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，适当地修改了一些操作，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-Chp6-r3。

        关于在Android Studio上配置OpenCV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开发一个图像拼接应用，涉及特征检测与匹配、图像匹配、光束平差法、自动校直、增益补偿、多频段融合等知识。但我们的应用只需要在C++部分调用OpenCV的Stitcher就可完成整个拼接过程。关于以上各算法的原理可以参考《深入OpenCV Android应用开发》第六章。

开发环境

        Windows 10 x64专业版

        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

代码及简略解释

        1. 创建Android Studio项目，包命名为net.johnhany.moaap_chp6。注意我们需要建立一个NDK项目，所以需要勾选Include C++ Support。导入OpenCV-android-sdk\sdk\java到项目中，并为app模块加载模块依赖。

        2. 在app\src\main\java目录中找到net.johnhany.moaap_chp6包，为MainActivity.java添加如下代码：

package net.johnhany.moaap_chp6;

import android.Manifest;
import android.app.ProgressDialog;
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.provider.MediaStore;
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.view.View;
import android.widget.Button;
import android.widget.ImageView;
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.CvType;
import org.opencv.core.Mat;
import org.opencv.core.Size;
import org.opencv.imgproc.Imgproc;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private final int CLICK_PHOTO = 1;
    private final int SELECT_PHOTO = 2;
    private Uri fileUri;
    private ImageView ivImage;
    private Mat src;
    private List<Mat> listImage	= new ArrayList<>();
    private static final String FILE_LOCATION = Environment.getExternalStorageDirectory() + "/Download/MOAAP/Chapter6/";
    static int REQUEST_READ_EXTERNAL_STORAGE = 11;
    static boolean read_external_storage_granted = false;

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ivImage = (ImageView)findViewById(R.id.ivImage);

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

        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;
        }

        Button bClickImage, bDone;

        bClickImage = (Button)findViewById(R.id.bClickImage);
        bDone = (Button)findViewById(R.id.bDone);

        bClickImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                File imagesFolder = new File(FILE_LOCATION);
                //imagesFolder.mkdirs();
                File image = new File(imagesFolder, "panorama_"+ (listImage.size()+1) + ".jpg");
                fileUri = Uri.fromFile(image);
                Log.d("MainActivity", "File URI = " + fileUri.toString());
                intent.putExtra(MediaStore.EXTRA_OUTPUT, fileUri); // set the image file name

                // start the image capture Intent
                startActivityForResult(intent, CLICK_PHOTO);
            }
        });

        bDone.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(listImage.size()==0){
                    Toast.makeText(getApplicationContext(), "No images clicked", Toast.LENGTH_SHORT).show();
                } else if(listImage.size()==1){
                    Toast.makeText(getApplicationContext(), "Only one image clicked", Toast.LENGTH_SHORT).show();
                    Bitmap image = Bitmap.createBitmap(src.cols(), src.rows(), Bitmap.Config.ARGB_8888);
                    Utils.matToBitmap(src, image);
                    ivImage.setImageBitmap(image);
                } else {
                    createPanorama();
                }
            }
        });
    }

    @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_image && read_external_storage_granted) {
            Intent photoPickerIntent = new Intent(Intent.ACTION_PICK);
            photoPickerIntent.setType("image/*");
            startActivityForResult(photoPickerIntent, SELECT_PHOTO);
            return true;
        } else if(!read_external_storage_granted) {
            Log.e("MainActivity", "pick image failed");
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

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

        Log.d("MainActivity", "request code " + requestCode + ", click photo " + CLICK_PHOTO + ", result code " + resultCode + ", result ok " + RESULT_OK);

        switch(requestCode) {
            case CLICK_PHOTO:
                if(resultCode == RESULT_OK){
                    try {
                        Log.d("MainActivity", fileUri.toString());
                        final InputStream imageStream = getContentResolver().openInputStream(fileUri);
                        final Bitmap selectedImage = BitmapFactory.decodeStream(imageStream);
                        src = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC4);
                        Imgproc.resize(src, src, new Size(src.rows()/4, src.cols()/4));
                        Utils.bitmapToMat(selectedImage, src);
                        Imgproc.cvtColor(src, src, Imgproc.COLOR_BGR2RGB);
                        listImage.add(src);
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
                break;
            case SELECT_PHOTO:
                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);
                        src = new Mat(selectedImage.getHeight(), selectedImage.getWidth(), CvType.CV_8UC4);
                        Imgproc.resize(src, src, new Size(src.rows()/4, src.cols()/4));
                        Utils.bitmapToMat(selectedImage, src);
                        Imgproc.cvtColor(src, src, Imgproc.COLOR_RGBA2BGR);
                        Log.d("MainActivity", "image height " + src.rows() + ", image width " + src.cols());
                        listImage.add(src);
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
                break;
        }
    }

    @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");
            }
        }
    }

    private void createPanorama(){
        new AsyncTask<Void, Void, Bitmap>() {
            ProgressDialog dialog;
            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                dialog = ProgressDialog.show(MainActivity.this, "Building Panorama", "Please Wait");
            }

