本书中文版链接：京东  当当

英文原版链接：亚马逊《Mastering OpenCV Android Application Programming》

        原书所给代码以章为单位，针对的Android版本从API 19到API 21不等，同时使用的OpenCV库版本也有2.4.9和2.4.10两种。本文给出的代码是在原书代码的基础上，针对Android 7.0（API 24）与OpenCV 3.2进行了修改，适当地增加了一些预处理等操作，以使代码整体上更合理。

        原书的完整代码可以在这里获取：https://www.packtpub.com/lcode_download/22299。

        更新后的代码托管在GitHub：https://github.com/johnhany/MOAAP/tree/master/MOAAP-Chp3-r3。

        关于在Android Studio上配置OpenCV 3.1开发环境的方法，请参考《在Android Studio上进行OpenCV 3.1开发》。

        关于在Android Studio上利用CMAKE配置OpenCV NDK开发环境的方法，请参考《Android Studio 2.3利用CMAKE进行OpenCV 3.2的NDK开发》。

        本章介绍如何在Android Studio 2上利用OpenCV 3.2的Java API和Native API，通过Android NDK r15b开发一个图像匹配应用，涉及的算法有SIFT，SURF，ORB，BRISK和FREAK。关于以上各算法的原理可以参考《深入OpenCV Android应用开发》第三章。

        本书中文版在62页介绍了向Android项目添加nonfree模块的方法，但对于OpenCV 3.2是不适用的。首先，在OpenCV 3.2中，SIFT等匹配算法被转移到了opencv_contrib项目的xfeatures2d模块中。其次，即使在项目的Android.mk中添加所需的源码文件，由于OpenCV 3.2本身的一个BUG，而且，由于匹配算法大多受专利保护，OpenCV源码的features2d_manual.hpp文件中默认没有声明相应的C++接口。所以就需要自己来编译支持SIFT等匹配算法的OpenCV Android SDK，并生成相应的OpenCV Manager安装包。所以，本文所给的代码与原书相比变化较大。

        此外，由于涉及的目标匹配算法计算量都比较大，所以本章引入了Android多线程编程技巧，利用AsyncTask执行核心计算，以避免阻塞主UI进程。具体细节请参考下文代码。

开发环境：

        Ubuntu 16.04 x64（用于OpenCV Android SDK的编译，也可直接下载我编译好的SDK）

        Windows 10 x64专业版（用于项目开发）

        Android Studio 2.3.3（Gradle 3.3，Android Plugin 2.3.3）

        Android 7.0（API 24）

        Android NDK r15b

        JDK 8u141

        OpenCV 3.2.0 Android SDK（需要自己编译）

编译OpenCV Android SDK：

        请参考《Ubuntu 16.04下为Android编译OpenCV 3.2.0 Manager》的过程为该项目编译一个支持SIFT，SURF和FREAK算法的OpenCV Android SDK，并将编译产生的OpenCV-android-sdk文件夹拷贝到某个位置，比如/home/john/Android/OpenCV-contrib-android-sdk（我把自己编译的OpenCV Android SDK重命名为OpenCV-contrib-android-sdk以区分官方的预编译库）。

        我的Android SDK目录为/home/john/Android/Sdk，Android NDK目录为/home/john/Android/Sdk/ndk-bundle。

        向Android项目导入自己编译的OpenCV Android SDK，显示的名称为openCVLibrary310dev，而且在项目结构视图里会出现两个模块，分别为openCVLibrary310和openCVLibrary310dev，其中openCVLibrary310dev需要加入app的Dependencies。如果是以3.2.0 Release源码编译的，则导入后的模块名称仍为openCVLibrary320。

