My development environment is set up as follows:

• android-ndk-r10d (install path: D:\adt-bundle-windows-x86_64-20140702\android-ndk-r10d\)
• OpenCV-2.4.10-android-sdk (install path: D:\CODE\OpenCV-2.4.10-android-sdk\), Download link
• OpenCV-2.4.10 (install path: D:\CODE\OpenCV-2.4.10\), Download link

Building the nonfree module

1. We actually only need to copy a few files from OpenCV-2.4.10 source code to OpenCV-2.4.10-android-sdk, namely:
   Copy the nonfree folder from OpenCV-2.4.10\sources\modules\nonfree\include\opencv2\to OpenCV-2.4.10-android-sdk\sdk\native\jni\include\opencv2.

2. Create a folder to hold our new project for libnonfree.so. Here, I call it libnonfree. Create a jni folder under libnonfree. Copy the following files from OpenCV-2.4.10\sources\modules\nonfree\src to libnonfree\jni\ folder:

   • nonfree_init.cpp
   • precomp.hpp
   • sift.cpp (use the original file)
   • surf.cpp (use the original file)

3. Building libnonfree.so:
   Create Android.mk and Application.mk scripts. This Android.mk is used to build libnonfree.so.

   • Application.mk
   • Android.mk (you should modify OPENCV_PATH where your OpenCV-2.4.10-android-sdkis)

   cd into the project folder libnonfree and type ndk-build to build the libnonfree.so.

So far, you have got libnonfree.so along with libopencv_java.so and libgnustl_shared.so in libnonfree\libs\armeabi-v7a folder.
You can easily build any SIFT or SURF applications using those libraries. If you want to use SIFT and SURF in JAVA code in your Android application, you only need to write JNI interfaces for the functions you want to use.

Building a sample application

1. Create a project folder call libnonfree_demo. Create a jni folder inside the project folder. Then copy libnonfree.so along with libopencv_java.so and libgnustl_shared.so into jni.

2. Create a nonfree_jni.cpp in jni. It is simple SIFT test program. It basically reads an image and detects the keypoints, then extracts feature descriptors, finally draws the keypoints to an output image.

3. Create Android.mk and Application.mk inside jni:

   • Application.mk
   • Android.mk (you should modify OPENCV_PATH where your OpenCV-2.4.10-android-sdkis)

   cd into the project folder libnonfree_demo and type ndk-build to build the libnonfree_demo.so.

At this point you can easily extend the sample app with your SVMDetector. Just copy the source and include files int to the folder libnonfree_demo\jni and add cpp files to LOCAL_SRC_FILES in Android.mk.

The whole source can be downloaded from: https://github.com/bkornel/opencv_android_nonfree.

Original source from: http://web.guohuiwang.com/technical-notes/sift_surf_opencv_android