Building OpenCV for Android

This project stands as an entry point for building OpenCV and OpenCV Contrib for the Android Platform. Besides the two bash scripts from this repository, other scripts, toolchains and SDKs are necessary. I compiled a list in the tutorial below.

Why do it yourself?

OpenCV already offers pre-build Android bundles, available on sourceforge. Those bundles do not contain all the (if any) modules from contrib.

This was an issue for me, as I intended to use the Face Recognizer class, so I ended up building both OpenCV and OpenCV Contrib from sources.

Although the scripts are supposed to work both on Linux and Windows, I encountered more difficulties on Windows. In order to fix and avoid future frustration, I adapted some existing tools, performed troubleshooting and compiled this tutorial.

Requirements

• for Windows: bash support
• Git
• Python >=2.4 ^[1]
• CMake >=2.8
• JDK
• Apache Ant
• Android SDK Tools (version 25.2.5 or earlier ^[2])
• Android NDK (will be downloaded by the android-ndk-downloader)
• android-ndk-downloader (will be downloaded by setup.sh)
• android-cmake (will be downloaded by setup.sh)
• for Windows: MinGW/ GNU Make ^[3] or Ninja ^[4]

[1] Android NDK comes with python in ANDROID_NDK_ROOT/prebuilt/linux-<variant>/bin/ (for Linux), or python.exe in ANDROID_NDK_ROOT/prebuilt/windows-<variant>/bin/ (for Windows)

[2] Starting with SDK Tools 23.5.0, Google removed Ant build support. See: opencv/opencv#8460

[3] Android NDK comes with make.exe, in ANDROID_NDK_ROOT/prebuilt/windows-<variant>/bin/ directory

[4] In my experience, Ninja performs builds faster than MinGW Make and much faster than GNU Make

Setup

Clone repository and run setup.sh

$ git clone https://github.com/JDFind/build-opencv-for-android.git
$ cd build-opencv-for-android
$ ./setup.sh [-v <opencv_version>]

This script will clone and checkout the specified version (default 3.3.0) of OpenCV and OpenCV Contrib and the latest versions of android-ndk-downloader and android-cmake.

The android-ndk-downloader will automatically be launched. After specifying the version, the Android NDK will be downloaded under ./android-ndk-downloader.

Environment Variables

For the toolchain to work correctly, the following environment variables are needed:

• JAVA_HOME set to the root of JDK
• JAVA_HOME/bin added to PATH
• ANDROID_SDK_HOME set to the root of Android SDK directory
• ANDROID_SDK_HOME/bin added to PATH
• ANDROID_NDK_ROOT set to the root of Android NDK directory
• ANT_HOME set to the root of Apache Ant directory
• ANT_HOME/bin added to PATH
• ANT_HOME/bin added to PATH
• git added to PATH
• python added to PATH
• cmake added to PATH
• make or ninja added to PATH

Enabling Modules

To include extra modules in the Java Wrapper, you must follow the steps below:

• navigate to opencv_contrib/modules/<target_module>
• open CMakeLists.txt
• check the parameters of ocv_define_module and add java after WRAP keyword if not present
• if the ocv_define_module does not contain WRAP, add that before java

ocv_define_module(face opencv_core opencv_imgproc opencv_objdetect WRAP python java)

Note: re-running setup.sh restores the contents of opencv, opencv-contrib, android-cmake and ndk-downloader to their original state, overriding any changes

Building

After completing the steps above, run the appropriate script for your machine (build-linux, build-win-make or build-win-ninja).

$ ./build-win-ninja.sh

The final library will be located in android_opencv/opencv directory.

If you have any suggestions or questions regarding this project, feel free to submit issues or create pull requests.