代码及简略解释：

        1.创建Android Studio项目，包命名为net.johnhany.moaap_chp3。

        2.在app\src\main\java目录中找到net.johnhany.moaap_chp3包，为MainActivity.java添加如下代码：

package net.johnhany.moaap_chp3;

import android.Manifest;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import org.opencv.android.BaseLoaderCallback;
import org.opencv.android.LoaderCallbackInterface;
import org.opencv.android.OpenCVLoader;
import org.opencv.android.Utils;
import org.opencv.core.Core;
import org.opencv.core.CvType;
import org.opencv.core.Mat;
import org.opencv.core.MatOfDMatch;
import org.opencv.core.MatOfKeyPoint;
import org.opencv.core.Point;
import org.opencv.core.Scalar;
import org.opencv.core.DMatch;
import org.opencv.features2d.DescriptorExtractor;
import org.opencv.features2d.DescriptorMatcher;
import org.opencv.features2d.FeatureDetector;
import org.opencv.features2d.Features2d;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

import java.io.FileNotFoundException;
import java.io.InputStream;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private final int SELECT_PHOTO_1 = 1;
    private final int SELECT_PHOTO_2 = 2;
    private ImageView ivImage1;
    private TextView tvKeyPointsObject1, tvKeyPointsObject2, tvKeyPointsMatches, tvTime;
    private int keypointsObject1, keypointsObject2, keypointMatches;
    Mat src1, src2, src1_gray, src2_gray;
    static int ACTION_MODE = 0;
    private boolean src1Selected = false, src2Selected = false;
    static int REQUEST_READ_EXTERNAL_STORAGE = 11;
    static int REQUEST_WRITE_EXTERNAL_STORAGE = 12;
    static boolean read_external_storage_granted = false;
    static boolean write_external_storage_granted = false;

    private BaseLoaderCallback mOpenCVCallBack = new BaseLoaderCallback(this) {
        @Override
        public void onManagerConnected(int status) {
            switch (status) {
                case LoaderCallbackInterface.SUCCESS:
                    //DO YOUR WORK/STUFF HERE
                    System.loadLibrary("xfeatures2d");
                    break;
                default:
                    super.onManagerConnected(status);
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_3_2_0, this, mOpenCVCallBack);

        if(getSupportActionBar() != null) {
            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        }

        ivImage1 = (ImageView)findViewById(R.id.ivImage1);
        tvKeyPointsObject1 = (TextView) findViewById(R.id.tvKeyPointsObject1);
        tvKeyPointsObject2 = (TextView) findViewById(R.id.tvKeyPointsObject2);
        tvKeyPointsMatches = (TextView) findViewById(R.id.tvKeyPointsMatches);
        keypointsObject1 = keypointsObject2 = keypointMatches = -1;
        tvTime = (TextView) findViewById(R.id.tvTime);
        Intent intent = getIntent();

        if(intent.hasExtra("ACTION_MODE")){
            ACTION_MODE = intent.getIntExtra("ACTION_MODE", 0);
        }

        if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.READ_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED) {
            Log.i("permission", "request READ_EXTERNAL_STORAGE");
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                    REQUEST_READ_EXTERNAL_STORAGE);
        }else {
            Log.i("permission", "READ_EXTERNAL_STORAGE already granted");
            read_external_storage_granted = true;
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();

        if (id == R.id.action_load_first_image && read_external_storage_granted) {
            Intent photoPickerIntent = new Intent(Intent.ACTION_PICK);
            photoPickerIntent.setType("image/*");
            startActivityForResult(photoPickerIntent, SELECT_PHOTO_1);
            return true;
        } else if (id == R.id.action_load_second_image && read_external_storage_granted) {
            Intent photoPickerIntent = new Intent(Intent.ACTION_PICK);
            photoPickerIntent.setType("image/*");
            startActivityForResult(photoPickerIntent, SELECT_PHOTO_2);
            return true;
        } else if(!read_external_storage_granted) {
            Log.e("APP", "pick image failed");
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent imageReturnedIntent) {
        super.onActivityResult(requestCode, resultCode, imageReturnedIntent);

