title: Building OpenCV for Android and using it with the NDK
body: <p>Context : I am currently developing an app on Android Studio for the Moverio BT 200 augmented reality glasses. I am using OpenCV, and specifically, the arUco module of the library. This module has to be used with the NDK. Also, it is not on the stable release, so I compiled the library myself (using this guide : <a href="https://zami0xzami.wordpress.com/2016/03/17/building-opencv-for-android-from-source/" rel="noreferrer">https://zami0xzami.wordpress.com/2016/03/17/building-opencv-for-android-from-source/</a>). The build of the library went well. After that, I made a android studio project (customOCVtest). I did it the way I always do when using OpenCV with Android Studio and the NDK, except this time it was with the custom build. I checked if the library was correctly loaded :</p>
  
  <pre><code>private static final String OCVdevTAG = "OCVmainAct";
  
  static {
      System.loadLibrary("native-lib");
      if(!OpenCVLoader.initDebug()) {
          Log.d(OCVdevTAG, "OpenCV not loaded");
      } else {
          Log.d(OCVdevTAG, "OpenCV loaded");
      }
  }
  </code></pre>
  
  <p>The library is indeed loaded when I build.</p>
  
  <p>Now comes the problem : When I try to actually use the arUco module in my native code (here is my native-lib.cpp) :</p>
  
  <pre><code>#include &lt;jni.h&gt;
  #include &lt;string&gt;
  #include &lt;opencv2/aruco.hpp&gt;
  
  extern "C" {
  jstring
  Java_com_jambonsama_customocvtest_MainActivity_stringFromJNI(
      JNIEnv *env,
      jobject /* this */) {
  std::string hello = "Hello from C++";
  cv::Ptr&lt;cv::aruco::Dictionary&gt; dict = cv::aruco::getPredefinedDictionary(cv::aruco::DICT_6X6_250);
  cv::Mat marker;
  cv::aruco::drawMarker(dict, 25, 200, marker, 1);
  return env-&gt;NewStringUTF(hello.c_str());
  }
  }
  </code></pre>
  
  <p>the gradle sync works, but I can't build. I get the following error :</p>
  
  <pre><code>Error:FAILURE: Build failed with an exception.
  
  * What went wrong:
  Execution failed for task ':app:externalNativeBuildDebug'.
  &gt; Build command failed.
  Error while executing 'C:\Users\JambonSama\AppData\Local\Android\Sdk\cmake\3.6.3155560\bin\cmake.exe' with arguments {--build C:\Users\JambonSama\AndroidStudioProjects\customOCVtest\app\.externalNativeBuild\cmake\debug\mips64 --target native-lib}
  [1/1] Linking CXX shared library ..\..\..\..\build\intermediates\cmake\debug\obj\mips64\libnative-lib.so
  FAILED: cmd.exe /C "cd . &amp;&amp; C:\Users\JambonSama\AppData\Local\Android\sdk\ndk-bundle\toolchains\llvm\prebuilt\windows-x86_64\bin\clang++.exe  -target mips64el-none-linux-android -gcc-toolchain C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64 --sysroot=C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/platforms/android-21/arch-mips64 -fPIC -g -DANDROID -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -Wa,--noexecstack -Wformat -Werror=format-security -fno-exceptions -fno-rtti -g -DANDROID -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -Wa,--noexecstack -Wformat -Werror=format-security -fno-exceptions -fno-rtti  -O0 -fno-limit-debug-info -O0 -fno-limit-debug-info  -Wl,--build-id -Wl,--warn-shared-textrel -Wl,--fatal-warnings -Wl,--no-undefined -Wl,-z,noexecstack -Qunused-arguments -Wl,-z,relro -Wl,-z,now -Wl,--build-id -Wl,--warn-shared-textrel -Wl,--fatal-warnings -Wl,--no-undefined -Wl,-z,noexecstack -Qunused-arguments -Wl,-z,relro -Wl,-z,now -shared -Wl,-soname,libnative-lib.so -o ..\..\..\..\build\intermediates\cmake\debug\obj\mips64\libnative-lib.so CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o  C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_aruco.a -llog -lm "C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/sources/cxx-stl/gnu-libstdc++/4.9/libs/mips64/libgnustl_static.a" &amp;&amp; cd ."
  C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Users/JambonSama/AppData/Local/Android/sdk/ndk-bundle/toolchains/mips64el-linux-android-4.9/prebuilt/windows-x86_64/lib/gcc/mips64el-linux-android/4.9.x/../../../../mips64el-linux-android/bin\ld: C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a(alloc.cpp.o): Relocations in generic ELF (EM: 40)
    C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a: error adding symbols: File in wrong format
    clang++.exe: error: linker command failed with exit code 1 (use -v to see invocation)
    ninja: build stopped: subcommand failed.
  
