# How to Import OpenCV in Android NDK
###### tags: `android` `opencv` `ndk`

## Pre-Requirement
* Please install Android SDK and NDK packages. (see [Android Native Development Kit (NDK)](https://hackmd.io/WO-nZYsfR1icmYbWy3jFrQ))

* Download and unzip OpenCV for Android [(link)](https://opencv.org/releases.html)

## Demo APP - OpenCVDemo
### Create a New Project
* Enable C++ support

* Enable Exception and RTTI Support

* Modify build.gradle(Module: app) to configure the ABI (armeabi-v7a as example)

    defaultConfig {
        applicationId "com.jd.opencvdemo"
        minSdkVersion 23
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags "-frtti -fexceptions"
            }
        }
        // Add below
        ndk {
            // Specifies the ABI configurations of your native
            // libraries Gradle should build and package with your APK.
            abiFilters  'armeabi-v7a'
        }
    }

### User Interface
* There are several UI components
* (Button) btn_load : open the Android Image Pick and load image from sd card.
* (Button) btn_proess : trigger our native image process function
* (Button) btn_save : save the current image into sd card
* (ImageView) img_main : display the image

### Request Permission (AndroidManifest.xml)
* Since this demo app requires the permission of accessing sd card to get the image, we need to add **<user-permission>** into the manifest file.

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.jd.opencvdemo">
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <application
    ...

* for more information about permission, see [Permission](https://developer.android.com/guide/topics/permissions/overview).

### Import Library
* Copy OpenCV-android-sdk/sdk/native/jni/include folder to YourProject/app/src/main/cpp/


* Copy the libopencv_java3.so of corresponsed ABI (or all) in OpenCV-android-sdk/sdk/native/libs to your Android project app/src/main/jniLibs
* arm64-v8a
* armeabi
* armeabi-v7a
* mips
* mips64
* x86
* x86_64


* Modify CmakeList.txt
* set the library path and its properties.

        include_directories(${CMAKE_SOURCE_DIR}/src/main/cpp/include)

        add_library(
            libopencv_java3
            SHARED
            IMPORTED
        )

        set_target_properties(
            libopencv_java3
            PROPERTIES IMPORTED_LOCATION
            ${CMAKE_SOURCE_DIR}/src/main/jniLibs/${ANDROID_ABI}/libopencv_java3.so
        )

* link *libopencv_java3*

        target_link_libraries(
                       native-lib
                       libopencv_java3
                       ${log-lib}
                       ${android-lib})

* Rebuild the project and you should find *libopencv_java3* is added.


### Build the Native Function (native-lib.cpp)
* include the .h file

    #include <opencv2/opencv.hpp>
    #include <opencv2/core.hpp>
    using namespace cv;

* define the native function : imgProcess

    extern "C"
    JNIEXPORT void JNICALL
    Java_com_jd_opencvdemo_MainActivity_imgProcess(
        JNIEnv *env,
        jobject, /*this*/
        jint h, jint w, jintArray RGBFrameData, jintArray ResFrameData) {

    // get data
    jint * pRGBFrameData = env->GetIntArrayElements(RGBFrameData, 0);
    jint * pResFrameData = env->GetIntArrayElements(ResFrameData, 0);

    // create the matrix of original image
    Mat mRGB(h, w, CV_8UC4, (unsigned char*)pRGBFrameData);

    // convert to gray image
    Mat mGray;
    cvtColor(mRGB, mGray, CV_RGBA2GRAY);

    // find edges by canny
    Mat mCanny;
    Canny(mGray,mCanny,10,100);

    // convert to RGBA image as result
    Mat mRes(h, w, CV_8UC4, (unsigned char*)pResFrameData);
    cvtColor(mCanny, mRes, CV_GRAY2RGBA);
    }

### Call the Native Function (OpenCV.java)
* load library

* declare the native function

    public class OpenCV {
        // Used to load the 'native-lib' library on application startup.
        static {
            System.loadLibrary("native-lib");
        }

        /**
         * A native method that is implemented by the 'native-lib' native library,
         * which is packaged with this application.
         */
        public native static void canny(int h, int w, int[] RGBFrameData, int[] ResFrameData);

    }

### Control UI Component and Trigger the Process (MainActivity.java)
* load image from Android gallery (btn_load)

    btn_load.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // open gallery
                Intent i = new Intent(Intent.ACTION_PICK);
                i.setType("image/*");
                startActivityForResult(i, SELECT_IMAGE);
            }
        });

* call the function by clicking button (btn_proc)

    btn_proc.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // get image
                Bitmap bitmap = ((BitmapDrawable)img_main.getDrawable()).getBitmap();
                int w = bitmap.getWidth();
                int h = bitmap.getHeight();

                // get rgb format data
                int[] rgb = new int[w * h];
                bitmap.getPixels(rgb, 0, w, 0, 0, w, h);

                // call native function to process the image
                int[] img_res = new int[w * h];
                OpenCV.canny(h, w, rgb, img_res);

                // show the result
                Bitmap bmp = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
                bmp.setPixels(img_res,0,w,0,0,w,h);
                img_main.setImageBitmap(bmp);
            }
        });

* save image in Pictures folder (btn_save)

    btn_save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // permission check
                if (ActivityCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
                    ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, REQUEST_SDCARD_PERMISSION);
                    return;
                }

                boolean sdAvailable = checkSDCard();

                if (!sdAvailable){
                    Log.e(LOG_TAG,"SD card is not available!");
                    return;
                }

                // find path
                File appDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
                if (!appDir.exists() && !appDir.mkdirs()) {
                    Log.e(LOG_TAG, "Directory not created");
                }

                String fileName = "test_"+System.currentTimeMillis() + ".jpg";
                File file = new File(appDir, fileName);

                // save file into gallery
                Bitmap bitmap = ((BitmapDrawable)img_main.getDrawable()).getBitmap();
                try {
                    FileOutputStream fos = new FileOutputStream(file);
                    bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos);
                    fos.flush();
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

                try {
                    MediaStore.Images.Media.insertImage(getApplication().getContentResolver(), file.getAbsolutePath(), fileName, null);
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                }

                // make a notification to system to update the gallery
                getApplication().sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE, Uri.parse(file.getAbsolutePath())));

                // check whether it is saved successfully
                if(file.exists()){
                    Toast.makeText(getApplicationContext(),"Success!", Toast.LENGTH_SHORT).show();
                }
                else{
                    Toast.makeText(getApplicationContext(), "Failed!", Toast.LENGTH_LONG).show();
                }
            }
        });

### Result
* Original Image

* Canny Image

## Reference
* [Android NDK学习笔记：Android Studio3.1+CMAKE+OpenCV3.4配置](https://blog.csdn.net/CV_Jason/article/details/79758823)

* [OpenCV使用Canny边缘检测器实现图像边缘检测](http://kongqw.com/2016/08/19/2016-08-19-OpenCV%E4%BD%BF%E7%94%A8Canny%E8%BE%B9%E7%BC%98%E6%A3%80%E6%B5%8B%E5%99%A8%E5%AE%9E%E7%8E%B0%E5%9B%BE%E5%83%8F%E8%BE%B9%E7%BC%98%E6%A3%80%E6%B5%8B/)