Develop Reference

Build OpenCV for Android with CMake's ExternalProject_Add

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:
Ninja
C++ compiler
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?

Fail to create new Android NDK project

I want to learn how to write code in Android NDK.
I open the 'android studio guide' and on the 'create new ndk project' I follow the instruction.
I just create new project and add support for C++ code and that's it .. all the IDE code is in red and nothing can be compile.
Can't find any problem and I don't know what to do.
Tried to sync by clicking 'Try again' in first - and still fail.
The error message is:
CMake Error at
C:/Users/AppData/Local/Android/sdk/ndk-bundle/build/cmake/android.toolchain.cmake:40
(cmake_minimum_required): CMake 3.6.0 or higher is required. You
are running version 3.4.1 Call Stack (most recent call first):
C:/Users/AppData/Local/Android/sdk/cmake/share/cmake-3.4/Modules/CMakeDetermineSystem.cmake:98
(include) CMakeLists.txt CMake Error: CMAKE_C_COMPILER not set,
after EnableLanguage CMake Error: CMAKE_CXX_COMPILER not set, after
EnableLanguage
-- Configuring incomplete, errors occurred! Build command failed. Error while executing process
C:\Users\AppData\Local\Android\sdk\cmake\bin\cmake.exe with arguments
{-HC:\WORKSPACE\Android\Practice\NDK\Practice_001\app
-BC:\WORKSPACE\Android\Practice\NDK\Practice_001\app.externalNativeBuild\cmake\debug\armeabi
-GAndroid Gradle - Ninja -DANDROID_ABI=armeabi -DANDROID_NDK=C:\Users\Rn\AppData\Local\Android\sdk\ndk-bundle -DCMAKE_LIBRARY_OUTPUT_DIRECTORY=C:\WORKSPACE\Android\Practice\NDK\Practice_001\app\build\intermediates\cmake\debug\obj\armeabi
-DCMAKE_BUILD_TYPE=Debug -DCMAKE_MAKE_PROGRAM=C:\Users\AppData\Local\Android\sdk\cmake\bin\ninja.exe
-DCMAKE_TOOLCHAIN_FILE=C:\Users\AppData\Local\Android\sdk\ndk-bundle\build\cmake\android.toolchain.cmake
-DANDROID_PLATFORM=android-24 -DCMAKE_CXX_FLAGS=-frtti -fexceptions} CMake Error at
C:/Users/AppData/Local/Android/sdk/ndk-bundle/build/cmake/android.toolchain.cmake:40
(cmake_minimum_required): CMake 3.6.0 or higher is required. You
are running version 3.4.1 Call Stack (most recent call first):
C:/Users/AppData/Local/Android/sdk/cmake/share/cmake-3.4/Modules/CMakeDetermineSystem.cmake:98
(include) CMakeLists.txt CMake Error: CMAKE_C_COMPILER not set,
after EnableLanguage CMake Error: CMAKE_CXX_COMPILER not set, after
EnableLanguage
-- Configuring incomplete, errors occurred!
Thanks for any help.

Click on the "Try again" in yellow banner. If you got the error still probably problem is with you do not have necessary library packages in your build.gradle file. Check if you have added necessary Support Library Packages in your build.gradle file. If doesn't then add it and "rebuild" your project.

Your error message is:
CMake 3.6.0 or higher is required. You are running version 3.4.1
I do not know whether you have your own CMake installation or you are attempting to use one that comes from Android Studio. My guess is that you have your own CMake installation. If so, upgrade it to 3.6.0 or higher.

Checking preprocessor directives with Android Studio Gradle Build, NDK and CMake

I'm passing in preprocessor directives via CMakeLists.txt for the build of a native android library using android NDK.
add_definitions(-DMY_DIRECTIVE=1)
It would be great to double check that those preprocessor directives are actually finding their way into the calls to the compiler (llvm ?)
But the gradle build output doesn't seem to include the calls to the compiler, I just get:
Building C object CMakeFiles/my_project.dir/home/me/projects/my_proj/src/my_native.c.o
Is there a means to make the gradle output more verbose such that I can see the actual compiler calls and check those preprocessor directives are present?

