How to compile the latest OpenCV + Aruco and import into Android Studio.

By following this guide, hopefully, you will have a working Android Studio project "template" from which you can build up on. Allowing you to create an AR app for Android using Java and/or c++ with JNI.

Credits goes to these links:

• https://stackoverflow.com/questions/40948953/building-opencv-for-android-and-using-it-with-the-ndk
• https://stackoverflow.com/questions/26016770/how-to-install-old-version-of-android-build-tools-from-command-line
• https://jeanvitor.com/cpp-opencv-windonws10-installing/

Tested on Windows 10 and MX Linux 18.2

Prerequisites:

Item	Link
Cmake 3.11 or greater	https://cmake.org/download/
Java JDK 1.8	https://adoptopenjdk.net/?variant=openjdk8&jvmVariant=hotspot
Python 3.* preferred	https://www.python.org/downloads/
latest OpenCV source	https://github.com/opencv/opencv
latest OpenCV contrib source	https://github.com/opencv/opencv_contrib
Android Studio 3.2	https://developer.android.com/studio/
android-ndk-r16b-windows-x86_64	Windows: https://dl.google.com/android/repository/android-ndk-r16b-windows-x86_64.zip Linux: https://dl.google.com/android/repository/android-ndk-r16b-linux-x86_64.zip
tools-r25.2.5	Windows: https://dl.google.com/android/repository/tools_r25.2.5-windows.zip Linux: https://dl.google.com/android/repository/tools_r25.2.5-linux.zip
Build tools	Windows: https://osdn.net/projects/mingw/releases/p15522 Linux: gcc, build-essential
latest Apache Ant	https://ant.apache.org/bindownload.cgi

• Notes: For Linux users, check out your package manager for available applications

Prologue; Initial setup:

• Notes:
  • If using MinGW install manager, choose: mingw32-base-bin and msys-base-bin
  • Add JAVA_HOME environment variable pointing to JDK installation directory
  • Add Python installation directory to PATH environment variable
  • After creating environment variables, restart PC.
• Setup project directory for building and easy access:
  • Create the following directories:
  • ../opencv_project (project root directory)
  • ../opencv_project/opencv
  • ../opencv_project/contrib
  • ../opencv_project/build
  • ../opencv_project/ndk-r16b
  • ../opencv_project/tools
  • ../opencv_project/ant
• Extract the following zip file contents into their directories:
  • opencv-master.zip into ../opencv_project/opencv
  • opencv_contrib-master.zip into ../opencv_project/contrib
  • android-ndk-r16b-windows-x86_64.zip into ../opencv_project/ndk-r16b
  • tools-r25.2.5.zip into ../opencv_project/tools
  • apache-ant-1.10.5-bin.zip into ../opencv_project/ant
• Download build-tools and platform using Windows: sdkmanager.bat or Linux: sdkmanager:
  • Open PowerShell or Terminal
  • Navigate to ../opencv_project/tools/bin
  • Execute Windows: .\sdkmanager.bat "build-tools;28.0.3" or Linux: ./sdkmanager "build-tools;28.0.3"
  • Execute Windows: .\sdkmanager.bat "platforms;android-27" or Linux: ./sdkmanager "platforms;android-27"
  • Execute Windows: .\sdkmanager.bat "platform-tools" or Linux: ./sdkmanager "platform-tools"
  • Note: To lookup package names use Windows: .\sdkmanager.bat --list or Linux: ./sdkmanager --list

1. step; Cmake gui; generating the project:

• Open Cmake gui
• Click Browse Source... find and add ../opencv_project/opencv
• Click Browse Build... find and add .../opencv_project/build
• Add the following entries by clicking Add Entry:
  • Name: ANDROID_NDK Type: Path Value: ../opencv_project/ndk-r16b
  • Name: ANDROID_NATIVE_API_LEVEL Type: String Value: 16
  • Name: ANDROID_SDK Type: Path Value: ../opencv_project/
  • Name: ANDROID_NDK_HOST_X64 Type: Bool Value: Ticked
  • Name: ANT_EXECUTABLE Type: File Path Value: Windows: ../opencv_project/ant/bin/ant.bat or Linux: ../opencv_project/ant/bin/ant
• Click Configure
• A window will pop up, make sure the following values are selected:
  • Windows: MinGW Makefiles or Linux: Unix Makefiles as selected generator
  • Specify toolchain file for cross-compiling option is selected
  • Click Next
  • Specify toolchain file location: ../opencv_project/opencv/platforms/android/android.toolchain.cmake
  • Click Finish
• Every thing is RED!
• Use Search box to check the following entries, if not found, add them:
  • ANDROID_SDK_TARGET Value: android-27 (the platform version that was downloaded in Prologue)
  • OPENCV_EXTRA_MODULES_PATH Value: ../opencv_project/contrib/modules
  • For Windows only: CMAKE_MAKE_PROGRAM Value: ../MINGW_INSTALL_DIR/bin/mingw32-make.exe
  • WITH_CAROTENE Value: Not ticked
  • BUILD_ZLIB Value: Ticked
• Click Configure
• If some entries are still RED, click Configure again
• Lastly Click Generate