            @Override
            protected Bitmap doInBackground(Void... params) {
                int	elems = listImage.size();
                long[] tempobjadr = new long[elems];
                for	(int i=0; i<elems; i++){
                    tempobjadr[i] = listImage.get(i).getNativeObjAddr();
                }
                Mat result = new Mat();

                int stitchstatus = StitchPanorama(tempobjadr, result.getNativeObjAddr());
                Log.d("MainActivity", "result height " + result.rows() + ", result width " + result.cols());
                if(stitchstatus != 0){
                    Log.e("MainActivity", "Stitching failed: " + stitchstatus);
                    return null;
                }

                Imgproc.cvtColor(result, result, Imgproc.COLOR_BGR2RGBA);
                Bitmap bitmap = Bitmap.createBitmap(result.cols(), result.rows(), Bitmap.Config.ARGB_8888);
                Utils.matToBitmap(result, bitmap);

                return bitmap;
            }

            @Override
            protected void onPostExecute(Bitmap bitmap) {
                super.onPostExecute(bitmap);
                dialog.dismiss();
                if(bitmap!=null) {
                    ivImage.setImageBitmap(bitmap);
                }
            }
        }.execute();
    }

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

    public native int StitchPanorama(long[]	imageAddressArray, long	outputAddress);

}

        原书给出的代码是保留一个空的MainActivity，创建一个StitchingActivity，然后所有的操作在StitchingActivity当中执行。我这里改为全部Java操作只在MainActivity中执行。另外，Java层与Native层之间图像数据的传递方式也产生了一些变化。而且，原书所给代码只提供了一个用于载入图像的菜单按钮，并没有实际提供对从内存中读取的图像进行拼接的功能。我修改为即可以通过摄像头拍照进行拼接，也可以通过读取已有图像的方式来拼接。

        3. 修改app\src\main\res\layout\activity_main.xml内容如下：

<?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_chp6.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/ivImage" />
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">
            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="0.5"
                android:id="@+id/bClickImage"
                android:text="Click more images"/>
            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_weight="0.5"
                android:id="@+id/bDone"
                android:text="Done"/>
        </LinearLayout>
    </LinearLayout>
</ScrollView>

        4. 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.
             stitcher

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/stitcher.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.
                       stitcher 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》。另外，我这里引用的是包含opencv_contrib模块的OpenCV Android SDK，其实直接引用官方预编译SDK也可。

        5. 将项目默认产生的app\src\main\cpp\native-lib.cpp文件更名为stitcher.cpp，然后将文件内容修改为：

#include <jni.h>
#include <vector>
#include <android/log.h>

#include "opencv2/opencv.hpp"

using namespace cv;
using namespace std;

char filepath1[100] = "/storage/emulated/0/Download/MOAAP/Chapter6/panorama_stitched.jpg";

#define  LOG_TAG    "MOAAP-CHP6"

#define  LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define  LOGE(...)  __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)

extern "C"
{
JNIEXPORT jint JNICALL
Java_net_johnhany_moaap_1chp6_MainActivity_StitchPanorama(JNIEnv *env, jobject,
                                                                   jlongArray imageAddressArray,
                                                                   jlong outputAddress) {
    jsize a_len = env->GetArrayLength(imageAddressArray);
    jlong *imgAddressArr = env->GetLongArrayElements(imageAddressArray,0);
    vector<Mat> imgVec;
    for(int	k=0; k<a_len; k++)
    {
        Mat	&curimage = *(Mat*)imgAddressArr[k];
        Mat	newimage;
        curimage.copyTo(newimage);
        float scale = 500.0f / curimage.rows;
        resize(newimage, newimage, Size((int)(scale*curimage.cols), (int)(scale*curimage.rows)));
        LOGD("Image height %d width %d", newimage.rows, newimage.cols);
        imgVec.push_back(newimage);
    }
    Mat	&result = *(Mat*)outputAddress;

    Stitcher stitcher = Stitcher::createDefault();
    Stitcher::Status status = stitcher.stitch(imgVec, result);

    LOGD("Result height %d width %d", result.rows, result.cols);

    imwrite(filepath1, result);

    return status;
}
}

        6. 在app\src\main\res目录下创建一个名为menu的Andorid resource directory，再在res\menu中创建一个menu_main.xml文件：

<?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">
    <item android:id="@+id/action_load_image"
        android:title="@string/action_load_image"
        android:orderInCategory="1"
        app:showAsAction="never" />
</menu>

        7. 修改app\src\main\res\values\strings.xml文件：

<resources>
    <string name="app_name">第六章 - 深入OpenCV Android应用开发</string>
    <string name="action_load_image">Load Image</string>
    <string name="title_activity_stitching">第六章 - 深入OpenCV Android应用开发</string>
    <string name="title_activity_home">第六章 - 深入OpenCV Android应用开发</string>
</resources>

        8. 修改app\src\main\res\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_chp6">

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

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

    <uses-feature
        android:name="android.hardware.camera"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.autofocus"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front.autofocus"
        android:required="false" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true">
        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_stitching"
            android:theme="@style/AppTheme">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>

        9. 将app\build.gradle文件修改为：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 24
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "net.johnhany.moaap_chp6"
        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')
}

        同样地，这里引用的是包含opencv_contrib模块的OpenCV Android SDK，读者可以直接引用官方预编译SDK。

        10. openCVLibrary320\build.gradle文件修改为：

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'
        }
    }
}

        11. 检查一下项目根目录的build.gradle文件是否为如下内容：

// 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

        测试图片来自https://github.com/opencv/opencv_extra/tree/master/testdata/stitching。

        原始图片：

        拼接后的效果：