The answer is to understand that Gradle utilises CMake to build the android NDK component (shared library) of an android project, and CMake utilises Ninja as a build system to handle the calls to the compiler. The compiler used by android NDK now defaults to LLVM->Clang.
So in order to actually see the Clang calls you have to find the build.ninja files for each target of your android project.
In my case I am only building for an armeabi-v7a target architecture. Therefore the relevant build.ninja files are found in:
/home/me/projects/my_proj/app/.externalNativeBuild/cmake/debug/armeabi-v7a
/home/me/projects/my_proj/app/.externalNativeBuild/cmake/release/armeabi-v7a
cd to either directory and run:
ninja -v
i.e. the -v option is the key to see all the calls to the clang compiler that the native build (android NDK) part of your android project generates.
Note if you have installed CMake via the android package manager, you may find that ninja is not installed in a location that is on your PATH. For me the ninja binary is located as follows:
~/Android/Sdk/cmake/3.6.3155560/bin/ninja
(same directory as cmake binary)
Therefore for me to see all the clang compiler calls for my android project's debug armeabi-v7a build I have to run:
cd /home/me/projects/my_proj/app/.externalNativeBuild/cmake/debug/armeabi-v7a
~/Android/Sdk/cmake/3.6.3155560/bin/ninja -v
Note if ninja tells you ninja: no work to do.
Then run:
~/Android/Sdk/cmake/3.6.3155560/bin/ninja clean
Relevant ninja documentation is -> https://ninja-build.org/manual.html#_extra_tools

Compile vtk and ves libraries for Android on Linux unbuntu64

I have followed the instructions in this link: http://www.vtk.org/Wiki/VES/Developers_Guide
I have downloaded VES, Andoid ndk-r8b, Android Studio (which has Android sdk), adb, ant.
And following the instructions:
export ANDROID_NDK=/home/ahrgm/Downloads/android-ndk-r8b
cmake -P configure.cmake
cd build
make -j4
the following error appears:
loading initial cache file
/home/ahrgm/VES/CMake/toolchains/TryRunResults.cmake CMake Error at
/home/ahrgm/VES/CMake/toolchains/android.toolchain.cmake:745
(message): Could not find any working toolchain in the NDK.
Probably your Android NDK is broken. Call Stack (most recent call
first): /usr/share/cmake-2.8/Modules/CMakeDetermineSystem.cmake:93
(include) CMakeLists.txt:3 (project)
CMake Error: Error required internal CMake variable not set, cmake may
be not be built correctly. Missing variable is:
CMAKE_C_COMPILER_ENV_VAR CMake Error: Error required internal CMake
variable not set, cmake may be not be built correctly. Missing
variable is: CMAKE_C_COMPILER CMake Error: Could not find cmake module
file:
/home/ahrgm/VES/Apps/Android/CMakeBuild/build/CMakeExternals/Build/vtk-android/CMakeFiles/2.8.12.2/CMakeCCompiler.cmake
CMake Error: Error required internal CMake variable not set, cmake may
be not be built correctly. Missing variable is:
CMAKE_CXX_COMPILER_ENV_VAR CMake Error: Error required internal CMake
variable not set, cmake may be not be built correctly. Missing
variable is: CMAKE_CXX_COMPILER CMake Error: Could not find cmake
module file:
/home/ahrgm/VES/Apps/Android/CMakeBuild/build/CMakeExternals/Build/vtk-android/CMakeFiles/2.8.12.2/CMakeCXXCompiler.cmake
CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage CMake
Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred! make[2]: * [CMakeExternals/Stamp/vtk-android/vtk-android-configure] Error 1
make[1]: * [CMakeFiles/vtk-android.dir/all] Error 2 make: *** [all]
Error 2
How can I solve this problem?

