It seems that NDK just doesn't have C++ library large enough to build OpenCV with it. – Eyjafl May 08 '22 at 19:44
  • I also tried to add CL arguments obtained by this (https://stackoverflow.com/a/48555098/16360266) method, but to no avail – Eyjafl May 08 '22 at 20:05

0 Answers0