        switch(requestCode) {
            case SELECT_PHOTO_1:
                if(resultCode == RESULT_OK && read_external_storage_granted){
                    try {
                        final Uri imageUri = imageReturnedIntent.getData();
                        final InputStream imageStream = getContentResolver().openInputStream(imageUri);
                        final Bitmap selectedImage = BitmapFactory.decodeStream(imageStream);
                        src1 = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC4);
                        src1_gray = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC1);
//                        ivImage1.setImageBitmap(selectedImage);
                        Utils.bitmapToMat(selectedImage, src1);
                        Imgproc.cvtColor(src1, src1_gray, Imgproc.COLOR_BGRA2GRAY);
                        src1Selected = true;
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
                break;
            case SELECT_PHOTO_2:
                if(resultCode == RESULT_OK && read_external_storage_granted){
                    try {
                        final Uri imageUri = imageReturnedIntent.getData();
                        final InputStream imageStream = getContentResolver().openInputStream(imageUri);
                        final Bitmap selectedImage = BitmapFactory.decodeStream(imageStream);
                        src2 = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC4);
                        src2_gray = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC1);
                        Utils.bitmapToMat(selectedImage, src2);
                        Imgproc.cvtColor(src2, src2_gray, Imgproc.COLOR_BGRA2GRAY);
                        src2Selected = true;
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
                break;
        }

        Toast.makeText(MainActivity.this, "First image: " + src1Selected + " Second image: " + src2Selected, Toast.LENGTH_SHORT).show();

        if(src1Selected && src2Selected){
            Log.i("APP", "Before Execute");
            new AsyncTask<Void, Void, Bitmap>() {
                private long startTime, endTime;
                @Override
                protected void onPreExecute() {
                    super.onPreExecute();
                    startTime = System.currentTimeMillis();
                }

                @Override
                protected Bitmap doInBackground(Void... params) {
                    return executeTask();
                }

                @Override
                protected void onPostExecute(Bitmap bitmap) {
                    super.onPostExecute(bitmap);
                    endTime = System.currentTimeMillis();
                    ivImage1.setImageBitmap(bitmap);
                    tvKeyPointsObject1.setText(getString(R.string.result_target_1, keypointsObject1));
                    tvKeyPointsObject2.setText(getString(R.string.result_target_2, keypointsObject2));
                    tvKeyPointsMatches.setText(getString(R.string.result_target_matches, keypointMatches));
                    tvTime.setText(getString(R.string.result_time_cost, endTime-startTime));
                }
            }.execute();
        }
    }

    private Bitmap executeTask(){
        Log.i("APP", "Execute");
        final int MAX_MATCHES = 50;
        FeatureDetector detector;
        MatOfKeyPoint keypoints1, keypoints2;
        DescriptorExtractor descriptorExtractor;
        Mat descriptors1, descriptors2;
        DescriptorMatcher descriptorMatcher;
        MatOfDMatch matches = new MatOfDMatch();
        keypoints1 = new MatOfKeyPoint();
        keypoints2 = new MatOfKeyPoint();
        descriptors1 = new Mat();
        descriptors2 = new Mat();
        Log.i("APP", "before switch");
        switch (ACTION_MODE){
            case HomeActivity.MODE_SIFT:
                detector = FeatureDetector.create(FeatureDetector.SIFT);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.SIFT);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_SL2);
//                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.FLANNBASED);
                break;
            case HomeActivity.MODE_SURF:
                detector = FeatureDetector.create(FeatureDetector.SURF);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.SURF);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_SL2);
                break;
            case HomeActivity.MODE_ORB:
                detector = FeatureDetector.create(FeatureDetector.ORB);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.ORB);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_HAMMING);
                break;
            case HomeActivity.MODE_BRISK:
                detector = FeatureDetector.create(FeatureDetector.BRISK);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.BRISK);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_HAMMING);
                break;
            case HomeActivity.MODE_FREAK:
                detector = FeatureDetector.create(FeatureDetector.FAST);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.FREAK);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_HAMMING);
                break;
            default:
                detector = FeatureDetector.create(FeatureDetector.FAST);
                descriptorExtractor = DescriptorExtractor.create(DescriptorExtractor.BRIEF);
                descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_HAMMING);
                break;
        }
        Log.i("APP", "After switch");

        detector.detect(src2_gray, keypoints2);
        detector.detect(src1_gray, keypoints1);
        Log.i("APP", CvType.typeToString(src1_gray.type())+" "+CvType.typeToString(src2_gray.type()));
        Log.i("APP", keypoints1.toArray().length+" keypoints");
        Log.i("APP", keypoints2.toArray().length+" keypoints");
        Log.i("APP", "Detect");

        keypointsObject1 = keypoints1.toArray().length;
        keypointsObject2 = keypoints2.toArray().length;

        descriptorExtractor.compute(src1_gray, keypoints1, descriptors1);
        descriptorExtractor.compute(src2_gray, keypoints2, descriptors2);

        descriptorMatcher.match(descriptors1, descriptors2, matches);