2. step; Compiling the project:

• For Windows:
  • Navigate to ../MINGW_INSTALL_DIR/msys/1.0
  • Run msys.bat
  • In the CMD window:
    • Execute cd "../opencv_project/build/"
    • Execute mingw32-make (takes a while to finish ~40min)
    • Execute mingw32-make install
• For Linux:
  • Execute cd "../opencv_project/build/"
  • Execute make (takes a while to finish ~40min)
  • Execute make install

3. step; Android Studio:

• Open Android Studio
• Create a new project with these parameters:
  • Select Include C++ support
  • Choose minimum API 23
  • Pick Empty Activity
  • Select C++ standart: C++11
• Note: During the following steps, Android Studio might ask you to install missing packages, do so
• Switch from Android to Project view
• Make Android Studio use the old ndk:
  • Click File-> Project Structure...
  • Select SDK Location
  • Set the Android NDK location as ../opencv_project/ndk-r16b
  • Click OK
• Import the compiled opencv project as a module:
  • Open File-> New-> Import Module...
  • As Source directory select ../opencv_project/build/install/sdk/java
  • Leave the module name as it is (further in this guide OPENCV_MOD)
  • Click Next
  • Click Finish
• Remove uses-sdk tag from AndroidManifest.xml located in opencv_module_name/src/main
• Add OPENCV_MOD as a dependency to the main app:
  • Open File-> Project Structure...:
  • Select app from the list
  • Select Dependecies tab
  • Click the + sign: Select Module dependecy
  • Select OPENCV_MOD from the list
  • Click OK
  • Click OK in Project Structure
• Set the following values in OPENCV_MOD/build.gradle to the same values as in app/build.gradle:
• Expand OPENCV_MOD and app directories in project view and open both build.gradle
• Check compileSdkVersion is the same
• Check minSdkVersion is the same
• Check targetSdkVersion is the same
• Click Sync Now
• Creating jniLibs directory:
  • Expand app/src/main
  • Right click main directory
  • Select New-> Folder-> JNI Folder
  • Select Change folder Location
  • Input src/main/jniLibs into New Folder Location
  • Click Finish
• Open File Explorer navigate to ../opencv_project/build/install/sdk/native/libs:
  • Copy the armeabi-v7a directory into app/src/main/jniLibs
  • Click OK
• Add the following line in app/build.gradle file, inside externalNativeBuild{...} block:
  • ndk { abiFilter "armeabi-v7a" }
  • Click Sync Now
• Linking the libraries, open app/CMakeList.txt:
  • Note: Full path is required here; Remember to change PROJECT_NAME
  • Add the following lines after add_library(...) and before find_library(...):
  • include_directories("../opencv_project/build/install/sdk/native/jni/include")
  • link_directories("../AndroidStudioProjects/PROJECT_NAME/app/src/main/jniLibs/armeabi-v7a")
  • Add the following lines after find_library(...) and before target_link_libraries(...):
  • file(GLOB PARTYLIBS "../opencv_project/build/install/sdk/native/3rdparty/libs/armeabi-v7a/*.a")
  • file(GLOB CVLIBS "../opencv_project/build/install/sdk/native/staticlibs/armeabi-v7a/*.a")
  • Add the following lines inside target_link_libraries(...) method between native-lib and ${log-lib}:
  • ${CVLIBS}
  • ${PARTYLIBS}
  • ${CVLIBS}
  • Click Sync Now

Epilogue; Make sure it works:

• C++
• Navigate to app/src/main/cpp and open native-lib.cpp:
  • Add #include <opencv2/aruco.hpp>
  • Replace the contents of stringFromJNI function with:
  • std::string hello = "Hello from C++";
  • cv::Mat test;
  • hello += " test";
  • return env->NewStringUTF(hello.c_str());
• If errors are present, high chance that the libraries are not linked:
  • Check CMakeList.txt for typos
  • In project view, check if libopencv_java{number}.so is under jniLibs/armeabi-v7a directory
  • In explorer, check if ../opencv_project/build/install/sdk/native/staticlibs/armeabi-v7a/ contains all the built libs aruco, etc
• Java
• Navigate to app/src/main/java/com.example.me.PROJECT_NAME and open MainActivity.java:
  • Add import org.opencv.core.Mat;
  • Add System.loadLibrary("opencv_java4"); after System.loadLibrary("native-lib");
  • Replace the contents of onCreate method with:
  • super.onCreate(savedInstanceState);
  • setContentView(R.layout.activity_main);
  • Mat test = new Mat();
  • TextView tv = (TextView) findViewById(R.id.sample_text);
  • tv.setText(stringFromJNI() + " " + test.depth());
• If errors are present:
  • Check if OPENCV_MOD is set as a dependency
  • Check if the jniLibs folder is used instead of jni