andengine compileReleaseNdk error

I want to use andengine in my android studio project but I have ndk error while building.
Error:Execution failed for task ':andEngine:compileReleaseNdk'.
> com.android.ide.common.internal.LoggedErrorException: Failed to run command:
D:\Android\android-ndk-r9d\ndk-build.cmd NDK_PROJECT_PATH=null APP_BUILD_SCRIPT=D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\Android.mk APP_PLATFORM=android-19 NDK_OUT=D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\obj NDK_LIBS_OUT=D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\lib APP_ABI=all
Error Code:
2
Output:
D:/Android/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/windows-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\obj/local/armeabi-v7a/objs/andengine_shared/D_\Android\workspace\simpleclock\simple_clock_as\andEngine\src\main\jni\src\GLES20Fix.o: in function Java_org_andengine_opengl_GLES20Fix_glVertexAttribPointer:GLES20Fix.c(.text.Java_org_andengine_opengl_GLES20Fix_glVertexAttribPointer+0x40): error: undefined reference to 'glVertexAttribPointer'
D:/Android/android-ndk-r9d/toolchains/arm-linux-androideabi-4.6/prebuilt/windows-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\obj/local/armeabi-v7a/objs/andengine_shared/D_\Android\workspace\simpleclock\simple_clock_as\andEngine\src\main\jni\src\GLES20Fix.o: in function Java_org_andengine_opengl_GLES20Fix_glDrawElements:GLES20Fix.c(.text.Java_org_andengine_opengl_GLES20Fix_glDrawElements+0x30): error: undefined reference to 'glDrawElements'
collect2: ld returned 1 exit status
make.exe: *** [D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\obj/local/armeabi-v7a/libandengine_shared.so] Error 1
I suppose I'm missing some OpenGL files?

The Android Gradle plugin's NDK task does not actually use any Android.mk file that you may have provided in your jni/ folder. This was a great source of confusion for me until I figured that out.
It generates a intermediate Android.mk file during the build, based on parameters that you have set in your Gradle build script and on the content of your jni/ folder.
You can see this for yourself by inspecting the source for its NdkCompile task at https://android.googlesource.com/platform/tools/base/+/55aebda607efcc29b8d9d5e1a99d446e320ff288/build-system/gradle/src/main/groovy/com/android/build/gradle/tasks/NdkCompile.groovy.
Note the writeMakeFile(...) method on line 126.
This is why the error you are getting from the ndk-build command that runs as part of your Gradle build references the build script APP_BUILD_SCRIPT=D:\Android\workspace\simpleclock\simple_clock_as\andEngine\build\intermediates\ndk\release\Android.mk, not something like APP_BUILD_SCRIPT=D:\Android\workspace\simpleclock\simple_clock_as\andEngine\src\main\jni\Android.mk as you might expect and want it to.
There is no way of getting the Android Gradle plugin's NDK task to use your own Android.mk file (believe me if there was I would have found it!).
You have two options for getting your NDK code compiling as part of Gradle:
Figure out the correct configuration to put in your build.gradle so that the generated Android.mk file contains the required LOCAL_LDLIBS := -lGLESv2 line and any of the other lines from https://github.com/nicolasgramlich/AndEngine/blob/GLES2/jni/Android.mk that are required.
Write a custom NDK compile task that uses the AndEnginge's Android.mk file directly. I've recently had to do this myself for an NDK source set that requires more parameters than the Android Gradle plugin currently supports passing via Gradle, so it if comes to this I can provide help.
I think in this case option 1 is open and so of course the preferable solution.
Something like this added to the android defaultConfig block should work:
android {
defaultConfig {
ndk {
moduleName "myNDKModule"
stl "stlport_shared"
ldLibs "lGLESv2"
cFlags "-Werror"
}
}
}
Unfortunately I am very much not a C/native code expert (I know practically nothing) so cannot guess whether AndEngine needs LOCAL_MODULE_FILENAME and LOCAL_EXPORT_C_INCLUDES to be set to build correctly. If it does, you'll need to go with approach 2 (at least until if/when the Android Gradle NDK task supports configuring them). Though I have just checked out the AndEngine git repo and successfully run ndk-build after having removed them from its Android.mk file, which is promising.
(I found which NDK properties can be parametrised via inspection of https://android.googlesource.com/platform/tools/base/+/55aebda607efcc29b8d9d5e1a99d446e320ff288/build-system/gradle/src/main/groovy/com/android/build/gradle/internal/dsl/NdkConfigDsl.java).

I had a similar issue, and this video https://www.youtube.com/watch?v=0-rYK2oh8oo helped me to fix the build issues. Basically, you need to download (and extract) NDK from here: http://developer.android.com/ndk/downloads/index.html and specify the NDK location in Module Settings. Also, you need to alter the andEngine's build.gradle file to include
sourceSets{
main{
jni.srcDirs = []
}
}

Android.mk actually includes the line?
https://github.com/nicolasgramlich/AndEngine/blob/GLES2/jni/Android.mk#L10
LOCAL_LDLIBS := -lGLESv2
These errors indicate it.
error: undefined reference to 'glVertexAttribPointer'
error: undefined reference to 'glDrawElements'