        Log.i("APP", matches.toArray().length+" matches");

        keypointMatches = matches.toArray().length;

        Collections.sort(matches.toList(), new Comparator<DMatch>() {
            @Override
            public int compare(DMatch o1, DMatch o2) {
                if(o1.distance<o2.distance)
                    return -1;
                if(o1.distance>o2.distance)
                    return 1;
                return 0;
            }
        });

        List<DMatch> listOfDMatch = matches.toList();
        if(listOfDMatch.size()>MAX_MATCHES){
            matches.fromList(listOfDMatch.subList(0,MAX_MATCHES));
        }

//        Mat src3 = src1.clone();
//        Features2d.drawMatches(src1, keypoints1, src2, keypoints2, matches, src3);
        Mat src3 = drawMatches(src1_gray, keypoints1, src2_gray, keypoints2, matches, false);

        Log.i("APP", CvType.typeToString(src3.type()));

        Bitmap image1 = Bitmap.createBitmap(src3.cols(), src3.rows(), Bitmap.Config.ARGB_8888);
        Utils.matToBitmap(src3, image1);
        Imgproc.cvtColor(src3, src3, Imgproc.COLOR_BGR2RGB);

        if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED) {
            Log.i("permission", "request WRITE_EXTERNAL_STORAGE");
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    REQUEST_WRITE_EXTERNAL_STORAGE);
        }else {
            Log.i("permission", "WRITE_EXTERNAL_STORAGE already granted");
            write_external_storage_granted = true;
        }

        if(write_external_storage_granted) {
            boolean bool = Imgcodecs.imwrite(Environment.getExternalStorageDirectory() + "/Download/" + ACTION_MODE + ".png", src3);
            Log.i("APP", bool + " " + Environment.getExternalStorageDirectory() + "/Download/" + ACTION_MODE + ".png");
        }
        return image1;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String permissions[], @NonNull int[] grantResults) {
        if (requestCode == REQUEST_READ_EXTERNAL_STORAGE) {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                // permission was granted
                Log.i("permission", "READ_EXTERNAL_STORAGE granted");
                read_external_storage_granted = true;
            } else {
                // permission denied
                Log.i("permission", "READ_EXTERNAL_STORAGE denied");
            }
        }else if(requestCode == REQUEST_WRITE_EXTERNAL_STORAGE) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                // permission was granted
                Log.i("permission", "WRITE_EXTERNAL_STORAGE granted");
                write_external_storage_granted = true;
            } else {
                // permission denied
                Log.i("permission", "WRITE_EXTERNAL_STORAGE denied");
            }
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
    }

    static Mat drawMatches(Mat img1, MatOfKeyPoint key1, Mat img2, MatOfKeyPoint key2, MatOfDMatch matches, boolean imageOnly){
        //https://github.com/mustafaakin/image-matcher/tree/master/src/in/mustafaak/imagematcher
        Mat out = new Mat();
        Mat im1 = new Mat();
        Mat im2 = new Mat();
        Imgproc.cvtColor(img1, im1, Imgproc.COLOR_GRAY2RGB);
        Imgproc.cvtColor(img2, im2, Imgproc.COLOR_GRAY2RGB);
        if ( imageOnly){
            MatOfDMatch emptyMatch = new MatOfDMatch();
            MatOfKeyPoint emptyKey1 = new MatOfKeyPoint();
            MatOfKeyPoint emptyKey2 = new MatOfKeyPoint();
            Features2d.drawMatches(im1, emptyKey1, im2, emptyKey2, emptyMatch, out);
        } else {
            Features2d.drawMatches(im1, key1, im2, key2, matches, out);
        }
        //Bitmap bmp = Bitmap.createBitmap(out.cols(), out.rows(), Bitmap.Config.ARGB_8888);
        Imgproc.cvtColor(out, out, Imgproc.COLOR_BGR2RGB);
        Imgproc.putText(out, "Frame", new Point(img1.width() / 2,30), Core.FONT_HERSHEY_PLAIN, 2, new Scalar(0,255,255),3);
        Imgproc.putText(out, "Match", new Point(img1.width() + img2.width() / 2,30), Core.FONT_HERSHEY_PLAIN, 2, new Scalar(255,0,0),3);
        return out;
    }
}

