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https://github.com/VlSomers/native-opencv-android-template

A tutorial for setting up OpenCV 4.6.0 (and other 4.x.y version) for Android in Android Studio with Native Development Kit (NDK) support for C++ development.
https://github.com/VlSomers/native-opencv-android-template

android android-ndk android-studio computer-vision cpp java java-native-interface jni kotlin kotlin-android ndk opencv opencv-android-release opencv-android-sdk opencv4android

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A tutorial for setting up OpenCV 4.6.0 (and other 4.x.y version) for Android in Android Studio with Native Development Kit (NDK) support for C++ development.

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# Native OpenCV for Android with Android NDK



A tutorial for setting up OpenCV 4.6.0 (and other 4.x.y versions) for Android in [Android Studio](https://developer.android.com/studio) with Native Development Kit (NDK) support.

[Android NDK](https://developer.android.com/ndk) enables you to implement your [OpenCV](https://opencv.org) image processing pipeline in C++ and call that C++ code from Android Kotlin/Java code through JNI ([Java Native Interface](https://en.wikipedia.org/wiki/Java_Native_Interface)).



This sample Android application displays a live camera feed with an OpenCV *adaptive threshold* filter applied on each frame. The OpenCV *adaptive threshold* call is performed in C++.



## Setup



| Tool      | Version |

| ---       |  ---    |

| [OpenCV](https://opencv.org) | 4.6.0

| [Android Studio](https://developer.android.com/studio) | 2021.2.1

| [Android Build Tool](https://developer.android.com/about) | 33.0

| [Android NDK](https://developer.android.com/ndk/guides) | 25.0

| [Kotlin](https://kotlinlang.org/docs/reference/) | 1.6.10

| [Gradle](https://gradle.org) | 7.2.1

| Mac OS | 12.4



## How to use this repository



1. [Download and Install Android Studio](https://developer.android.com/studio)



2. [Install NDK and CMake](https://developer.android.com/studio/projects/install-ndk.md)



3. Clone this repository as an Android Studio project :

     * In Android Studio, click on `File -> New -> Project from Version Control -> Git`

     * Paste this repository *Github URL*, choose a *project directory* and click next.



4. Install *OpenCV Android release* :

    * Download [OpenCV 4.6.0 Android release](https://sourceforge.net/projects/opencvlibrary/files/4.6.0/opencv-4.6.0-android-sdk.zip/download) or download latest available Android release on [OpenCV website](https://opencv.org/releases/).

    * Unzip downloaded file and put **OpenCV-android-sdk** directory on a path of your choice.



5. Link your *Android Studio* project to the *OpenCV Android SDK* you just downloaded :

    * Open [gradle.properties](gradle.properties) file and edit following line with your own *OpenCV Android SDK* directory path :



          opencvsdk=/Users/Example/Downloads/OpenCV-android-sdk



6. Sync Gradle and run the application on your Android Device!



Note: MainActivity is written in Kotlin but you can comment out the [Kotlin](/app/src/main/kotlin/com/example/nativeopencvandroidtemplate/MainActivity.kt) file and uncomment the [Java](/app/src/main/java/com/example/nativeopencvandroidtemplate/MainActivity.java) file to use Java.



## Bootstrap a new Android project with Native OpenCV support



Here are the steps to follow to create a new Android Studio project with native OpenCV support :



1. [Download and Install Android Studio](https://developer.android.com/studio)



2. [Install NDK and CMake](https://developer.android.com/studio/projects/install-ndk.md)



3. Create a new *Native Android Studio project* :

    * Select `File -> New -> New Project...` from the main menu.

    * Click `Phone and Tablet tab`, select `Native C++` and click next.

    * Choose an `Application Name`, select your favorite `language` (Kotlin or Java), choose `Minimum API level` (28 here) and select next.

    * Choose `Toolchain default` as *C++ standard* and click Finish.



4. Install *OpenCV Android release* :

    * Download [OpenCV 4.6.0 Android release](https://sourceforge.net/projects/opencvlibrary/files/4.6.0/opencv-4.6.0-android-sdk.zip/download) or download latest available Android release on [OpenCV website](https://opencv.org/releases/).

    * Unzip downloaded file and put **OpenCV-android-sdk** directory on a path of your choice.

    

5. Add *OpenCV Android SDK* as a module into your project :

    * Open [setting.gradle](settings.gradle) file and append these two lines.

    

          include ':opencv'

          project(':opencv').projectDir = new File(opencvsdk + '/sdk')

        

    * Open [gradle.properties](gradle.properties) file and append following line. Do not forget to use correct *OpenCV Android SDK* path for your machine. 

    

          opencvsdk=/Users/Example/Downloads/OpenCV-android-sdk

          

    * Open [build.gradle](app/build.gradle) file and add `implementation project(path: ':opencv')` to dependencies section :

    

          dependencies {

              ...

              implementation project(path: ':opencv')

          }

    

    * Click on `File -> Sync Project with Gradle Files`.

    

6. Add following config to app [build.gradle](app/build.gradle) file :

    * In `android -> defaultConfig -> externalNativeBuild -> cmake` section, put these three lines :

    

          cppFlags "-frtti -fexceptions"

          abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a'

          arguments "-DOpenCV_DIR=" + opencvsdk + "/sdk/native"

        

7. Add following config to [CMakeLists.txt](app/src/main/cpp/CMakeLists.txt) file :

    * Before `add_library` instruction, add three following lines :

    

          include_directories(${OpenCV_DIR}/jni/include)

          add_library( lib_opencv SHARED IMPORTED )

          set_target_properties(lib_opencv PROPERTIES IMPORTED_LOCATION ${OpenCV_DIR}/libs/${ANDROID_ABI}/libopencv_java4.so)

        

    * In `target_link_libraries` instruction arguments, add following line :

    

          lib_opencv

        

8. Add following *permissions* to your [AndroidManifest.xml](app/src/main/AndroidManifest.xml) file :



       

       

       

       

       

    

9. Create your *MainActivity* :

    * You can copy paste MainActivity [Kotlin](/app/src/main/kotlin/com/example/nativeopencvandroidtemplate/MainActivity.kt) or [Java](/app/src/main/java/com/example/nativeopencvandroidtemplate/MainActivity.java) file. Do not forget to adapt package name.

    

10. Create your *activity_main.xml* :

    * You can copy paste [activity_main.xml](/app/src/main/res/layout/activity_main.xml) file.

    

11. Add native code in *native-lib.cpp* :

    * You can copy paste [native-lib.cpp](app/src/main/cpp/native-lib.cpp) file. Do not forget to adapt the method name : 

    `Java_com_example_nativeopencvtemplate_MainActivity_adaptiveThresholdFromJNI`

    should be replaced with 

    `Java__MainActivity_adaptiveThresholdFromJNI`.

    

12. Sync Gradle and run the application on your Android Device!



![alt text](images/native-opencv-android-template-screenshot.jpg)



## Questions and Remarks



If you have any question or remark regarding this tutorial, feel free to open an issue.



## Acknowledgments



This tutorial was inspired by this very good [Github repository](https://github.com/leadrien/opencv_native_androidstudio).



## Keywords



Tutorial, Template, OpenCV 4, Android, Android Studio, Native, NDK, Native Development Kit, JNI, Java Native Interface, C++, Kotlin, Java