  
  * Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
  </code></pre>
  
  <p>In particular, this error message tells me that I tried to build with the following argument :</p>
  
  <pre><code>{--build C:\Users\JambonSama\AndroidStudioProjects\customOCVtest\app\.externalNativeBuild\cmake\debug\mips64 --target native-lib}
  </code></pre>
  
  <p>This argument is automatically generated by AS, and I can't find the file where it is passed. I believe that if I can find it, I can build for arm (which is what I want), instead of mips. The thing is, I can't find it. (And I'm not even sure that is indeed what I'm looking for. I'm just trying to make my project build for my glasses.)</p>
  
  <p>Here after is my CMakeLists.txt :</p>
  
  <pre><code># Sets the minimum version of CMake required to build the native
  # library. You should either keep the default value or only pass a
  # value of 3.4.0 or lower.
  
  cmake_minimum_required(VERSION 3.4.1)
  
  # Creates and names a library, sets it as either STATIC
  # or SHARED, and provides the relative paths to its source code.
  # You can define multiple libraries, and CMake builds it for you.
  # Gradle automatically packages shared libraries with your APK.
  
  add_library( # Sets the name of the library.
               native-lib
  
               # Sets the library as a shared library.
               SHARED
  
               # Provides a relative path to your source file(s).
               # Associated headers in the same location as their source
               # file are automatically included.
               src/main/cpp/native-lib.cpp )
  
  include_directories(C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/jni/include)
  
  # Searches for a specified prebuilt library and stores the path as a
  # variable. Because system libraries are included in the search path by
  # default, you only need to specify the name of the public NDK library
  # you want to add. CMake verifies that the library exists before
  # completing its build.
  
  find_library( # Sets the name of the path variable.
                log-lib
  
                # Specifies the name of the NDK library that
                # you want CMake to locate.
                log )
  
  # Specifies libraries CMake should link to your target library. You
  # can link multiple libraries, such as libraries you define in the
  # build script, prebuilt third-party libraries, or system libraries.
  
  
  target_link_libraries( # Specifies the target library.
                         native-lib
                         C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_core.a
                         C:/Libs/opencv_src/opencv/platforms/build_android_arm/install/sdk/native/libs/armeabi-v7a/libopencv_aruco.a
  
                         # Links the target library to the log library
                         # included in the NDK.
                         ${log-lib} )
  </code></pre>
  
  <p>here is my app build.gradle :</p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 25
      buildToolsVersion "25.0.1"
      defaultConfig {
          applicationId "com.jambonsama.customocvtest"
          minSdkVersion 14
          targetSdkVersion 25
          versionCode 1
          versionName "1.0"
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
          externalNativeBuild {
              cmake {
                  cppFlags ""
              }
          }
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
      externalNativeBuild {
          cmake {
              path "CMakeLists.txt"
          }
      }
      sourceSets { main { jni.srcDirs = ['src/main/jni', 'src/main/jniLibs/'] } }
  }
  
  dependencies {
      compile fileTree(include: ['*.jar'], dir: 'libs')
      androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
          exclude group: 'com.android.support', module: 'support-annotations'
      })
      compile 'com.android.support:appcompat-v7:25.0.1'
      testCompile 'junit:junit:4.12'
      compile project(':openCVLibrary310dev')
  }
  </code></pre>
  
  <p>and my openCVLibrary310dev build.gradle</p>
  
  <pre><code>apply plugin: 'com.android.library'
  
  android {
      compileSdkVersion 25
      buildToolsVersion "23.0.2"
  
      defaultConfig {
          ndk {
              abiFilter("armeabi-v7a")
          }
          minSdkVersion 14
          targetSdkVersion 25
      }
  
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
          }
      }
  }
  </code></pre>
  
  <p>What I tried, mainly by just copying stuff that helped other people solve problems that seemed similar to mines : </p>
  
  <pre><code>#set(CMAKE_SYSTEM_NAME Android)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>#set(CMAKE_TOOLCHAIN_FILE "Toolchain file" CACHE FILEPATH "C:/Libs/opencv_src/opencv/platforms/android.toolchain.cmake")
  </code></pre>
  
  <p>or</p>
  
  <pre><code>#set( CMAKE_CXX_COMPILER "C:/Libs/android-ndk-r13b/toolchains/arm-linux-androideabi-4.9/prebuilt/windows-x86_64/bin/arm-linux-androideabi-g++.exe" )
  </code></pre>
  
  <p>in the CMakeLists.txt</p>
  
  <p>and </p>
  
  <pre><code>splits {
      abi {
          enable true
          reset()
          include 'armeabi-v7a'
          universalApk true
      }
  }
  </code></pre>
  
  <p>or </p>
  
  <pre><code>tasks.getByPath(":app:linkMipsDebugRemoteDesktopSharedLibrary").enabled = false
  </code></pre>
  
  <p>in the build.gradle (I tried both because I didn't know which one was supposed to have it).</p>
  
  <p>Except for the very last bit of code (the task.getByPath...), the sync worked, but I couldn't build. For the last one, I couldn't even sync.</p>
  
  <p>I also tried to rebuild the library for mips, since the architecture seems to be the problem, but I have no idea how to do that.</p>
  
  <p>Where can I find the file where the following argument : </p>
  
  <pre><code>{--build C:\Users\JambonSama\AndroidStudioProjects\customOCVtest\app\.externalNativeBuild\cmake\debug\mips64 --target native-lib}
  </code></pre>
  
  <p>is passed when building? And / or how do I make Android Studio build my project for arm architecture?</p>