        与原书的代码相比，变化主要有：

        (1)去掉了一些无用的import，增加了一些必要的import；

        (2)在onCreate()中增加了动态申请外部存储读取权限的机制，并增加了相应变量REQUEST_READ_EXTERNAL_STORAGE和read_external_storage_granted；在需要保存输出图像的代码之前增加了申请外部存储写入权限的代码，并增加了相应变量REQUEST_WRITE_EXTERNAL_STORAGE和write_external_storage_granted；

        (3)将部分Log.d()替换为Log.i()，便于查看调试信息；

        (4)更新了OpenCV的版本，OPENCV_VERSION_2_4_9改为OPENCV_VERSION_3_2_0；

        (5)把jni模块nonfree重命名为xfeatures2d。

        (6)考虑到FREAK方法默认对单通道图像进行匹配，所以加入将彩色图像转换为灰度图像的代码，强制对所有方法以单通道显示结果。

        3.在app\src\main\java目录中添加一个名为HomeActivity.java的文件，其内容如下：

package net.johnhany.moaap_chp3;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class HomeActivity extends Activity {

    public static final int MODE_SIFT = 1;
    public static final int MODE_SURF = 2;
    public static final int MODE_ORB = 3;
    public static final int MODE_BRISK = 4;
    public static final int MODE_FREAK = 5;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        Button bSIFT, bSURF, bORB, bBRISK, bFREAK;
        bSIFT = (Button) findViewById(R.id.bSIFT);
        bSURF = (Button) findViewById(R.id.bSURF);
        bORB = (Button) findViewById(R.id.bORB);
        bBRISK = (Button) findViewById(R.id.bBRISK);
        bFREAK = (Button) findViewById(R.id.bFREAK);
        bSIFT.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getApplicationContext(), MainActivity.class);
                i.putExtra("ACTION_MODE", MODE_SIFT);
                startActivity(i);
            }
        });
        bSURF.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getApplicationContext(), MainActivity.class);
                i.putExtra("ACTION_MODE", MODE_SURF);
                startActivity(i);
            }
        });
        bORB.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getApplicationContext(), MainActivity.class);
                i.putExtra("ACTION_MODE", MODE_ORB);
                startActivity(i);
            }
        });
        bBRISK.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getApplicationContext(), MainActivity.class);
                i.putExtra("ACTION_MODE", MODE_BRISK);
                startActivity(i);
            }
        });
        bFREAK.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getApplicationContext(), MainActivity.class);
                i.putExtra("ACTION_MODE", MODE_FREAK);
                startActivity(i);
            }
        });
    }
}

        4.在修改app/CMakeLists.txt内容如下：

cmake_minimum_required(VERSION 3.4.1)

set(CMAKE_VERBOSE_MAKEFILE on)
set(ocvlibs "E:/dev-lib/opencv-contrib-android-sdk/sdk/native/libs")

include_directories(E:/dev-lib/opencv-contrib-android-sdk/sdk/native/jni/include)

add_library(libopencv_java3 SHARED IMPORTED )
set_target_properties(libopencv_java3 PROPERTIES
                      IMPORTED_LOCATION "${ocvlibs}/${ANDROID_ABI}/libopencv_java3.so")

add_library( # Sets the name of the library.
             xfeatures2d

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/xfeatures2d_init.cpp
             src/main/cpp/sift.cpp
             src/main/cpp/surf.cpp
             src/main/cpp/freak.cpp)

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

target_link_libraries( # Specifies the target library.
                       xfeatures2d android log libopencv_java3

                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

        关于该文件中各项的含义，请参考《Android Studio 2.3利用CMAKE进行OpenCV 3.2的NDK开发》。

        5.把/home/john/Downloads/opencv-master/opencv_contrib/modules/xfeatures2d/src目录下的freak.cpp，precomp.hpp，sift.cpp，surf.cpp，surf.hpp，xfeatures2d_init.cpp共6个文件拷贝到app\src\main\cpp文件夹中。其中，precomp.hpp文件需要做如下修改：

