Native OpenCV for Android with Android NDK

A tutorial for setting up OpenCV 4.5.5 (and other 4.x.y versions) for Android in Android Studio with Native Development Kit (NDK) support. Android NDK enables you to implement your OpenCV image processing pipeline in C++ and call that C++ code from Android Kotlin/Java code through JNI (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

How to use this repository

1. Download and Install Android Studio
2. Install NDK, CMake and LLDB
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 4.5.5 Android release or download latest available Android release on OpenCV website.
   • 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 file and edit following line with your own OpenCV Android SDK directory path :

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

Note: MainActivity is written in Kotlin but you can comment out the Kotlin file and uncomment the 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
2. Install NDK, CMake and LLDB
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 4.5.5 Android release or download latest available Android release on OpenCV website.
   • 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 file and append these two lines.

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

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

dependencies

cppFlags "-frtti -fexceptions" abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a' arguments "-DOpenCV_DIR=" + opencvsdk + "/sdk/native" 

include_directories($/jni/include) add_library( lib_opencv SHARED IMPORTED ) set_target_properties(lib_opencv PROPERTIES IMPORTED_LOCATION $/libs/$/libopencv_java4.so) 

lib_opencv 

• You can copy paste MainActivity Kotlin or Java file. Do not forget to adapt package name.

• You can copy paste activity_main.xml file.

• You can copy paste 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 .

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.

Keywords

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