        注释掉第52-53行的

#include "opencv2/core/private.hpp"
#include "opencv2/core/private.cuda.hpp"

        再注释掉第62行的

#include "opencv2/core/private.hpp"

        6.修改app/src/main/res/layout/activity_main.xml文件内容为：

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="net.johnhany.moaap_chp3.MainActivity">

    <LinearLayout android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <ImageView
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="0.5"
            android:id="@+id/ivImage1" />
        <TextView
            android:id="@+id/tvKeyPointsObject1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
        <TextView
            android:id="@+id/tvKeyPointsObject2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
        <TextView
            android:id="@+id/tvKeyPointsMatches"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
        <TextView
            android:id="@+id/tvTime"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
    </LinearLayout>
</ScrollView>

        7.在app/src/main/res/layout目录中添加activity_home.xml文件，其内容如下：

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <LinearLayout
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:orientation="vertical" >

        <Button
            android:id="@+id/bSIFT"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="SIFT" />
        <Button
            android:id="@+id/bSURF"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="SURF" />
        <Button
            android:id="@+id/bORB"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="ORB" />
        <Button
            android:id="@+id/bBRISK"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="BRISK" />
        <Button
            android:id="@+id/bFREAK"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:text="FREAK" />
    </LinearLayout>
</ScrollView>

        8.在app/src/main/res目录下新建一个名为menu的文件夹，在其中添加一个menu_main.xml文件，内容如下：

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context="net.johnhany.moaap_chp3.MainActivity">
    <item android:id="@+id/action_load_first_image"
        android:title="@string/action_load_first_image"
        android:orderInCategory="1"
        app:showAsAction="never" />
    <item android:id="@+id/action_load_second_image"
        android:title="@string/action_load_second_image"
        android:orderInCategory="2"
        app:showAsAction="never" />
</menu>

        9.修改app/src/main/res/values/strings.xml文件，内容为：

<resources>
    <string name="app_name">第三章 - 深入OpenCV Android应用开发</string>
    <string name="action_load_first_image">Load First Image</string>
    <string name="action_load_second_image">Load Second Image</string>
    <string name="title_activity_main">第三章 - 深入OpenCV Android应用开发</string>
    <string name="title_activity_home">第三章 - 深入OpenCV Android应用开发</string>
    <string name="result_target_1">目标1：%1$d</string>
    <string name="result_target_2">目标2：%1$d</string>
    <string name="result_target_matches">关键点匹配：%1$d</string>
    <string name="result_time_cost">耗费时间：%1$d ms</string>
</resources>

        10.修改app/src/main/AndroidManifest.xml文件为如下内容：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="net.johnhany.moaap_chp3">

    <supports-screens
        android:anyDensity="true"
        android:largeScreens="true"
        android:normalScreens="true"
        android:resizeable="true"
        android:smallScreens="true" />

    <uses-permission
        android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission
        android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_main"
            android:parentActivityName=".HomeActivity" >
        </activity>

        <activity android:name=".HomeActivity"
            android:label="@string/title_activity_home">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

        11.修改app/build.gradle文件为：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 24
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "net.johnhany.moaap_chp3"
        minSdkVersion 16
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags "-std=c++11", "-frtti", "-fexceptions"
                abiFilters 'x86', 'x86_64', 'armeabi', 'armeabi-v7a', 'arm64-v8a', 'mips', 'mips64'
            }
        }
    }
    sourceSets {
        main {
            jniLibs.srcDirs = ['E:\\dev-lib\\opencv-contrib-android-sdk\\sdk\\native\\libs']
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:24.2.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'
    compile project(':openCVLibrary320')
}

        12.修改openCVLibrary320项目下的build.gradle文件为：

apply plugin: 'com.android.library'

android {
    compileSdkVersion 24
    buildToolsVersion "26.0.1"

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion 24
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
}

        13.确认一下项目根目录下的build.gradle文件是否为：

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

运行效果：